Transcript Energy

Energy
What is it?
What is it?
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Here are some examples:
Definition
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Energy is the ability to cause change
• Two types of energy
 Kinetic
 Potential
Kinetic
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Energy in the form of motion
• Running
• Throwing
• Flying
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How much kinetic energy depends on
what?
• Mass and Velocity
• Question: What has more kinetic energy: a
truck traveling at 100 km/hr or a motorcycle
traveling at 100 km/hr
• How about a truck traveling at 100 km/hr or a
truck traveling at 80 km/hr
Potential Energy
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Stored Energy
• Students sitting in their seats
• A bullet in a gun
• Battery
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Potential energy changes into kinetic
energy
Work
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Transfer of energy through motion
• In order for work to take place, a force
must be exerted through distance
• Calculating Work:
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Work = Force x Distance
or
W=FxD
Units
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Work is expressed in Newton-meters
(N x m)
Or
Joules (j)
Named after British Scientist James
Prescott Joule
Example
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A student’s full backpack weighs 30
N. She lifts it from the floor to a shelf
1.5 m high. How much work is done
on the pack full of books?
W=FxD
F= 30 N
D = 1.5 m
W=?
W= 30N x 1.5 m
W = 45 N x m
or
45 J
Try This One
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A carpenter lifts a 4.5 kg beam 1.2
meters. How much work is done on
the beam?
• What do you know?
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Distance = 1.2 m
Force = ? You must calculate force
•F=MxA
M = 4.5 kg
A=?
• A = gravity = 9.8 m/s2
• F = 4.5 kg x 9.8 m/s2
F = 44.1 N
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W = 44.1N x 1.2 m
W = 52.92 J
Another Example
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A man pushes a lawnmower with a
force of 500 N for 200 m. How much
work is this if the force of friction
acting on the lawnmower is 150 N?
•W = F x D
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F = 500 N + 150 N
D = 200 m
• W = 650 N X 200 m
• W = 130000 Joules
One more example
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Cheerleader A is holding up cheerleader B.
Cheerleader B is exerting a force a 80 N on
cheerleader A. If cheerleader A walks
forward 2 meters while holding cheerleader
B, how much work has cheerleader A done?
No work!!!! Why?
Because work was not done over a distance
W=FxD
W = 80 N x 0 m
W = 0 N x m or 0 Joules