STUDENTS WILL:•DETERMINE WHEN WORK IS BEING

DONE ON AN OBJECT•CALCULATE THE AMOUNT OF WORK

DONE ON AN OBJECT•EXPLAIN THE DIFFERENCE BETWEEN

WORK AND POWER

Chapter 8 Section 1: Work and Power

Vocabulary

WorkJoulePowerWatt

What is Work?

The transfer of energy to an object using a force that causes the object to move in the direction of the force.

2 things need to happen for there to be work done on an object: 1. The object moves as a force is applied to it 2. The direction of the object’s motion is the same

as the direction of the force. Ex. Pushing a table across a room is an example

of work. Carrying a book across a room is NOT an

example of work because the book is not moving in the direction of the force

Transfer of Energy and Difference between force and work

Work’s “transfer of energy” is kinetic energy This is energy of motion or “moving” energy

When ever you apply a force to an object and the object moves in the direction of the force, you have done work on the object

REFER to figure 2 on page 211

HOW MUCH WORK?

Weight and distance are the 2 ways to increase the amount of work done.

Work depends upon distance as well as forceYou need the same amount of work to push a car up a hill as

to raise it up a cliff This is because work depends upon distance and force. The straight

cliff is a shorter distance but takes more force. The slope is a longer distance, but takes less force.

If you go up a slope to climb a hill, you need less force to do the work.

Formula to calculate work W= F x d Work = force times distance The unit used to express work is a joule ( symbol J )

Power: How fast work is done

Power is the rate at which energy is transferred

Watt is the unit used to express powerPower measures how fast work

happens or how quickly energy is transferred

Power output is greater when more work is done in a given amount of time and when the time it takes to do a certain amount of work is decreased

Examples

Ex 1. If you sand a shelf by hand, the energy needed is the same as if you sanded it with an electric sander. The difference between the 2 methods is the power output is lower when you sand the shelf by hand.

Ex 2. A powerful engine affects the performance of a car because a more powerful engine will move the car more quickly. For a given speed, a more powerful engine can move a heavier car than a less powerful engine can.

How to calculate power

P = W/tPower = work divided by time

Answers to Ch.8 Sec.1 Review 3. A) force times distance 4. Work is done when a force causes an object to move in the

direction of the force, and power is the rate at which work is done.

5. W = F x d = 10 N x 10 m = 100 J

6. P = W/t = 100 J / 5 s = 20 W

7. No; once the ball leaves the pitcher’s hand, work is no longer being done on the ball. The motion of the ball after that point is due to kinetic energy already given to it by the pitcher.

8. W = F x d = 50 N x 0.5 m = 25 J

No further work has been done on the chair once it has been lifted.

Monday Welcome Work 1. Work is the transfer of __________ to

an object. 2. Force is expressed in SI unit

called ________. 3. If you increase weight or

____________, this will increase the amount of work done.

4. For work to be done on an object, the object must move in the same direction as the ________.

5. Power measures how fast ________ is done.

Tuesday Welcome Work:1. What is the SI unit for force?2. What is the SI unit for work?3. What is the SI unit for power?4. What is the SI unit for time?5. What is the SI unit for distance?

Chapter 8 Section 2What is a machine

Students will:Explain how a machine makes

work easierDescribe and give examples of the

force-distance trade-off that occurs when a machine is used

Calculate mechanical advantageExplain why machines are not

100% efficient