Work and Power

Objectives:

• Define work and power• Calculate work done on an object and the rate

at which work is done• Use ‘work’ and ‘power’ as science terms

Work and Power notes:

• In science, ‘work’ has a specific meaning

• Work is ONLY done when a force causes a change in the position or motion of an object in the direction of the applied force

• i.e. making something move because of force

Work or no work?

Work or no work?

Lifting a barbell over your head?

Work or no work?

Holding a barbell over your head?

Work or no work?

Is there work being done ON the tray?

THINK!

• What’s the difference between doing work and not doing work?

• How do you think you might calculate ‘work’? (Hint: what HAS to happen for work to be done?)

WORK EQUATION:

• Work = Force x distance W = F x d

• Work is measured in joules (J).– 1 J = 1 N•m = 1 kg•m2/s2

– 1 joule is roughly equal to the energy required to lift a small apple 1 meter

*N measures force/weight*m measures mass... To find weight in N, need to use w=mg

James Prescott Joule

THINK!

• Remember the bus-pull challenge? At what point in this clip is work being done? Why?

Practice:

• A crane uses an average force of 5200 N to lift a crate 25 m. How much work does the crane do on the crate?

• An apple weighing 1N falls a distance of 1m. How much work is done on the apple by the force of gravity?

Challenge!

• A mechanic uses a hydraulic lift to raise a 1200 kg car 0.5 m off the ground. How much work does the lift do on the car?

THINK!

• Does running up a flight of stairs require more work than walking? Why?

• What is the difference then?

POWER

• POWER is how much work is done in a given time

POWER: the rate at which work is done or energy is converted.

– power = work / time P = W / t– Measured in watts (W)– 746 watts = 1 horsepower

Practice

• 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?

• Using a jack, a mechanic does 5350 J of work to lift a car 0.500 m in 50.0 s. What is the mechanic’s power output?

• On the bottom of your notes:

Using their scientific meanings, make a sentence using ‘work’ and a sentence using ‘power’.

Exit Ticket #1-4

1. What is the equation to find work?a. W=N x db.W=F x dc. W=N/dd.W=F/d

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? a. 8.9 Jb. 0.112 Nc. 1750 Jd. 1750 N

3. It takes 100 J or work to lift a box 18 m. If this is done in 20 s, what is the power output?

a. 5.55 Wb.5 Wc. 90 Wd.3.6 W

4. Anna walks up the stairs on her way to class. She weighs 565 N and the stairs go up 3.25 m vertically. calculate her power output if she climbs the stairs in 12.6 s.

a. 146 Wb.44.8 Wc. 173.8 Wd.7119 W