9.1 Work & Power

1. 9.1Work & Power

2. IS IT WORK?

3. What is WORK? • Transfer of energy through motion! • For work to occur, an object must move. • We know work is being done when: • a force is applied, and the object moves in the same direction as the applied force. • Amount of work depends on: • the force exerted • distance over which the force is applied

4. Work & energy are related! • Energy is always transferred. . . • from the object doing the work • to the object on which the work is done. • Both work & energy are measured in Joules, named after the English physicist James Prescott Joule.

5. Calculating Work Work = Force × distance W = F d F = W / d d = W / F UNITS: F: N (Newtons) d: m (meters) W: N • m = J (Joules)

6. Practice Problems The brakes on a bicycle apply 125 N of frictional force to the wheels as the bicycle travels 14.0 m. How much work have the brakes done on the bicycle? G) F = 125 N, d = 14.0 m U) W = ? E) W = Fd S) W = (125 N)(14.0 m) S) W = 1,750 J G) U) E) S) S)

7. A hydraulic lift performs12,500 J of work raising a car0.5 m. How muchforcehas the lift applied to the car? G) W = 12500 J, d = 0.5 m U) F = ? E) F = W/d S) F = 12500 J / 0.5 m S) F = 25,000 N

8. POWER • The amount of work done in a certain amount of time (rate at which work is done). • Power is measured in Watts (W), named after the Scottish inventor James Watts. • A horsepower is a common unit used for power which originally described the average power output of a draft horse. (1 hp = 746 W)

9. Calculating Power Power = Work ÷ time P = W / t W = P t t = W / P UNITS: t: s (seconds) W: J (Joules) P: J / s = W (Watts)

10. Practice Problems While rowing in a race, John does 3960 J of work on the oars in 60.0 s. What is his power output in watts? G) W = 3960 J, t = 60.0 s U) P = ? E) P = W / t S) P = 3960 J / 60.0 s S) P = 66 W

11. While carrying a box of books for 20.0 s, your power output is 720 W. How much work have you done? G) P = 720 W, t = 20.0 s U) W = ? E) W = Pt S) W = (720 J)(20.0 s) S) W = 14,400 J