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Work & Power

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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. 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

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

- Examples
- Non-Examples

- Equation:
work = force x distance

W = F x d

W = F x d

unit = kg*m2/s2

unit = N*m

unit = J(joule)

Example:

Three push-ups require about 1000 J of work

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

- Examples

- Equation:
power = work/time

P = W /t

P = W /t

unit = J / s

unit = W(watt)

Example:

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

- 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.

- 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

- 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

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

input forceoutput 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