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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

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Work & Power

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Work power

Work & Power

Chapter 8.1


Work power

  • How do you define “work,” or what comes to mind when you hear that word?

  • How about the word “power,” anything else?


Chapter 8 1 outline

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

Chapter 8.1 Outline


What is work

What is Work?

  • Non-science definitions

  • Definition:

    • The effect of a force acting over a distance

  • Examples

  • Non-Examples


Calculating work

Calculating Work

  • Equation:

    work = force x distance

    W = F x d


The units of work

The Units of Work

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

What is Power?

  • Non-science definitions

  • Definition:

    • The rate at which work is done

  • Examples


Calculating power

Calculating Power

  • Equation:

    power = work/time

    P = W /t


The units of power

The Units of Power

P = W /t

unit = J / s

unit = W(watt)

Example:

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


Machines

Machines

  • 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

Machines and Work

  • 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

Mechanical Advantage

  • Definition:

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

      In other words, how much is the machine benefiting you?


Mechanical advantage1

Mechanical Advantage

  • The Equation:

    Mech. Adv. =output force = input distance

    input forceoutput distance

    It is a ratio, therefore there are no units attached


Math skills putting it all together

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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