Download Work, Impulse, Momentum and Power

Work, Impulse, Momentum
and Power
Physics 2
Unit 1
Work
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Definition: the transfer of energy to an
object by the application of a force
Formula: W = F x d (work equals force
times distance)
Units: Units for work are Joules
Work is only done when a force causes a
change in position or displacement
As a weightlifter holds a barbell over his
head, is he doing any work? Why or why
not?
Example problem 1
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Imagine a father playing with his
daughter by lifting her repeatedly in the
air. How much work does he do with
each lift assuming he lifter her 2 meters
and exerts an average force of 190N?
W=Fxd
W = 190N x 2m
W= 380J
Example Problem 2
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The brakes on a bicycle apply 125N of
frictional force to the wheels as the
bicycle travels 14m. How much work
have the brakes done on the bicycle?
W=Fxd
W = 125N x 14m
W = 1750J
Power
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Definition: power is a quantity that
measures the rate at which work is
done
Formula: P = W / t (Power equals
work divided by time)
Units: Watts
As time decreases while applying the
same amount of work, what happens to
the power output?
Horsepower
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Horsepower is referred to the average
power output of a draft horse
1hp = 746 watts
With that much power, a horse could
raise a load of 746 apples, weighing 1lb
each, by 1 meter every second
Example Problem 1
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While rowing across the lake during a
race, John does 3960J of work on the
oars in 60 seconds. What is his power
output in watts?
P=W/t
P = 3960J / 60 sec
P = 66 watts
Example Problem 2
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Using a jack, a mechanic does 5350J of
work to lift a car 0.5m in 50 seconds.
What is the mechanic’s power output?
P=W/t
P = 5350J / 50sec
P = 107 watts
Kilowatts
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Often times power is expressed in
kilowatts
1kW is 1000 Watts
Impulse
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Definition: Impulse is the amount of
force exerted on an object per unit of
time
Formula: I = F x t (Impulse equals
force multiplied by time)
Units: Ns (Newton seconds)
Example Problem 1
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Calculate the impulse of a car crash that
exerts a force of 250N over a time of
0.8 seconds.
I=Fxt
I = 250N x 0.8sec
I = 200 Ns
Example Problem 2
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Calculate the impulse of a collision of a
ball and baseball bat with a force of
100N over a contact time of 1.25
seconds.
I=Fxt
I = 100 x 1.25 sec
I = 125Ns
Momentum
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Definition: The product of the mass
times the velocity of an object
Formula: p = m x v (momentum
equals mass multiplied times velocity)
Units: kg m/s (kilograms meters per
second)
The greater the mass, the greater the
momentum, the greater the velocity the
greater the momentum
Which has more momentum?
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Which has more momentum, a
skateboarder skating in a skatepark or a
semi truck stopped at a rest stop? Why?
Example Problem 1
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Calculate the momentum of a 75kg
speed skater moving forward at 16m/s
P=mxv
P = 75kg x 16m/s
P = 1200 kgm/s
Example Problem 2
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Calculate the momentum of a 6kg
bowling ball moving at 10m/s down the
alley towards the pins.
P=mxv
P = 6kg x 10m/s
P = 60kgm/s