Applied Force and Friction Force
Applied Force and Friction Force April-23-12 9:09 AM
What happens when you rub your hands together really quickly? You generate heat!
Ff x d = Eheat Questions 1. Describe the velocity of an object if we move it over a surface applying a force equal to the frictional force. 2. Do I do any work on the object? Work and Energy Page 1
2. Do I do any work on the object? 3. What is my net force? Is it equal to my applied force? 1. Constant velocity 2. Yes, but only with applied force 3. Zero. No. FA x d = Work (total) done on an object
What if we apply a force larger than the frictional force? What will happen to its velocity? It'll accelerate. A change in velocity means a change in kinetic energy. FNET x d = Ek Let's derive this:
Lastly, we can also change the potential energy of an object. How can we do that? We do work against gravity. Work and Energy Page 2
We do work against gravity.
Fg x d = Ep Alas, we have these four equations: FA x d = ET Ff x d = Eheat FNET x d = Ek Fg x d = Ep And we know: Total E = KE + PE KE = 1/2mv2 PE = mgh
The cart's mass is 50 kg and it has an initial velocity of 0 m/s. What is its velocity Work and Energy Page 3
initial velocity of 0 m/s. What is its velocity at each point (3 sig figs)? A = 0 m/s B = 27.6 m/s C = 11.7 m/s D = 19.3 m/s Three bullets question:
The three guns are fired at the same height, each bullet leaving the nozzle with the same velocity. Which bullet will hit the ground with the greatest velocity Homework Do you really understand? Work and Energy Work and Energy Page 4
Energy
Work and Energy Page 5
What happens when you rub your hands together really quickly? You generate heat!
Ff x d = Eheat Questions 1. Describe the velocity of an object if we move it over a surface applying a force equal to the frictional force. 2. Do I do any work on the object? Work and Energy Page 1
2. Do I do any work on the object? 3. What is my net force? Is it equal to my applied force? 1. Constant velocity 2. Yes, but only with applied force 3. Zero. No. FA x d = Work (total) done on an object
What if we apply a force larger than the frictional force? What will happen to its velocity? It'll accelerate. A change in velocity means a change in kinetic energy. FNET x d = Ek Let's derive this:
Lastly, we can also change the potential energy of an object. How can we do that? We do work against gravity. Work and Energy Page 2
We do work against gravity.
Fg x d = Ep Alas, we have these four equations: FA x d = ET Ff x d = Eheat FNET x d = Ek Fg x d = Ep And we know: Total E = KE + PE KE = 1/2mv2 PE = mgh
The cart's mass is 50 kg and it has an initial velocity of 0 m/s. What is its velocity Work and Energy Page 3
initial velocity of 0 m/s. What is its velocity at each point (3 sig figs)? A = 0 m/s B = 27.6 m/s C = 11.7 m/s D = 19.3 m/s Three bullets question:
The three guns are fired at the same height, each bullet leaving the nozzle with the same velocity. Which bullet will hit the ground with the greatest velocity Homework Do you really understand? Work and Energy Work and Energy Page 4
Energy
Work and Energy Page 5
