Thursday, February 18th

AgendaPass Back/Go Over Chapter 7 TestsBegin Chapter 8: Work, Power, and Machines

Work, joules, power, wattsIn-class assignments:

Pg. 251: #1-4Pg. 253: #1-5

Homework:Muscles and Work Worksheet

We will finish section 8.1 next time…

Chapter 7: Motion & ForcesTests

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Chapter 8Work, Power and Machines

WorkImagine trying to lift the front of a car to

change a tire without using a jack.You could exert a lot of force without moving

the car at all.It would seem like you were working very

hard.But in science, the word

work has a very specific meaning…

WorkWork: a quantity that measures the effects of

a force acting over a distance.In science, work is done only when force

causes a change in the motion of an object.So, were you really working when you were

trying to lift the front end of the car?Answer:

NO, not if the car didn’t move.

WorkThis Olympic weightlifter is applying a force to the barbell as he hold it overhead but the barbell is not moving.

Question: Is the weightlifter doing any work on the barbell?

Answer: NO, not if the barbell is not moving.

Work Equation

The equation used to calculate work is

Work = Force X DistanceW = F x d

The Unit for Work is the Joule (J)Because work is calculated as force times distance,

it is measured in units of newtons times meters, N•m.

1 N•m = 1 Joule

The Joule was named in honor of the English physicist James Prescott Joule who discovered the foundations of the law of conservation of energy.

Sample ProblemPg. 251

Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N?

Equation: Work = force X distanceForce = 190 NDistance = 2.0 mSolve: Work = (190 N) (2.0 m)Answer: 380 N•m 380 J

Additional PracticeHow much work does a pitcher do if he throws a

baseball with a force of 45.0 N through a distance of 1.50 m?

Equation: Work = force X distanceForce = 45.0 NDistance = 1.50 mSolve: Work = (45.0 N) (1.50 m)Answer: 67.5 N•m 67.5 J

In-Class Assignment

Take about 10-15 minutes to complete the following assignment:Pg 251: Practice #1-4

Be sure to include correct units!

PowerRunning up a flight of stairs doesn’t require

any more work than walking up slowly, but it is definitely more exhausting.

The amount of time is takes to get work done is another important factor when talking about work.

PowerPower: a quantity that measures the rate at

which work is done.Power is defined as how much work is done in

a certain amount of time.

Power Equations

Power = work time

P = W t

SI Unit for Power

The SI unit for power is the watt, W.A watt is the amount of power needed to do

1 Joule of work in 1 second.The watt was named for James Watt, a

Scottish inventor who played an important part in the development of the steam engine.

Sample ProblemPg. 252

It takes 100,000 J of work to lift an elevator 18 m. If this is done in 20 s, what is the average

power of the elevator during the process?Equation: Power = Work

timeWork = 100,000 JTime = 20 sSolve: Power = 100,000 J = 5,000 W

20 s

Additional PracticeA student lifts a 12 N textbook 1.5 m in 1.5 s.a)How much work does the student do on the

book?Equation: Work = force X distanceSolve: Work = (12 N) (1.5 m) = 18 J

b)What is the power output of the student?Equation: Power = work

timeSolve: Power = 18 J = 12 W

1.5 s

In-Class Assignment

Take about 10-15 minutes to complete the following assignment:Pg 253: Practice #1-5

Be sure to include correct units!

Homework

Worksheet: “Muscles and Work”