Section 1: Work. Explain the meaning of work. Describe how work and energy are related. Calculate work. Calculate power.

Work and MachinesSection 1: Work

Learning Goals

Explain the meaning of work.Describe how work and energy are related.

Calculate work.Calculate power.

Warm-up:Backpacking is a lot of fun, but it also can be a lot of work. Whether hoisting the pack onto your back to start the hike or trudging up a long hill, you’ll need to exert a good deal of effort to get to the next camp.1. Compare the effort exerted

by a backpacker moving over level ground to that exerted by a backpacker moving uphill.

2. How do you think the weight of the backpack affects the amount of force needed to move it?

Work

Work: transfer of energy that occurs when a force makes an object move

Work

For work to occur, two conditions must be met:

1. an object must move

2. the motion of the object must be in the same direction as the applied force on the object.

Work and Energy

Work and energy are related. Energy is always transferred from the object doing the work to the object on which the work is being done.

Work and Energy

Work and Energy

Work is done on an object only when a force is being applied to the object and the object moves. If the object does not move, then no work is being done!

Calculating Work

Work equals force (in newtons) times distance

W = Fd W work, measured in joules (J)

F force, measured in newtons (N)

d distance, measured in meters (m)

Calculating Work

You push a refrigerator with a force of 100 N. If you move the refrigerator a distance of 5 m while you are pushing, how much work are you doing?

Calculating Work

F = W/dd = W/f

Calculating Work

A lawn mower is pushed with a force of 80 N. If 12,000 J of work are done in mowing a lawn, what is the total distance the lawn mower was pushed?

Calculating Work

The brakes on a car do 240,000 J of work in stopping the car. If the car travels a distance of 50 m while the brakes are being applied, what is the force the brakes exert on the car?

Power

Power: amount of work done in a certain amount of time rate at which work is done

Calculating Power

Power equals work divided by time.P = W/t

P power, measured in watts (W)

W work, measured in joules (J)

t time, measured in seconds (s)

Calculating Power

You do 900 J of work in pushing a sofa. If it took 5 s to move the sofa, how much power did you use?

Calculating Power

W = Ptt = PW

Calculating Power

If a runner’s power is 130 W as she runs, how much work is done by the runner in 10 minutes?

Calculating Power

The power produced by an electric motor is 500 W. How long will it take the motor to do 10,000 J of work?