Work and MachinesChapter 3

Lesson 1- Work and Power

What is work?➔ any time a force is exerted on an object

that causes the object to move◆ the object must move in the same

direction in which the force is exerted.◆ No movement of object = no work done

no matter how much force is used

Different directions = NO WORK...

Some work is done.

Most work is done.

Work = Force X DistanceWork = (N) X (m)Work = _____ N.m

Therefore the SI unit for Work is called a JOULE. (1 J = force of 1N to move an object a distance of 1 meter)

How do you calculate work?

90N X 30 m = 2700 J (force X Distance)

180 N X 60 m = 10800 J ( 4 times more)

What is Work Demonstration…Worksheet 71 with

discussion

What is Power?

★ the rate at which work is done○ Power = Work Force X Distance

Time Time

• Unit for Power is watt o 1J/s = 1 W

Pinwheel Demo… (15 seconds)

1. Is work done on the pinwheel? Explain.

2. How are the two situations different?

3. Which situation involved more power? Explain.

Investigating Power: pg 72

You and your partner will be completing this investigation.

Use your time wisely!

Lesson 2Understanding Machines

Input force

• This is the force you exert.o You exert this force over a specific distance.

(input work)

Output Force

• This is the force the machine exertso The machine also moves a specific distance

(output work)

Output distance and force

What does a machine do?

❖Makes work easier by➢ changing the amount of force your exert OR➢ changing the distance over which you exert

your force OR➢ changing the direction in which you exert

your force

Machines don’t change the amount of work you do, but they do change the way you do the work.

Changing Force

If a machine allows you to use less input force, you must apply that force over a greater distance.

Changing Force

Work = Force X Distance● Let’s Say: My force is 5 N and the distance is 2 m;

○ 5 N x 2 m = 10 J of work● If the amount of work stay the same, a decrease of force means an increase of distance.

● If I put 2N of force, then my distance would be 5m to equal 10 J of work.

Changing Distance

If a machine allows you to move your input force over a shorter distance, than you need to apply a greater input force.

Changing Distance

Largeinputforce

Largeoutputdistance

Change Direction

• Some machines don’t change in either force or distance, they change the direction of the force.

Small input force

large input distance

Small output force

large output distance

Mechanical Advantage

• The number of times a machine increases a force exerted on it.

• The ratio of output force to input force

Mechanical Advantage = output force input force

What happens when you increase Force?

When the output force is greater than the input force - mechanical advantage > 1

YES

15N / 10N = 1.5

Greater

What happens when distance increases?

• When a machine increases distance, the output force (machine’s force) is less than the input force (your force).

• Mechanical advantage is < 1

What happens when direction changes?

• Input force = output forceo Mechanical advantage is 1.

The larger the mechanical advantage, the easier a machine makes your work.

What is efficiency?

• The machine’s (output) work is always less than your (input) work because the machine has to overcome the force of friction.

• The less work a machine has to do to overcome friction, the more efficient it is.

To calculate efficiency...

Output work X 100%

Input work

Lesson 3Inclined Planes and Levers

Simple Machines:

Inclined Planes - Flat sloped surface

Wedge - device that is thick on one end and tapers to a thin edge at the other

Screw - be thought of as an inclined plane wrapped around a cylinder.

Levers:

• Rigid bar that is free to pivot, or rotate on a fixed point.

• That fixed point is called a fulcrum

• Types of levers are classified according to the location of the fulcrum relative to the input and output forces.

Types of Levers: 1st Class

Types of Levers: 2nd Class

Types of Levers: 3rd Class

More Examples of Levers:

Lesson 4Putting Machines Together

PULLEY Systems

Simple Machine made of a grooved wheel with a cable or rope wrapped around it.

Types of Pulleys: Fixed

Changes the direction of the force but not the amount applied:

Types of Pulleys: Movable

Decreases the amount of input force needed. It doesn’t change the direction of the force.

Type of Pulleys: Block and Tackle

A pulley system make up of fixed and movable pulleys.

Wheel and Axle

Two connected objects that rotate about a common axis.