Download Chapter 9 Section 1 Notes

Chapter 9 Section 1 Notes
Work, Power, and Machines
What is Work?
Work is done only when a force causes a
change in motion of an object.
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Work is done by a force on an object.
Work is calculated by multiplying the force
by the distance over which the force is
applied.
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Work = Force x distance; W = F x d
Work
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If the distance
something moves is
zero, there has been
no work done
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Example: If you are trying
to push a car stuck in the
mud & it doesn’t go
anywhere, you have
done no work because
the distance moved is
zero. But, you have
applied a force.
Work
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Work is measured in Joules.
All of these units are equivalent: 1 N • m; 1 J;
1 kg • m2/s2
You do 1 Joule of work when you lift an
apple, which weighs 1 N, from your arm’s
length down at your side to the top of your
head (1 meter).
Work sample problem:
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Nick lifts a 0.150 kg sandwich 0.30 m from
the table to his mouth. How much work does
he do?
Solution:
Power
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Power measures the rate at which work is
done, or, how much work is done in a
certain amount of time.
Power = work/time; P = W/t
SI unit for power: Watt (W)
A watt is the amount of power required to do
1 J of work in 1 s.
Power sample problem:
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While rowing across the lake during a race,
John does 3960 J of work on the oars in
60.0 s. What is his power output in watts?
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Solution:
Machines and Mechanical
Advantage
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Machines multiply and
redirect forces.
Machines help to do
work by redistributing
the work we put into
them.
Machines
Machines can:
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Change the direction of the input force
Increase output force by changing the
distance over which the force is applied;
called multiplying the force.
Machines
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You can do the same
amount of work while
applying a different force.
Why?
As force decreases, the
distance increases, so
you are doing the same
amount of work.
Machines make work easier
by increasing the distance
over which force is applied.
Mechanical Advantage
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Mechanical Advantage (abbreviated MA): tells us
how much a machine multiplies force or
increases distance.
Equation: MA = output force = input distance
input force
output distance
If MA >1: multiplies the input force; helps you
move or lift a heavy object, such as a car.
If MA <1: increases distance and speed.
Mechanical Advantage
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Example: Find the MA of a ramp that is 6.0 m
long and 1.5 m tall.
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Example: Alex pulls on the handle of a claw
hammer with a force of 15 N. If the hammer
has a MA of 5.2, how much force is exerted
on the nail in the claw?
G Force
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G Force is a measurement of an object's
acceleration expressed in g's. It may also
informally refer to the reaction force resulting
from an acceleration, with the causing
acceleration expressed in g's.
G force acts on all body parts, including
organs, which are only loosely connected
together
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In 1954, John Paul Stapp experienced 46.2 g
all for science. He was strapped into a
rocket-powered sled on train tracks and
decelerated from 630 mph to 0 in 1.25
seconds. This is the same as hitting a brick
wall at 120mph! He survived, but his eyes
filled with blood and he was temporarily
blinded in what is called a “red out”.
Video of John Paul Stapp