Work and Power

Chapter 5!! The End of the ROAD is near…..!!

WORK

I. Work: a transfer of energy that occurs when a force makes an object move over a distance

a. Two requirements for work: 1. Applying a force to an object HAS to

make it move; if it doesn’t move no work is done 2. The distance the object moves MUST be in the same direction as the force applied to the object

WORK

NO WORK WORKDirection of applied force

Direction of motion

Carrying a Box

Direction of applied force

Direction of motion

Lifting a Box

What’s Work?

A scientist delivers a speech to an audience of his peers.A body builder lifts 350 pounds above his head.A mother carries her baby from room to room.A father pushes a baby in a carriage.A woman carries a 20 kg grocery bag to her car.

What’s Work?

A scientist delivers a speech to an audience of his peers. NOA body builder lifts 350 pounds above his head. YESA mother carries her baby from room to room. NOA father pushes a baby in a carriage. YESA woman carries a 20 kg grocery bag to her car. NO

WORK

b. Equation: Work = Force x distance

c. Unit: Joule (J) = N x m

W

F d

Practice Calculations

1. The force needed to lift an object is equal in size to the gravitational force on the object. How much work is done in lifting an object with a mass of 5 kg a vertical distance of 2.0 m?

2. 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?

m = 5 kgd = 2.0 mW = ?

W = F x d

W = 49 N x 2.0 m

W = 98 J

F = 80 NW = 12,000 Jd = ?

d = W F

d = 12,000J 80 N

d = 150 m

F = 5 kg x 9.8 m/s/s = 49 N

How much work if the object is then carried a horizontal distance of 2m?

None! Force applied is not in the same direction as movement

POWER

W

P t

POWER

c. 1000 watts = 1 kilowatt

d. How does a light bulb get its power rating? a 50 w light bulb does 50 J of work per second

e. 1 horsepower= 750 watts

Check for Understanding

Two students, Ben and Bonnie, are in the weightlifting room. Bonnie bench-presses the 50 kg barbell (a distance of 0.6m) 10 times in 1 minute. Ben bench-presses the 50 kg barbell the same distance 10 times in 10 seconds. Which student does the most work?Which student delivers the most power?

Check for Understanding

Ben and Bonnie do the same amount of work. They apply the same force to lift the barbell the same distance.Ben is more powerful since he does the same amount of work in less time.

Practice Calculations1. In lifting a baby from a crib, 50 J of work are done. How much power is needed if the

baby is lifted in 2 s?

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

3. How much work can be done by a 150 horsepower engine in 1 minute? (HINT: Use

dimensional analysis to convert horsepower to watts and be careful with the time)

W = 50 Jt = 2 sP = ?

P = W t

P = 50 J 2 s

P = 25 w

P = 500 WW = 10,000 Jt = ?

t = W P

t = 10,000 J 500 W

t = 20 s

W = ?P = 150 horsepowert = 60 s

W = P x tW = 112,500 w x 60 sW = 6,750,000 J

x 750 w = 112500 w 1 horsepower