CHAPTER 14.1WORK & POWER

Pgs 426-430

LEARNING OBJECTIVES

Recognize when work is done.

Calculate how much work is done.

Explain the relationship between work and power.

NEW VOCABULARY

Work: done when a force causes an object to move in the same direction that a force is applied

Power: rate at which work is done

WHAT IS WORK?

In science work has a different meaning that just chores or jobs that must be completed

Work: done when a force causes on object to move in the same direction that a force is applied

In what ways have you done work today?

WORK & MOTION In order for you to do work, two things must

happen You must apply a force to an object The object must move in the same direction as

your applied force You do work on an object only when the object

moves as a result of the force you exert.

APPLY FORCE AND DOING WORK

To do work, an object must move in the direction a force is applied

Example: Lifting a laundry basket (fig. 2, pg 427) Arms exert a force upward as you lift the basket;

the basket moves in the direction of the force your arms applied

If you carry the basket forward, do you arms still do the work?

No, they do not. Your legs are now doing work to move your body & the basket forward

No work is done when the motion of an object is at a right angle to the applied force

FORCE IN TWO DIRECTIONS Sometimes only part of the force you exert moves

an object Example: pushing a lawn mower (fig.3, pg 427)

You push at an angle to the ground Part of the force is to the right and part is down Only part of the force that’s in the same direction of

the mower (to the right) does the work

CALCULATING WORK

Work is done when force makes an object move More work is done when the force is

increased or the object is moved a greater distance

Work is calculated using the following equation: • Applying Math: pg 428• Practice problems 1 & 2The SI unit of measure

for work is Joules

Work= joulesForce = NewtonsDistance = meters

To calculate work, the distance is not the distance the object moved total, but the distance the object moves while the force is being applied.

WHAT IS POWER?

Power is the rate at which work is done

What does it mean to be powerful?

If two weightlifters both lift the same amount of weight the same vertical distance, the both do the same amount of work.

How much power do they use – the same or different?

The weightlifter who lifted the fastest is more powerful

CALCULATING POWER

Power can be calculated by dividing the amount of work done by the time needed to do the work.

Power is measured in watts Applying Math, pg 429 Complete 1 & 2

Work is measured in JoulesTime is measured in seconds

WORK & ENERGY

If you push a chair and make it move, you do work on the chair and change its energy

When something is moving it has energy of motion (kinetic energy)

You also change the energy of an object when you do work and lift it higher

An object has potential energy that increases when it is higher above Earth’s surface You are increasing its potential energy

POWER AND ENERGY

When you do work on an object you increase the energy of the object

Energy can never be created or destroyed

If an object gains energy, it must lose energy When you do work on an object you

transfer energy to the object and your energy decreases

The amount of work done is the amount of energy transferred

Power is also equal to the amount of energy transferred in a certain amount of time

ASSESS YOUR LEARNING

Describe a situation in which work is done on an object.

Evaluate which of the following situations involves more power: 200 J of work done in 20 sec or 50 J of work done in 4 sec. Explain your answer

Determine two ways power can be increased. Calculate how much power, in watts, is needed

to cut a lawn in 50 min if the work involved is 100,000 J

Applying Math: complete problems 7 pg 430