Quiz 4a (Phys 101 - 11.30am, Spring 2005)
1. How much work do you do when you raise a 2kg block to a height of 3m? (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 = 2x10x3 = 60J
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 = 60J (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 60J (this is what conservation means).
The energy has now been converted to a different form of energy, namely, KE = 60J.
(Note that the total energy = GPE + KE + sound energy + heat energy = 0 + 60 + 0 + 0 = 60J still remains constant. After the block hits the ground sound + heat energy = 60J while GPE = 0 and KE = 0.)