21
Period 6 Activity Sheet Solutions:
Energy and Work
6.1
How Do We Measure Work and Energy?
a)
Work is done when a force moves an object.
Work and energy are measured in units of joules.
1 joule
=
1 kg m
2
/s
2
Force is measured in units of newtons.
1 newton
=
1 kg m/s
2
Explain why a joule can also be called a newton meter.
You can write the units of energy as 1 kg m
2
/s
2
= 1 kg m/s
2
m = 1 N m
The unit of energy and work (joule) is the produce of units of force
(newton) times units of distance (meter).
b)
This activity illustrates the amount of energy in one joule.
Lift the 1 newton plastic apple up a distance of 1 meter.
How much work did
you do on the apple?
__
1 joule
_
How much energy did you exert?
(Assume no energy was wasted.) _
1 joule
_
6.2
How Does the Work Done Compare to Potential Energy Gained?
a)
Work
:
Your instructor will demonstrate potential energy with a pile driver.
1)
The pile driver mass is 4 kilograms.
What is its weight in newtons?
Weight
=
F
=
M g
=
4 kg
x
9.8 m/s
2
=
39.2 N
2)
How much force is necessary to raise the mass at a constant velocity?
39.2 N
For motion at a constant velocity, the force pushing the
mass up equals the force of gravity pulling the mass down.
3)
To what height was the pile driver mass raised?
___________
4)
How much work was done to raise the mass to this height?
Use
W
=
M
g
h
to find the joules of work done.
b)
Potential Energy
:
1)
How much potential energy did the mass have before it was raised?
__
0
__
2)
How much potential energy does the mass store when it is raised?
Ignoring wasted energy, the amount of stored energy equals the
amount of work done to raise the mass.
(Same answer as 6.2.a.4)
3)
The mass is now allowed to drop.
What form of energy does the mass have
just before it hits the metal stand at the bottom? __
kinetic energy
_
c)
Group discussion question:
Is the amount of energy the mass had just before it
hits the same as the amount of gravitational potential energy it had when it was
raised?
Why or why not?
The mass has less kinetic energy just before it hits than gravitational
potential energy before it falls.
As it falls, most of its gravitational
potential energy is converted into kinetic energy.
However some
potential energy is converted into wasted thermal energy from friction
between the mass and the air molecules it pushes aside as it falls.
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6.3
What Happens to Stored Potential Energy?
a) Your instructor will demonstrate the pile driver mass hitting a nail into a board.
1)
If this pile driver exerts a force of 500 N to drive a nail into a board a
distance of 3 cm, how much work does has the pile driver done on the nail?
W
=
F
D
=
500 N
x 0.03 m
=
15 joules
2)
If the 4 kg mass is raised to a height of 0.75 meters, how much potential
energy does it store?
E
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 Spring '10
 Staff
 Energy, Force, Kinetic Energy, Potential Energy, Work

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