1. Work and Power

2. Work-Energy Theorem Energy is the ability to do work.
Work is applying a force to an object that causes displacement of the object.
The work-energy theorem states that without energy, work cannot be done.

3. What are some examples of work?
A force acting upon an object to cause a displacement (movement in a direction)
For a force to qualify it must
1. Displacement MUST happen
2. Force MUST cause the displacement
What are some examples of work?

5. Examples a horse pulling a plow through the fields
a father pushing a grocery cart down the aisle of a grocery store
(Hmmmm… work or a miracle?)
a freshman lifting a backpack full of books upon her shoulder
(would a junior do work like this?)
a weightlifter lifting a barbell above her head
a shot-put launching the shot, etc.

7. Work or NOT? Discuss with your partner, alter the cases in which work is not done to illustrate work
A teacher applies a force to a wall and becomes exhausted.
A book falls off a table and free falls to the ground.
A waiter carries a tray full of meals above his head by one arm across the room.
A rocket accelerates through space.

8. Answers No. The wall is not displaced.
Yes! The is a downward force (gravity) which acts on the book to displace it.
No. There is an upward force, and there is a horizontal displacement but the force does not cause the displacement
Yes the expelled gas is the force which accelerates the rocket through space.

9. Describing Work Mathematically
W = fd
W = force x distance
Symbol: W
Unit: J, Joule

10. Units of Work (and Energy)
The Joule
One joule =
1 Newton of force  displacement of 1 meter
1 Joule = 1 Newton*meter
Each set of units is equivalent to a force unit times a displacement unit.

11. Example Problem
You push a refrigerator with a force of 100 N. If you move the refrigerator a distance of 5 m, how much work do you do?

12. Power
Power is the rate at which work is done or the rate at which energy is transformed.
work
Force x distance
P =
P =
time
time
For example: If you and a friend push two boxes (identical in mass) for the same distance = YOUR WORK DONE IS EQUAL
BUT: If your friend pushes the box faster than you, your friend is MORE POWERFUL!

13. Power
Symbol: P
Unit: W, Watt
1W = 1J/s

14. Example Problem
You do 900 J of work in pushing a sofa. If it took 5 sec to move the sofa, what was your power?
W = 900 J
t = 5 sec
P = ??Watt