PHY 2020
Fall Sept 28, 2015
Homework Set #6: Energy and Work
1. A brick of mass 2 kg is on a cliff which is 20m above the ground. Recall that the ground
is the place where the potential energy is chosen to be zero.
(a) How much potential energy does the brick have? Recall; that energy is measured
in units of Joules, where 1 J = 1 kg(m/ s)2 .
(b) Use the conservation of energy to determine the amount of kinetic energy the
brick would have just before it hits the ground, if it were to fall off edge of the
cliff.
(c) And, what would the speed of the 2 kg brick be just before it hits the ground?
2. I have a rock of mass 0.1 kg tied to a string of length 1m, and the rock is initially
held horizontally. If I were to release the rock (while holding onto the other end of the
string), it would swing downwards in an arc. What is the maximum speed that the
rock would have while it is swinging?
3. A roller-coaster of mass M = 2, 000 kg starts from rest at the highest point on its very
curvy track.
(a) How fast is the roller-coaster going when it is a vertical distance of 10m below
its initial starting point?
(b) How much additional energy would be required to raise the roller coaster back
to its initial height?
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