Quiz 4b (Phys 101 - 11.30am, Spring 2005)
Name: ________________Answers______________________
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.)