Questions on conversation of energy?

Question

Energy cannot be created or destroyed.

If I had a basketball, every time I bounce it off the floor some kinetic energy is turned to heat and sound energy.

Kinetic energy loss = heat + sound energy gain

1.Where does the sound energy go to? It can't simply disappear?

2. If the floor was spongy ( longer time of impact ), there is little or no sound when the ball hit the floor. Less sound energy is released and more kinetic energy is lost ( ball does not bounce as high ), does more energy change to heat?

3.If I lift and move a briefcase for a distance and then put it back down. What is the change of energy?

4. I have two boards of the same thickness, one is harder than the other. A bullet is shot through them one at a time. Which bullet has a higher velocity after piercing through them?

( IMO, the bullet that shot through the harder board should have a lower velocity because the impulsive force is higher, unlike the basketball case where soft floor decreased the kinetic energy more)

Answer 1

1. friction. Waves are coherent movement thtough the air: when coherence is lost you get heat. This happens through distances or reflection (or absorption ) of the walls.

2. True

3. Not clear what the question is. Ex: its temperature can change through friction, or it can have a different potential energy (different height). If anything changes (you assume nothing you do to it changes its internal status) than the energy is the same

4. It depends on a number of factors. A harder board can break more easily than another one, because most of the time strenght=lesser change in shape before it breaks. If it doesn't break simply, but also change shape (through greater increase on temperature) this change in structure can absorb less or more energy. A major problem is if the deformation of the medium is given back or dispersed.

Answer 2

1. the sound energy radiates outward as sound waves in the air, and get continually weaker as the waves travel. eventually another, larger sound comes from the opposite direction and the big wave cancels out the small one, and becomes weaker in the process.

2. the energy goes into moving the spongy floor.

3. kinetic to potential

4. you just answered it.