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Practice Problem answers:
1.
Richard wants to know how much potential energy his cat has when it climbs to the top of the
tree near his house. The tree is 15 meters high and the cat has a mass of 5 kilograms. How much
potential energy does the cat have?
Solution: 735 J
PE
=
mgh
PE
= (5 kg)(9.8 m/s
2
)(15 meters)
PE
= (5 kg)(147 m
2
/s
2
)
PE
= 735 kg m
2
/s
2
or 735 joules
2.
John has an object suspended in the air. It has a mass of 50 kilograms and is 50 meters
above the ground. How much (and what kind of) energy does the object have?
Solution: 24516.375 joules Potential energy
PE
=
mgh
PE
of the object is (50 kg)(9.80655 m/s
2
)(50 m)
PE
= (2500 kg m)(9.80655 m/s
2
)
PE
= 24516.375 kg m
2
/s
2
or 24516.375 joules
If it was dropped it could do 24516.375 joules of work/energy
3.
Mrs. Jacobs dropped an object from 10 meters. She knows it did 50 joules of work. How
much did it weigh?
Solution: .5 kilograms
PE
=
mgh
PE
of the object is 50 joules
50 kg m
2
/s
2
=
m
(9.80655 m/s
2
)(10 m)
Solve for
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 Fall '11
 HOPKINS

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