1. WORK AND POWER
WHEN OBJECTS MOVE

2. Bellwork Pick up notes and activity sheet from the back
Bring your October Sky Book and a calculator to class tomorrow
Test is Thursday!

3. Changes in Motion
Any change in an object’s motion is caused by a FORCE.
When we apply a force on an object to cause this motion, we do WORK on the object
Work done is a result of a force over a DISTANCE

4. Work
Work is done only if the force causes an object to move in the SAME direction as the force

5. Concept Check State whether work is done in each of the following situations:
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 straight across the room at a constant speed. (Careful! This is a difficult question.)
A rocket accelerates through space.

6. Doing Work
When a weight lifter raises a heavy barbell, he does work on the barbell.
When a weight lifter simply holds a barbell overhead, he does NO work on the barbell.

7. Calculating Work

8. 1 J = 1 Nm The unit for work is the Newton-meter
UNIT of Work
The unit for work is the Newton-meter
We call the newton-meter the joule (J).
One joule of work is done when a force of one Newton is exerted over a distance of one meter.
1 J = 1 Nm

9. Example 1
Together, two students push on a car with 825 N of force. If the students push the car 35 m, how much work do the students do on the car?

10. Example Problem 2
Bud, a very large man of mass 130 kg, stands on a pogo stick. How much work is done as Bud compresses the spring of the pogo stick 0.50 m?

11. Work done at an ANGLE Remember that a force is a vector quantity
The work done is parallel to the direction of the objects motion.
This is the horizontal component of the force if the object moves in a horizontal direction or the vertical component if the object moves in a vertical direction

12. Calculating Work at an Angle

13. Example Problem 3
A force of 50 N acts on the block at the angle shown in the diagram. The block moves a horizontal distance of 3.0 m. How much work is done by the applied force?
W = F (cosɵ)d
W = (50 N)(cos 30)(3 m) W = Joules  

14. Power

15. Power
The quantity that deals with the rate at which work is done is called power
Power is the amount of work done in a given amount of time.
Compare the work done between two climbers making their way up a mountain- one a rock climber, and one a hiker- which one has more power
The hiker has a greater power rating because it takes less time for her to reach the top of the mountain

16. Power The SI unit for power is the watt, W.
1 watt is equal to a joule per second.

17. Power Equation

18. Example 1
A tired squirrel (mass of approximately 1 kg) does push-ups by applying a force to elevate its center-of-mass by 5 cm in order to do a mere 0.50 Joule of work. If the tired squirrel does all this work in 2 seconds, then determine its power.

19. Example 2
When doing a chin-up, a physics student lifts her 42.0-kg body a distance of 0.25 meters in 2 seconds. What is the power delivered by the student's biceps?