Thursday, February 18 th Agenda  Pass Back/Go Over Chapter 7 Tests  Begin Chapter 8: Work, Power, and Machines  Work, joules, power, watts  In-class assignments:  Pg. 251: #1-4  Pg. 253: #1-5  Homework:  Muscles and Work Worksheet  We will finish section 8.1 next time…

Chapter 7: Motion & Forces Tests  Overall, we were pleased with the test scores.  Questions/Problems on your test?  Please pass them back in…

Chapter 8 Work, Power and Machines

Work  Imagine 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…

Work  Work: 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.

Work  This 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 Distance W = 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, Nm. 1 Nm = 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 Problem Pg. 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 distance Force = 190 N Distance = 2.0 m Solve: Work = (190 N) (2.0 m) Answer: 380 Nm 380 J

Additional Practice How 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 distance Force = 45.0 N Distance = 1.50 m Solve: Work = (45.0 N) (1.50 m) Answer: 67.5 Nm 67.5 J

In-Class Assignment  Take about minutes to complete the following assignment: Pg 251: Practice #1-4  Be sure to include correct units!

Power  Running 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.

Power  Power: 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 Problem Pg. 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 time Work = 100,000 J Time = 20 s Solve: Power = 100,000 J = 5,000 W 20 s

Additional Practice A 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 distance Solve: Work = (12 N) (1.5 m) = 18 J b)What is the power output of the student? Equation: Power = work time Solve: Power = 18 J= 12 W 1.5 s

In-Class Assignment  Take about minutes to complete the following assignment: Pg 253: Practice #1-5  Be sure to include correct units!

Homework  Worksheet: “Muscles and Work”