1. Work, Power & Simple Machines

2. 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

3. Work or Not Work Example Direction of Force Direction of Motion
Doing work?
Pushing a box across the floor
Yes
A waiter carrying a tray of food to a table
Pushing down on the pedal of a go cart
Pushing on a locked door

4. Work or Not Work, Continued
Example
Direction of Force
Direction of Motion
Doing work?
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

5. 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

6. 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

7. A large force over a small distance
In this diagram the same amount of work is done using two different methods:
A large force over a small distance
A small force over a large distance

8. 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 = F x D
W = 50 N x 2 m
W = 100 N-m or 100 J

9. 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?

10. 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

11. P = (force x distance) / time
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

12. 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?

13. 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?

14. 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

15. 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