Download Work and Power Notes

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Townsend- PAP Physics
Work and Power Notes
Work (I Can #1, 5)
 Work = Force x (cosine of the angle between the force and displacement) x displacement
W = F (cos ) d
The Physics End of Course Exam and TAKS test use a more simplified form.
Work = (Force)(distance)


W = Fd
Units: N*m = Joule (rhymes with “cool”) named after English Physicist James Prescott Joule
1 J of work is done when a force of 1 N is exerted over a distance of 1m.
(I CAN #1) Examples:
Ex 1: A box is pulled across a horizontal surface with a horizontal force of 10N. How much work is done on the box by the
pulling force after it has traveled 20m?
Ex 2: A box is pulled across a horizontal surface with a force of 10N directed at an angle of 30 o above the horizontal. How much
work is done on the box by the force after it has traveled 20m?
Ex. 3: A monkey a 6kg load of bananas 1m straight up at a constant speed.
a) What is the angle between the lifting force and the displacement?
b) How much work does he do?
c) The same monkey then lifts a 12 kg load of bananas 1m straight up at a constant speed. How much work does he do this
time?
Ex. 4: A girl twirls her 1.5 kg doll around in a circle by her arms with a constant speed of 1.75 m/s. If the distance between her
and her doll is 0.75m how much work is she doing?
Ex. 5: How much work does a weightlifter do when he holds a 1000 N barbell 2m high in the air?
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Townsend- PAP Physics

Work generally falls into two categories
o
Work done
another force
 Ex. When you lift an object you do work against the gravitational force
o
Work done to
 The friction force can do work to bring an object to a stop.

Work is
( = 0o)

Work is
( = 180o)

Work is
( = 90o or 270 o)
of an object
when the force is in the
when the force is in the
when the force is in the
of the displacement.
of the displacement.
to the displacement.
Ex. 6: (I CAN #1)The box is pulled across a rough surface to the right. Draw a free-body diagram of the box and
state whether each force will do a positive, negative or zero work as the box travels to the right.
Graphing Work (I CAN #5)

Just as the area under the Velocity vs. Time graph gave us the displacement of an object,
the
under the Force vs. Displacement graph gives us the amount
Area of Rectangle= base x height
Area of Triangle= ½ (base) (height)
.
Townsend- PAP Physics
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Power ( I Can #6-8)



The definition of work says nothing about how long it takes to do the work.
The same amount of work will be done when carrying something up the stairs whether you run or
walk.
Power is the
at which work is done.
Power =

Units are measured in Watts after the Scottish engineer James Watt= 1 Watt=

Many of equations in Physics can be rearranged and substituted in for each other. For example, to
calculate the Power required to lift an object and of the equations below could be used to calculate
it.

And if you know the constant speed of the object you could also calculate the power required if
you know the force being used to make it go at that constant speed.
Ex. 7: (I CAN #8) You lift a 2kg book to a height of 2m. It took you 10 seconds. How much power did you
generate?
Ex. 8: (I Can #6) How much power is require to lift a 20 N object 2 m into the air with a constant speed of
2.5 m/s?
Ex. 9: (I Can #7) How much work does your 1200 Watt microwave perform in the 2 minutes you spend
cooking your hot pocket for lunch?
Ex. 10: (I Can #6) How much power is required to keep a 1500kg car moving at a constant speed of 30
m/s if the coefficient of friction between the car and the road is 0.45?
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Townsend- PAP Physics
Partner Warm-up
1. If a neighbor pushed a lawnmower four times as far as you but only exerts half the force, which
one of you does more work and by how much?
Simple Machines

A machine is a device used to multiply forces or simply to change the direction of the forces.
o The concept that underlies every machine is the conservation of energy.

Consider a lever.
o Work is done on one end and the other end does work
o The direction of the
o The
is changed.
is the pivot point.
o If you push down, the object goes
.
o
=

Pulleys
o A pulley is basically another kind of lever that can used to
change the direction of the force.
.
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Townsend- PAP Physics
 Inclined Planes
o When you use a ramp you will travel more distance (the hypotenuse) but you will exert less
force.
o Two men have identical boxes to move. One man uses the ramp and anther lifts the load
straight up. The same amount of work is done by both people. So why use a ramp?
d&F
h= d sin 

Man Lifting
Man Pushing
W= F(cos ) h
W= F(cos ) d
W=
W=
W=
W=
The same amount of work was done by each man.
The man pushing the box up the ramp exerted less force over a longer distance.
Efficiency ( I Can #14, 16, 17)



We have been talking about
machines. All the work input has been
transferred to work output.
An ideal machine would have an efficiency of 100%.
In reality, there is no such as thing as an ideal machine.
Work will be lost as other forms of energy such as
% efficiency =
x 100
.
No machine could ever ‘produce’ more work than
what was put in, yet alone transfer all the work put
in to work put out! Some work (energy) will always
be lost to the surroundings!
Ex. 11: (I Can #17) What is the efficiency of a machine when the input is 263 J and the output is 142J?
Ex. 12: (I can #16, 17) A crane moved a 200kg hot tub from the ground to the top of the hotel, 53 m above
the ground. If the crane used 200,000 J of energy, how efficient was the crane?