Physics Conceptual Word Problems I am confused with.

Question

1. A ball is dropped from an unknown height and allowed to bounce until it comes to rest.  Each bounce is lower than the previous one.  Does the law of conservation of energy hold true for this example? Explain.

 

2.  As an object is thrown into the air and returns to the thrower's hand, describe what happens to the gravitational potential and kinetic energy.

 

3. Describe power.  How would a high power object compare to a low power object?

 

4. What effect will friction have on the kinetic energy of an object?

Answer 1

Hi Samuel, here are some answers to your questions!

 

1) No, conservation of energy does not hold, because every time the ball bounces, it loses some of its energy.

 

2) As the object is thrown it starts with lots of kinetic energy.  As it goes up, the KE turns into PE as its height increases.  Finally, as it falls back down, gravity does positive work on the object and increases its KE.  When it returns to the thrower's hand, neglecting friction, it will have the same speed as when it started.

 

3) Power is the rate at which work is done (P = W/t).  If something has high power, it is performing work or transferring energy faster than something with low power.

 

4) Since friction usually opposes the motion of an object, friction tends to do negative work.  Due to the work-energy theorem, this means that the KE of the object will usually decrease, since work is negative.