4.3 Gravitational Potential Energy
Gravitational Potential Energy (measured in Joules): Energy due to elevation above the ground.
NEAR THE EARTH you can use the following formula (for distances greater than 200km the error
becomes too large):
E g  mgy
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Where ∆y is the vertical displacement in metres.
Note: to use this equation g (9.8 m/s2) must not change over the vertical displacement.
When objects are lifted up, kinetic energy is converted to gravitational potential energy
When objects are dropped, gravitational potential energy is converted to kinetic energy. (i.e. roller
coasters)
Example 1:
A 27 g pencil is dropped from rest vertically downward from a height of 1.35 m above the ground.
a) Determine the gravitational potential energy of the pencil relative to the ground before it is dropped,
and as it hits the ground.
b) How much work has the force of gravity done on the pencil at the instant the pencil hits the ground?
W = (Fcosθ) ∆d
c) How does the work done from b) compare to the change in the gravitational potential energy in (a)?
Note: A Loss in Eg = Gain in EK and vice versa.
Homework: p.181 #1-7