Work and Power

The Joule (J) = 1 Newton * meter Work Work is defined as the product of the applied force and the displacement through which that force is exerted. W = F*d FORCE (N) WORK DISTANCE (m) The Joule (J) = 1 Newton * meter is the unit of work

How much work is done lifting a 3 N cabbage 2 meters into the air? Try These... How much work is done lifting a 3 N cabbage 2 meters into the air? W = Fd = 3N(2m) = 6 N·m (or joules) How much work is done by holding it there for 5 seconds? none (why?)

*NOTE 1. Work is only done on an object when the object is moved (displaced). Though your muscles are exerting a force to hold up the cabbage, there is no displacement of the object. 2. In addition, work is only done when there is a component of the force in the direction of the displacement.

Now, Try This… Ff = mFN F = 10 N and d = .5 m W = 10 N(.5 m) = 5 J Pete fills a pitcher full of Pepsi and pushes the 5 N pitcher forward with a 10 N force over a distance of 0.5 m to a customer at the end of the counter. a. Determine the work done by Pete on the pitcher. b. Determine the work done by friction upon the pitcher. c. Determine the total work done upon the pitcher. F = 10 N and d = .5 m W = 10 N(.5 m) = 5 J Ff = mFN = .4(5 N) = 2 N to the left and d = .5 m W = 2N (.5m) = -1 J [cos(180) = -1] Total Work = 5 J + (-1 J) = 4.0 J

POWER Power is defined as work done divided by the time used to do the work. P = F*d = W t t The SI unit for power is the joule/sec = watt (W)

Back to Pete… d = 0.5m 5 N m = 0.4 If Pete’s pitcher of Pepsi takes 2 seconds to reach the customer, what was Pete’s power output? (Remember Pete’s Work on pitcher = 5 J) Answer… P = W/t = 5 J/2 s = 2.5 W

A machine is capable of a power output of 1,000 watts A machine is capable of a power output of 1,000 watts. How far can it lift a 50N weight straight up in 3s? Problem HINT: Solve formula for “d” = 60 meters

putting it all together... How much work is done holding a 1,200 N piano at shoulder height for 120 seconds (your shoulder is 1.5 m above the ground)? NONE What if we lifted the piano straight up 1 meter over that time? 1,200 J How much power was exerted? 10 Watts