File - 8th Grade Physical Science
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Transcript File - 8th Grade Physical Science
Work, Power
& Simple Machines
What is “WORK?”
Work is defined as
A force acting through a distance
In order for “work” to be done
There must be a force (push, pull, lift, throw…)
There must be movement over some distance
The motion must be in the same direction as
the force applied
Work or Not Work
Example
Pushing a box across the
floor
A waiter carrying a tray
of food to a table
Pushing down on the
pedal of a go cart
Pushing on a locked door
Direction of
Force
Direction
of Motion
Doing
work?
Yes
Work or Not Work, Continued
Example
Pulling a suitcase on
wheels through the
airport
Pushing a car
down an incline
Writing an essay in
cursive
Writing an essay
on the computer
Direction
of Force
Direction
of Motion
Doing
work?
Is work done?
Your friend pushes his book to you by
sliding it across the desk
You lift the book off of the desk
You hold the book in your hands
You pass the book to another classmate
You walk the book to another classmate
Measuring Work
Work is determined by considering Force and
Distance
Work is equal to the amount of force applied times
the distance the object moves
Work = force (newton) x distance (meter)
W = F x D
The unit for Work is the “newton-meter (N-m)” or the
“joule(J)”
If a force of 1 newton is applied to move an object 1
meter, then 1 newton-meter, or 1 Joule of work is
done
In this diagram the same amount of work is done using two different
methods:
A small
force over
a large
distance
A large
force
over a
small
distance
Solving work problems
If I push an object with a force of
50 Newton's, and it moves two meters as
a result of that push, how much work is
done?
W=FxD
W = 50 N x 2 m
W = 100 N-m or 100 J
Copy and complete…..
If I lift an object 0.5 meters and the
object has a weight of 150 N, how much
work is done?
If I carry the same object across the
room 5 meters, how much work is done?
What is “Power?”
Power tells the rate, or “how fast,” work
is being done;
Power measures work in relation to time;
Power is equal to the amount of work
divided by the amount of time over which
the work is done
Calculating Power
Power = work/time
P = w / t
P = (force x distance) / time
The unit for work is the Joule
The unit for time is the second
The unit for Power is joules/sec (J/s)
or the Watt (W)
1 Watt of power means that 1 joule of work is being done in
1 second
P=w/t
If more work is done in the same amount
of time, what happens to the Power?
It increases
If the same amount of work is done in
less time, what happens to Power?
It increases
Common Power Terms
A 75 watt light bulb does 75 Joules of
work in 1 second; it takes 75 watts of
power to make the light bulb glow
A kilowatt is 1000 watts
How many Kilowatts of power are used
by a 1200 Watt hair dryer?
Solving Power Problems
How much power is used when a person
does 500 Joules of work in 20 seconds?
P=w/t
P = 500 / 20
P = 25 Joules/sec or 25 Watts
Solving Power Problems
A person weighing 600 N gets on an
elevator. The elevator lifts the person
6m in 10 seconds. How much power
was used to lift the person?
P=w/t
P = (f x d) / t
P = (600 x 6) / 10
P = 3600 / 10
P = 360 J/s or 360 Watts