thursday, february 18 th agenda pass back/go over chapter 7 tests begin chapter 8: work, power,…
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Chapter 7: Motion & Forces Tests Overall, we were pleased with the test scores. Questions/Problems on your test? Please pass them back in…TRANSCRIPT
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
Overall, we were pleased with the test scores.
Questions/Problems on your test?
Please pass them back in…
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”