ENERGY,WORK, POWER,
AND MACHINES
IPC Spring 2014
Energy, Work, Power &
Machines
1. Energy is the ability to do work. Potential
energy – stored energy or energy due to
position
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PE=m g h
m is mass (kg)
g is gravity (9.8 m/s2)
h is the height of the object (m)
Kinetic energy – energy of motion
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KE= mv2 /2
m is mass (kg)
v is velocity (m/s)
Energy, Work, Power, &
Machines
 2. Energy conversion is a changing of
one form of energy to another.
 3. The Law of Conservation of
Energy states that energy cannot be
created nor destroyed, but is only
changed from one form to another.
1. After you place a book on a high
shelf, we say the book has
increased _____.
A. gravitational potential
energy.
B. elastic potential energy.
C. chemical energy.
D. kinetic energy.
2. An empty truck traveling at 10 km/h
has kinetic energy. How much kinetic
energy does it have when it is loaded
so its mass is twice, and its speed is
increased to twice?
A.
B.
C.
D.
the same KE
twice the KE
four times the KE
more than four times the
KE
3. When you ride a playground
swing, your potential energy is
greatest at the highest point.
A. True
B. False
4. As mass decreases, kinetic
energy increases.
A. True
B. False
Energy, Work, Power, &
Machines
 4. Work is done when a object
moves through a distance because
of a force acting upon the object.
 5. The following formula is used to
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calculate work:
W=F×d.
W stands for the work done (J)
F stands for the force (N)
d stands for distance (m)
The SI unit for work is the joule (J).
Energy, Work, Power, &
Machines
 6. Power - the rate at which work is
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done.
7. Power can be calculated by using the
following formula:
P = W/t or
P = F×d/t
P stands for power (W)
W stands for work (J)
t stands for time. (s)
The SI unit for power is the watt (W).
5. A student weighing 900 N runs to the
top of an 8 m staircase in 5 sec. How
much power was produced?
A. 56.25 W
B. 11.25 W
C. 1440 J
D. 1440 W
6. The metric unit for work is the
_________________.
A. Watt
B. Joule
C. Newton
D. Foot-Pound
7. The metric unit for power is a
_____________.
A. a. Joule
B. Watt
C. Newton
D. Horsepower
8. How much work does Billy perform if
he pushes the 8000 N stalled car a
distance of 25 meters?
A. 320 J
B. 100,000 J
C. 200,000 J
D. 8025 J
Simple Machines
Inclined Plane
 A plane is a flat surface. When that
plane is inclined, or slanted, it can help
you move objects across distances
and that's work! A common inclined
plane is a ramp. Lifting a heavy box
onto a loading dock is much easier if
you slide the box up a ramp--a simple
machine.
Wedge
 You can use the edge of an inclined
plane to push things apart. Then, the
inclined plane is a wedge. So, a wedge
is actually a kind of inclined plane. An
axe blade is a wedge. Think of the edge
of the blade. It's the edge of a smooth
slanted surface.
Screw
A screw an inclined plane wrapped around
a cylinder
A screw can convert a rotational force
(torque) to a linear force and vice versa.
Lever
 Any tool that pries something loose is a
lever. A lever is a rigid bar that "pivots" (or
turns) against a "fulcrum" (or a fixed
point).
Wheel and Axle
 It is two circular objects attached
together about a common axis
 Wheel is the large cylinder
 Axle is the small cylinder
Pulley
 In a pulley, a cord wraps around a wheel.
As the wheel rotates, the cord moves in
either direction. Now, attach a hook to
the cord, and you can use the wheel's
rotation to raise and lower objects.
Energy, Work, Power, &
Machines
 8.The mechanical advantage is the number
of times a machine multiplies an effort force.
Mechanical advantage can be calculated by the
following equations:
 AMA =
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Fr
IMA
=
de
Fe
dr
AMA = actual mechanical advantage (real life)
IMA = ideal mechanical advantage (pretend)
Fr = resistance force
de = effort distance
Fe = effort force d r = resistance distance
 9. The mechanical advantage of a pulley
system is determined by counting the number
of strands of rope that support the resistance
force.
Energy, Work, Power, &
Machines
 10.Work input is always greater than
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work output due to friction.
Win= W out
Fe x de = Fr x d r
Win = work input (J)
W out = work output (J)
Fe = effort force (N)
de = effort distance (m)
Fr =force of the resistance (N)
d r=how far the resistance moved (m)
Energy, Work, Power, &
Machines
 11.Efficiency
is a measure of how
much of the work put into a machine is
changed to work put out by the
machine.
 Efficiency
is calculated using the
following formula:

efficiency = W out x 100%
Win
 efficiency = AMA x 100%
IMA
Efficiency of a Machine
12.The amount of work obtained from a machine
is always less than the amount of work put
into it. This is because work is lost to friction.
Efficiency = output work / input work x 100
Remember that
work = force x distance
Energy, Work, Power, &
Machines
 13. Compound machines
consist of two or more simple
machines.
9. When 100 J are put into a
device that puts out 40 J, the
efficiency of the device is
A.
B.
C.
D.
40%.
50%.
60%.
140%.
10. Which pulley has the
greatest mechanical
advantage?
A. A. Picture 1
B. B. Picture 2
C. C. Picture 3
D. D. Picture 4