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### Work & Power

Chapter 8.1

- How do you define “work,” or what comes to mind when you hear that word?
- How about the word “power,” anything else?

I. hear that word?What is Work?

A. Definition

B. Equation

C. Unit

II. What is Power?

A. Definition

B. Equation

C. Unit

III. Machines

A. Forces

B. Input/Output

C. Mech. Advantage

1. Definition

2. Equation

IV. Math Skill Practice

A. Work

B. Power

C. Mechanical Advantage

Chapter 8.1 OutlineWhat is Work? hear that word?

- Non-science definitions
- Definition:
- The effect of a force acting over a distance

- Examples
- Non-Examples

Calculating Work hear that word?

- Equation:
work = force x distance

W = F x d

The Units of Work hear that word?

W = F x d

unit = kg*m2/s2

unit = N*m

unit = J(joule)

Example:

Three push-ups require about 1000 J of work

What is Power? hear that word?

- Non-science definitions
- Definition:
- The rate at which work is done

- Examples

Calculating Power hear that word?

- Equation:
power = work/time

P = W /t

The Units of Power hear that word?

P = W /t

unit = J / s

unit = W(watt)

Example:

Three push-ups done in 3 sec. is about 330 W of power

Machines hear that word?

- Why do we use machines?
- Which is easier, lifting a car by hand or using a car jack?
- Which requires more work?
Key Statement:

Machines multiply and redirect forces.

Machines and Work hear that word?

- The work that you put into a machine equals the work put out by a machine
- What things qualify as machines?

- p.254 – Figure 8-3
Key Statement:

Work input equals work output

Mechanical Advantage hear that word?

- Definition:
- A measurement of how much a machine multiplies a force or distance
In other words, how much is the machine benefiting you?

- A measurement of how much a machine multiplies a force or distance

Mechanical Advantage hear that word?

- The Equation:
Mech. Adv. = output force = input distance

input force output distance

It is a ratio, therefore there are no units attached

You and two friends apply a force of 425 N to push a piano up a 2.2 m long ramp.

How much work in joules has been done when you reach the top of the ramp?

W = F x d

= 425 N x 2.2 m

= 935 J

= 940 J

If you make it to the top in 5.0 s, what is your power output?

P = W / t

= 940 J / 5.0 s

= 188 W

= 190 W

If lifting the piano straight up would require 1700 N of force, what is the MA of the ramp?

MA = output F/ input F

= 1700 N / 425 N

= 4.0

Math Skills – Putting it all together!
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