Strengthening teaching and learning of forces: Teaching and learning ...
The National Strategies | Secondary Strengthening teaching and learning of forces: Teaching and learning about forces and motion
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Strengthening teaching and learning of forces: Teaching and learning about forces and motion The reason why force is an important idea in science is because the motion of any object can be explained by considering the forces acting on the object. We are going to examine a procedure, supported by two rules, to analyse and explain the motion of an object and the forces acting on the object. This procedure was included in the Introductory section of this study guide. 1. Identify all the forces acting on the object you are interested in, noting their directions. 2. Add the forces acting on the object to find the resultant (or total) force acting on it. 3. Apply the two rules: −− if there is a resultant force acting on an object, this will cause a change in its motion, in the direction of the force −− if the resultant force acting on an object is zero, its motion does not change. Both rules also apply in the other direction; that is if you know what the motion of an object is, you can deduce what the resultant force acting on it is. It should be emphasised that these rules are completely general, that is they apply to all examples of motion, without exception. They are a way of stating Newton’s First Law of Motion. At this point let’s recall Newton’s Laws, which are formally stated as: 1. Every body continues in a state of rest or uniform motion in a straight line unless acted upon by an external impressed force. 2. The rate of change of momentum is proportional to the impressed force, and takes place in the direction of the force. 3. Action and reaction are equal and opposite. Task 8: Newton’s First Law of Motion Restate Newton’s First Law of Motion, in your own words, as though explaining it to a pupil. Here is an example that might help. If the forces on a mass are balanced (no resultant force), then if it starts off: ––
at rest, it will stay at rest
––
moving in a straight line, it will keep on moving at a constant speed in a straight line.
The great value of the First Law, and indeed also the Second and Third Laws, is that they are absolutely precise and can be used to make exact numerical predictions about the motion of objects. However, the most important objective in teaching about forces and motion at Key Stage 3 is a qualitative understanding of these ideas. Quantitative use of this law usually occurs at Key Stage 4. The real purpose of Newton’s Laws is to extend and formalise the kind of everyday understanding that everything has of motion. When pupils understand this, it opens up a completely new way of looking at, and explaining, how and why things move the way they do.
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The National Strategies | Secondary Strengthening teaching and learning of forces: Teaching and learning about forces and motion
Forces and motion Let us continue with the use of the procedure we started with. Remember it ended with the two rules:
• •
if there is a resultant force acting on an object, this will cause a change in its motion, in the direction of the force if the resultant force acting on an object is zero, its motion does not change.
Applying the two rules requires that pupils understand what counts as a ‘change of motion’. A change of motion means that the object changes its speed or the direction in which it is moving. Therefore, an object at rest (stationary) or one that is moving at a constant speed in a straight line is not changing its motion. In these situations, the resultant force acting on the object is zero. Conversely, an object that is moving in a curved path (e.g. in a circle) at a constant speed is experiencing a change in its motion (a change in the direction in which it is moving). In this situation, there is a resultant force acting on the object. For objects moving in a straight line, the rules lead to the following conclusions.
•
When there is a resultant force (which is not zero) acting on an object: −− a stationary object will start to move in the direction of the resultant force, and its speed will steadily increase −−
•
an object moving in the direction of the resultant force will continue moving in that direction with its speed steadily increasing
−− an object moving in the opposite direction to the resultant force will continue moving in that direction with its speed steadily decreasing to zero. If the resultant force continues to act, the object will then start moving in the opposite direction (i.e. in the direction of the resultant force) with its speed steadily increasing. When the resultant force acting on an object is zero: −− if the object is stationary, it will remain stationary −− if the object is moving, it will continue moving at a steady speed in the same direction.
Task 9: Forces in action Time to check your understanding of the forces acting on balls after they have been kicked, hit or thrown. It is essential to be clear that the ‘propulsion’ force is not carried by the ball once it is no longer in contact with the source of the force, i.e. the foot, bat or hand.
• •
How would you explain to a pupil that a ball continues to move after it has been thrown even though there is no longer any propulsion force acting upon it? If you are unsure, talk through your explanation with a colleague more experienced in teaching about forces.
If you are unsure about your explanation, the parts that follow will help you further.
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Understanding constant speed (uniform motion) Almost all pupils at Key Stage 3 will think that an object moving with steady speed requires a net force to keep it moving. Their experience, and that of most adults, is of a world where they have come to terms with friction. In order to move something they have to continue to apply a force so that the object moves in the direction of the force. Newton’s First Law of Motion can be described in a number of ways, but generally states that every object moves with uniform motion, in a straight line, unless acted upon by some external force. Such conditions only exist in the type of ‘perfect’ world (one without friction) that physicists use to help explain the motion of objects. However, in the ‘real’ world frictional forces act on an object, usually with an opposing force to the one that is trying to move the object. When an object is moving at constant speed the resultant force (or total force) is zero, that is the driving force and opposing forces balance each other out. A stationary object is therefore just a special case of steady motion. In terms of resultant forces, there is no difference between being at rest, and moving uniformly at a constant speed in a straight line.
Objects that have been kicked, hit or thrown A particular situation that pupils find difficult to interpret is the movement of an object that has been set in motion and is now slowing down. Examples include a football that has been kicked and is rolling along the ground, or a ball that has been thrown vertically upwards. In situations like these, many pupils mark a force in the direction of motion. But this force exists only during the interaction that set the object in motion. Once it has left the foot or hand of the person who made it move, there is no force in the direction of motion. The resultant force is in the opposite direction, making the object slow down (and eventually stop, and perhaps start moving in the opposite direction). If you are feeling more confident you should be able to tackle a couple of simple problems: 1. During a ten-pin bowling game, describe the forces acting on the bowling ball: −− when it is being bowled, with the bowler’s hand still in contact −− as it moves down the bowling lane −− when it strikes the first pin. 2. How do skateboarders keep moving along a horizontal surface?
Task 10: False statements In this task you are going to look back at the first five of the pupils’ statements about forces from earlier, which are printed below. If there is motion there is a force acting If there is no motion, then there is no force acting There cannot be a force without motion When an object is moving, there is a force in the direction of its motion A constant speed results from a constant force For each statement apply the procedure and rules we have been using. Can you use the procedure to identify the fallacy in each statement?
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We will now explore an approach that involves developing a deeper level of understanding by reasoning in the other direction. Task 11: Filling in the gaps In the following five sentences suggest what should go into each space. Possible choices are given in the sentences to help you.
• • • •
If an object is moving in a straight line with increasing speed, there is _________ (no/a resultant) force acting on the object, in the direction __________(of/opposite to) its motion. If an object is moving in a straight line and is slowing down, there is __________ (no/a resultant) force acting on the object, in the direction ___________ (of/opposite to) its motion. If an object is stationary, the resultant force acting on it is _________ (positive/negative/zero). If an object is moving at a steady speed in a straight line, the resultant force acting on it is ____________ (positive/negative/zero).
In the first statement, the phrase increasing speed should trigger the response that a resultant force must be acting in the direction of the motion. In the second statement, the phrase slowing down should trigger the response that there is a change in speed; therefore a resultant force is acting. The change is a reduction in speed, so the direction of the resultant force must be opposite to the movement. The third and fourth statements are deliberately set to challenge your thinking in the context of the ‘perfect’ world and are based upon an understanding of Newton’s First Law. The third statement is less challenging than the last one and is a situation where the resultant force is zero. Zero is the correct response for the last statement also. The clue is in the words steady speed within this statement.
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Applying the approach to a range of practical situations Tasks 12 and 13 provide opportunities to look at activities more directly related to teaching. You may be able to try out some of these activities with small groups of pupils and to reflect on the teaching of forces and motion. Task 12: Classroom activities This task focuses on some fairly typical classroom activities used in teaching about forces and motion. Four such activities are suggested below, but you could substitute activities from your own department’s scheme of learning.
Activity 1
Activity 2
For the activity described below follow the procedure outlined.
For the activity described below follow the procedure outlined.
A parachutist jumps from a plane. She free falls for a few moments then opens her parachute. Some time later she reaches the ground.
Procedure
• • •
Identify all the forces acting on the object you are interested in, noting their directions. Add the forces acting on the object to find the resultant (or total) force acting on it. Apply the following rules:
You have a flexible track with a ball bearing. Place the ball bearing near the top at one side of the curve and predict where it will reach on the opposite side after it has been released. Try it and see if you were correct. Change the shape of the track and try again.
Procedure
• • •
Identify all the forces acting on the object you are interested in, noting their directions. Add the forces acting on the object to find the resultant (or total) force acting on it. Apply the following rules:
−−
if there is a resultant force acting on an object, this will cause a change in its motion, in the direction of the force
−−
if there is a resultant force acting on an object, this will cause a change in its motion, in the direction of the force
−−
if the resultant force acting on an object is zero, its motion does not change.
−−
if the resultant force acting on an object is zero, its motion does not change.
1. What are the forces acting?
1. What are the forces acting?
2. What is the resultant force and in what direction does it act?
2. What is the resultant force and in what direction does it act?
3. How does the motion of the object change?
3. How does the motion of the object change?
4. What are the main learning points?
4. What are the main learning points?
5. What are the main teaching points?
5. What are the main teaching points?
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The National Strategies | Secondary Strengthening teaching and learning of forces: Teaching and learning about forces and motion
Activity 3
Activity 4
For the activity described below follow the procedure outlined.
For the activity described below follow the procedure outlined.
You have been given an office chair with wheels. Sit on the chair with your feet against a wall. Now push against the wall and experience the resulting motion.
Procedure
• • •
Identify all the forces acting on the object you are interested in, noting their directions. Add the forces acting on the object to find the resultant (or total) force acting on it. Apply the following rules:
You have been given an empty bottle and sink of water. Place the empty bottle on the water and observe what happens. Then add a few marbles (or a little sand) at a time and put the bottle back into the water.
Procedure
• • •
Identify all the forces acting on the object you are interested in, noting their directions. Add the forces acting on the object to find the resultant (or total) force acting on it. Apply the following rules:
−−
if there is a resultant force acting on an object, this will cause a change in its motion, in the direction of the force
−−
if there is a resultant force acting on an object, this will cause a change in its motion, in the direction of the force
−−
if the resultant force acting on an object is zero, its motion does not change.
−−
if the resultant force acting on an object is zero, its motion does not change.
1. What are the forces acting?
1. What are the forces acting?
2. What is the resultant force and in what direction does it act?
2. What is the resultant force and in what direction does it act?
3. How does the motion of the object change?
3. How does the motion of the object change?
4. What are the main learning points?
4. What are the main learning points?
5. What are the main teaching points?
5. What are the main teaching points?
• • • •
If possible set up these activities in a room at school. Try out the activities yourself and try to answer the questions at the foot of each activity. Try it out with a small group of pupils who have already studied forces and motion. Listen carefully to their discussions. Try to probe their thoughts and understanding to give you an idea of their learning about forces and motion.
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Forces in pairs: Newton’s Third Law The questions in the next task probe pupils’ understanding of the fact that forces always come in pairs. If A pushes B, then B pushes back on A, with an equal force. This applies to both steady forces, and to short-lived forces such as those that arise in a collision or sudden impact. The forces are always equal, at every instant during an interaction, and regardless of the sizes of the objects involved, or their motion. Diagnosing Pupils’ Understanding of Forces and Motion
Some understanding that forces arise from interactions and always come in pairs is essential to make sense of the ideas required at Key Stage 3. There is a strong case for teaching these ideas earlier than Key Stage 4. Common misunderstandings are likely to include the following.
• • • •
Whilst you experience a push ‘back’ if you push an object and it doesn’t move, there is less force back (or none at all) if it does move. If you push an object in a situation where there is very little friction, there is no ‘push back’ by the object. The ‘passive’ partner in an interaction does not exert a force. The sizes of the forces in a pair depend on the masses of the objects, or their motion.
Diagnosing Pupils’ Understanding of Forces and Motion
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The National Strategies | Secondary Strengthening teaching and learning of forces: Teaching and learning about forces and motion
Task 13: Diagnostic questions
• • • •
Use the diagnostic questions listed below to check your confidence. You will find it more convenient if you print the page of questions onto paper. If possible try the questions on a small group of pupils to gather an insight into their understanding. Listen to their discussion and reasoning as they answer. If this is not possible directly, consider how you might use them with pupils in future.
Questions 1–3 These look at very similar situations – pushing a box. Pupils are most likely to recognise a push back force in Q1, where the box is not moving. If you ask more than one of these questions at the same time, you should be aware that pupils have a tendency to change the answer they give to a second question that seems similar to a previous one – assuming (not unreasonably) that the questioner would not have asked the second one if the situation was not different in some way. It takes confidence to give the same answer several times to a series of similar looking questions. 1. A large box is sitting on a level floor. Sam exerts a force forwards on the box but it does not move.
box not moving
Think about the force exerted by the box on Sam while he is pushing. Which of the following statements is correct? Tick ONE box.
While Sam is pushing, he exerts a force on the box. The box does not exert a force on Sam.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The two forces are the same size.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The force exerted on Sam by the box is bigger.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The force exerted on Sam by the box is smaller.
Diagnosing Pupils’ Understanding of Forces and Motion - Forces in pairs: Newton’s Third Law ©University of York, 2003. Used with kind permission.
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2. A large box is sitting on a level floor. Sam exerts a force forwards on the box, and it moves at a steady speed across the floor.
moving at steady speed
Think about the forces exerted by the box on Sam while he is pushing. Which of the following statements is correct? Tick ONE box.
While Sam is pushing, he exerts a force on the box. The box does not exert a force on Sam.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The two forces are the same size.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The force exerted on Sam by the box is bigger.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The force exerted on Sam by the box is smaller.
3. A large box is sitting on an ice rink. Sam stands on a path at the edge of the ice and exerts a force forwards on the box, and it moves across the ice. The ice is very smooth. The friction force on the box is so small that it can be ignored. moving across the ice
friction is so small it can be ignored
Think about the forces acting on Sam while he is pushing. Which of the following statements is correct? Tick ONE box.
While Sam is pushing, he exerts a force on the box. The box does not exert a force on Sam.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The two forces are the same size.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The force exerted on Sam by the box is bigger.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The force exerted on Sam by the box is smaller.
Diagnosing Pupils’ Understanding of Forces and Motion - Forces in pairs: Newton’s Third Law ©University of York, 2003. Used with kind permission.
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The National Strategies | Secondary Strengthening teaching and learning of forces: Teaching and learning about forces and motion
Answers To help you check how you have done here are the answers, with a little explanation. 1. Tick third box. 2. Tick third box. This answer may feel counter-intuitive for some pupils. But this is an absolutely fundamental idea about forces: forces arise from an interaction involving two objects; every interaction gives rise to a force pair; the two forces of a pair are equal in size, opposite in direction, and one acts on each of the interacting objects. 3. Tick third box. See comment on question two above. When the box is on ice, with no friction, this may be even more counterintuitive. But the same reasoning applies. Note that, as there is no counter-force due to friction in this case, the speed of the box will increase steadily whilst Sam is pushing it. Diagnosing Pupils’ Understanding of Forces and Motion - Forces in pairs: Newton’s Third Law ©University of York, 2003. Used with kind permission.
Acknowledgement Extract from Diagnosing Pupils’ Understanding of Forces and Motion – Forces in pairs: Newton’s Third Law ©University of York 2003. Used with kind permission.
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Strengthening teaching and learning of forces: Teaching and learning about forces and motion The reason why force is an important idea in science is because the motion of any object can be explained by considering the forces acting on the object. We are going to examine a procedure, supported by two rules, to analyse and explain the motion of an object and the forces acting on the object. This procedure was included in the Introductory section of this study guide. 1. Identify all the forces acting on the object you are interested in, noting their directions. 2. Add the forces acting on the object to find the resultant (or total) force acting on it. 3. Apply the two rules: −− if there is a resultant force acting on an object, this will cause a change in its motion, in the direction of the force −− if the resultant force acting on an object is zero, its motion does not change. Both rules also apply in the other direction; that is if you know what the motion of an object is, you can deduce what the resultant force acting on it is. It should be emphasised that these rules are completely general, that is they apply to all examples of motion, without exception. They are a way of stating Newton’s First Law of Motion. At this point let’s recall Newton’s Laws, which are formally stated as: 1. Every body continues in a state of rest or uniform motion in a straight line unless acted upon by an external impressed force. 2. The rate of change of momentum is proportional to the impressed force, and takes place in the direction of the force. 3. Action and reaction are equal and opposite. Task 8: Newton’s First Law of Motion Restate Newton’s First Law of Motion, in your own words, as though explaining it to a pupil. Here is an example that might help. If the forces on a mass are balanced (no resultant force), then if it starts off: ––
at rest, it will stay at rest
––
moving in a straight line, it will keep on moving at a constant speed in a straight line.
The great value of the First Law, and indeed also the Second and Third Laws, is that they are absolutely precise and can be used to make exact numerical predictions about the motion of objects. However, the most important objective in teaching about forces and motion at Key Stage 3 is a qualitative understanding of these ideas. Quantitative use of this law usually occurs at Key Stage 4. The real purpose of Newton’s Laws is to extend and formalise the kind of everyday understanding that everything has of motion. When pupils understand this, it opens up a completely new way of looking at, and explaining, how and why things move the way they do.
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The National Strategies | Secondary Strengthening teaching and learning of forces: Teaching and learning about forces and motion
Forces and motion Let us continue with the use of the procedure we started with. Remember it ended with the two rules:
• •
if there is a resultant force acting on an object, this will cause a change in its motion, in the direction of the force if the resultant force acting on an object is zero, its motion does not change.
Applying the two rules requires that pupils understand what counts as a ‘change of motion’. A change of motion means that the object changes its speed or the direction in which it is moving. Therefore, an object at rest (stationary) or one that is moving at a constant speed in a straight line is not changing its motion. In these situations, the resultant force acting on the object is zero. Conversely, an object that is moving in a curved path (e.g. in a circle) at a constant speed is experiencing a change in its motion (a change in the direction in which it is moving). In this situation, there is a resultant force acting on the object. For objects moving in a straight line, the rules lead to the following conclusions.
•
When there is a resultant force (which is not zero) acting on an object: −− a stationary object will start to move in the direction of the resultant force, and its speed will steadily increase −−
•
an object moving in the direction of the resultant force will continue moving in that direction with its speed steadily increasing
−− an object moving in the opposite direction to the resultant force will continue moving in that direction with its speed steadily decreasing to zero. If the resultant force continues to act, the object will then start moving in the opposite direction (i.e. in the direction of the resultant force) with its speed steadily increasing. When the resultant force acting on an object is zero: −− if the object is stationary, it will remain stationary −− if the object is moving, it will continue moving at a steady speed in the same direction.
Task 9: Forces in action Time to check your understanding of the forces acting on balls after they have been kicked, hit or thrown. It is essential to be clear that the ‘propulsion’ force is not carried by the ball once it is no longer in contact with the source of the force, i.e. the foot, bat or hand.
• •
How would you explain to a pupil that a ball continues to move after it has been thrown even though there is no longer any propulsion force acting upon it? If you are unsure, talk through your explanation with a colleague more experienced in teaching about forces.
If you are unsure about your explanation, the parts that follow will help you further.
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Understanding constant speed (uniform motion) Almost all pupils at Key Stage 3 will think that an object moving with steady speed requires a net force to keep it moving. Their experience, and that of most adults, is of a world where they have come to terms with friction. In order to move something they have to continue to apply a force so that the object moves in the direction of the force. Newton’s First Law of Motion can be described in a number of ways, but generally states that every object moves with uniform motion, in a straight line, unless acted upon by some external force. Such conditions only exist in the type of ‘perfect’ world (one without friction) that physicists use to help explain the motion of objects. However, in the ‘real’ world frictional forces act on an object, usually with an opposing force to the one that is trying to move the object. When an object is moving at constant speed the resultant force (or total force) is zero, that is the driving force and opposing forces balance each other out. A stationary object is therefore just a special case of steady motion. In terms of resultant forces, there is no difference between being at rest, and moving uniformly at a constant speed in a straight line.
Objects that have been kicked, hit or thrown A particular situation that pupils find difficult to interpret is the movement of an object that has been set in motion and is now slowing down. Examples include a football that has been kicked and is rolling along the ground, or a ball that has been thrown vertically upwards. In situations like these, many pupils mark a force in the direction of motion. But this force exists only during the interaction that set the object in motion. Once it has left the foot or hand of the person who made it move, there is no force in the direction of motion. The resultant force is in the opposite direction, making the object slow down (and eventually stop, and perhaps start moving in the opposite direction). If you are feeling more confident you should be able to tackle a couple of simple problems: 1. During a ten-pin bowling game, describe the forces acting on the bowling ball: −− when it is being bowled, with the bowler’s hand still in contact −− as it moves down the bowling lane −− when it strikes the first pin. 2. How do skateboarders keep moving along a horizontal surface?
Task 10: False statements In this task you are going to look back at the first five of the pupils’ statements about forces from earlier, which are printed below. If there is motion there is a force acting If there is no motion, then there is no force acting There cannot be a force without motion When an object is moving, there is a force in the direction of its motion A constant speed results from a constant force For each statement apply the procedure and rules we have been using. Can you use the procedure to identify the fallacy in each statement?
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We will now explore an approach that involves developing a deeper level of understanding by reasoning in the other direction. Task 11: Filling in the gaps In the following five sentences suggest what should go into each space. Possible choices are given in the sentences to help you.
• • • •
If an object is moving in a straight line with increasing speed, there is _________ (no/a resultant) force acting on the object, in the direction __________(of/opposite to) its motion. If an object is moving in a straight line and is slowing down, there is __________ (no/a resultant) force acting on the object, in the direction ___________ (of/opposite to) its motion. If an object is stationary, the resultant force acting on it is _________ (positive/negative/zero). If an object is moving at a steady speed in a straight line, the resultant force acting on it is ____________ (positive/negative/zero).
In the first statement, the phrase increasing speed should trigger the response that a resultant force must be acting in the direction of the motion. In the second statement, the phrase slowing down should trigger the response that there is a change in speed; therefore a resultant force is acting. The change is a reduction in speed, so the direction of the resultant force must be opposite to the movement. The third and fourth statements are deliberately set to challenge your thinking in the context of the ‘perfect’ world and are based upon an understanding of Newton’s First Law. The third statement is less challenging than the last one and is a situation where the resultant force is zero. Zero is the correct response for the last statement also. The clue is in the words steady speed within this statement.
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Applying the approach to a range of practical situations Tasks 12 and 13 provide opportunities to look at activities more directly related to teaching. You may be able to try out some of these activities with small groups of pupils and to reflect on the teaching of forces and motion. Task 12: Classroom activities This task focuses on some fairly typical classroom activities used in teaching about forces and motion. Four such activities are suggested below, but you could substitute activities from your own department’s scheme of learning.
Activity 1
Activity 2
For the activity described below follow the procedure outlined.
For the activity described below follow the procedure outlined.
A parachutist jumps from a plane. She free falls for a few moments then opens her parachute. Some time later she reaches the ground.
Procedure
• • •
Identify all the forces acting on the object you are interested in, noting their directions. Add the forces acting on the object to find the resultant (or total) force acting on it. Apply the following rules:
You have a flexible track with a ball bearing. Place the ball bearing near the top at one side of the curve and predict where it will reach on the opposite side after it has been released. Try it and see if you were correct. Change the shape of the track and try again.
Procedure
• • •
Identify all the forces acting on the object you are interested in, noting their directions. Add the forces acting on the object to find the resultant (or total) force acting on it. Apply the following rules:
−−
if there is a resultant force acting on an object, this will cause a change in its motion, in the direction of the force
−−
if there is a resultant force acting on an object, this will cause a change in its motion, in the direction of the force
−−
if the resultant force acting on an object is zero, its motion does not change.
−−
if the resultant force acting on an object is zero, its motion does not change.
1. What are the forces acting?
1. What are the forces acting?
2. What is the resultant force and in what direction does it act?
2. What is the resultant force and in what direction does it act?
3. How does the motion of the object change?
3. How does the motion of the object change?
4. What are the main learning points?
4. What are the main learning points?
5. What are the main teaching points?
5. What are the main teaching points?
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The National Strategies | Secondary Strengthening teaching and learning of forces: Teaching and learning about forces and motion
Activity 3
Activity 4
For the activity described below follow the procedure outlined.
For the activity described below follow the procedure outlined.
You have been given an office chair with wheels. Sit on the chair with your feet against a wall. Now push against the wall and experience the resulting motion.
Procedure
• • •
Identify all the forces acting on the object you are interested in, noting their directions. Add the forces acting on the object to find the resultant (or total) force acting on it. Apply the following rules:
You have been given an empty bottle and sink of water. Place the empty bottle on the water and observe what happens. Then add a few marbles (or a little sand) at a time and put the bottle back into the water.
Procedure
• • •
Identify all the forces acting on the object you are interested in, noting their directions. Add the forces acting on the object to find the resultant (or total) force acting on it. Apply the following rules:
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if there is a resultant force acting on an object, this will cause a change in its motion, in the direction of the force
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if there is a resultant force acting on an object, this will cause a change in its motion, in the direction of the force
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if the resultant force acting on an object is zero, its motion does not change.
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if the resultant force acting on an object is zero, its motion does not change.
1. What are the forces acting?
1. What are the forces acting?
2. What is the resultant force and in what direction does it act?
2. What is the resultant force and in what direction does it act?
3. How does the motion of the object change?
3. How does the motion of the object change?
4. What are the main learning points?
4. What are the main learning points?
5. What are the main teaching points?
5. What are the main teaching points?
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If possible set up these activities in a room at school. Try out the activities yourself and try to answer the questions at the foot of each activity. Try it out with a small group of pupils who have already studied forces and motion. Listen carefully to their discussions. Try to probe their thoughts and understanding to give you an idea of their learning about forces and motion.
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Forces in pairs: Newton’s Third Law The questions in the next task probe pupils’ understanding of the fact that forces always come in pairs. If A pushes B, then B pushes back on A, with an equal force. This applies to both steady forces, and to short-lived forces such as those that arise in a collision or sudden impact. The forces are always equal, at every instant during an interaction, and regardless of the sizes of the objects involved, or their motion. Diagnosing Pupils’ Understanding of Forces and Motion
Some understanding that forces arise from interactions and always come in pairs is essential to make sense of the ideas required at Key Stage 3. There is a strong case for teaching these ideas earlier than Key Stage 4. Common misunderstandings are likely to include the following.
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Whilst you experience a push ‘back’ if you push an object and it doesn’t move, there is less force back (or none at all) if it does move. If you push an object in a situation where there is very little friction, there is no ‘push back’ by the object. The ‘passive’ partner in an interaction does not exert a force. The sizes of the forces in a pair depend on the masses of the objects, or their motion.
Diagnosing Pupils’ Understanding of Forces and Motion
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The National Strategies | Secondary Strengthening teaching and learning of forces: Teaching and learning about forces and motion
Task 13: Diagnostic questions
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Use the diagnostic questions listed below to check your confidence. You will find it more convenient if you print the page of questions onto paper. If possible try the questions on a small group of pupils to gather an insight into their understanding. Listen to their discussion and reasoning as they answer. If this is not possible directly, consider how you might use them with pupils in future.
Questions 1–3 These look at very similar situations – pushing a box. Pupils are most likely to recognise a push back force in Q1, where the box is not moving. If you ask more than one of these questions at the same time, you should be aware that pupils have a tendency to change the answer they give to a second question that seems similar to a previous one – assuming (not unreasonably) that the questioner would not have asked the second one if the situation was not different in some way. It takes confidence to give the same answer several times to a series of similar looking questions. 1. A large box is sitting on a level floor. Sam exerts a force forwards on the box but it does not move.
box not moving
Think about the force exerted by the box on Sam while he is pushing. Which of the following statements is correct? Tick ONE box.
While Sam is pushing, he exerts a force on the box. The box does not exert a force on Sam.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The two forces are the same size.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The force exerted on Sam by the box is bigger.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The force exerted on Sam by the box is smaller.
Diagnosing Pupils’ Understanding of Forces and Motion - Forces in pairs: Newton’s Third Law ©University of York, 2003. Used with kind permission.
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2. A large box is sitting on a level floor. Sam exerts a force forwards on the box, and it moves at a steady speed across the floor.
moving at steady speed
Think about the forces exerted by the box on Sam while he is pushing. Which of the following statements is correct? Tick ONE box.
While Sam is pushing, he exerts a force on the box. The box does not exert a force on Sam.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The two forces are the same size.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The force exerted on Sam by the box is bigger.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The force exerted on Sam by the box is smaller.
3. A large box is sitting on an ice rink. Sam stands on a path at the edge of the ice and exerts a force forwards on the box, and it moves across the ice. The ice is very smooth. The friction force on the box is so small that it can be ignored. moving across the ice
friction is so small it can be ignored
Think about the forces acting on Sam while he is pushing. Which of the following statements is correct? Tick ONE box.
While Sam is pushing, he exerts a force on the box. The box does not exert a force on Sam.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The two forces are the same size.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The force exerted on Sam by the box is bigger.
While Sam is pushing, he exerts a force on the box. The box also exerts a force on Sam in the opposite direction. The force exerted on Sam by the box is smaller.
Diagnosing Pupils’ Understanding of Forces and Motion - Forces in pairs: Newton’s Third Law ©University of York, 2003. Used with kind permission.
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The National Strategies | Secondary Strengthening teaching and learning of forces: Teaching and learning about forces and motion
Answers To help you check how you have done here are the answers, with a little explanation. 1. Tick third box. 2. Tick third box. This answer may feel counter-intuitive for some pupils. But this is an absolutely fundamental idea about forces: forces arise from an interaction involving two objects; every interaction gives rise to a force pair; the two forces of a pair are equal in size, opposite in direction, and one acts on each of the interacting objects. 3. Tick third box. See comment on question two above. When the box is on ice, with no friction, this may be even more counterintuitive. But the same reasoning applies. Note that, as there is no counter-force due to friction in this case, the speed of the box will increase steadily whilst Sam is pushing it. Diagnosing Pupils’ Understanding of Forces and Motion - Forces in pairs: Newton’s Third Law ©University of York, 2003. Used with kind permission.
Acknowledgement Extract from Diagnosing Pupils’ Understanding of Forces and Motion – Forces in pairs: Newton’s Third Law ©University of York 2003. Used with kind permission.
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