# Chapter 12: Work & Energy

Work & Energy Unit Part 1: Work, Power, & Machines What is Work? Work has a very specific meaning in physics Work is only done when force causes a change in the ____________ or

_________ of an object in the direction of the applied force Finding Work Mathematically Work is equal to ____ times _______ If the object to which force is being applied does not move or change motion, then work is equal to

_______________. The Measurement of Work Work is measured in _____ A joule is equivalent to: ___________________ ___________________ The amount of work it takes to take to lift an apple from your waist to the top

of your head Historical Perspective The unit of work and energy was named in honor of the English physicist _______________________ (18181889) Joule was one of the first scientists to recognize that mechanical work and nonmechanical energy could be interchangeable

This was the foundation of the __________________________________ Work Problem Practice 1. A crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do on the girder? 2. 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? Power Is the rate at which ______ is done Mathematically defined as:

SI Unit of Power Is the watt, W A _______ is defined to be 1 joule per second (J/s) ___________ (hp) is another unit of power that is sometimes used 1 hp = _____________ Practice

1. While rowing across the lake during a race, John does 3960 J of work on the oars in 60.0 s. What is his power output in watts? 2. Using a jack, a mechanic does 5350 J of work to lift a car 0.500 m in 50.0 s. What is the mechanics power output? 3. Suppose you are moving a 300 N box of books. Calculate your power output in the

following situations: You exert a force of 60.0 N to push the box across the floor 12.0 m in 20.0 s. You lift the box 1 m onto a truck in 3 s. Machines

They help us by redistributing the work that we put into them They can change the ________ of an _________________ They can increase or decrease ________ by changing the _______ over which the force is applied (multiplying the force)

Different Forces can do the Same Amount of Work Mechanical Advantage Tells how much a machine multiples force or increases distance Mathematically defined as: ________ must be ignored

Practice A bus driver applies a force of 55.0 N to the steering wheel, which in turn applies 132 N of force on the steering column. What is the mechanical advantage of the steering wheel?

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