Work and Power Objectives Define work and power

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=Fxd

• Work is measured in joules (J). – 1 J = 1 N • m = 1 kg • m 2/s 2 – 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 1 N falls a distance of 1 m. 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 – Measured in watts (W) – 746 watts = 1 horsepower P=W/t

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 d b. W=F x d c. W=N/d d. 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 J b. 0. 112 N c. 1750 J d. 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. b. c. d. 5. 55 W 5 W 90 W 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 W b. 44. 8 W c. 173. 8 W d. 7119 W