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