Quiz 4b   (Phys 101 - 11.30am, Spring 2005)

1. How much work do you do when you raise a 2kg block to a height of 2m? (In this problem take g = 10m/s2.)

Work = Fd where the force F that you must apply to lift the block must overcome the weight of the block

i.e. F = W = mg. Therefore Work = mgd = 2x10x2 = 40J

2. How much gravitational potential energy does the block possess at this height?

The total energy = GPE + KE + sound energy + heat energy + ...

By conservation of total energy, the total energy is conserved.

Hence the work done on the object must be converted into some other form of energy, in this case

GPE = 40J (because you cannot lose any energy).

3. If you let the block fall, what would the block`s kinetic energy be just before it hit the ground?

Just before the block hits the ground GPE = mgh = 0J because the height h = 0m.

Recall that the total energy must stay constant and is equal to 40J (this is what conservation means).

The energy has now been converted to a different form of energy, namely, KE = 40J.

(Note that the total energy = GPE + KE + sound energy + heat energy = 0 + 40 + 0 + 0 = 40J still remains constant. After the block hits the ground sound + heat energy = 40J while GPE = 0 and KE = 0.)