How do machines

work?

What is a machine?

• Allows you to do work in a way that is easier

• Can be quite simple

• A machine changes at least one of three factors…

The 3 Factors of Work

1. Amount of force you exert

2. The distance over which you exert your force

3. The direction in which you exert your force

Input and Output Forces

• Remember… when you use a machine to do work, you exert a force over some distance.

• The force you exert on the machine is the input force.

• The force the machine exerts on an object is called the output force.

The output work done by the shovel can never be greater than the input work done by the gardener.

Input and Output Work

• Input force times input distance is called the input work.– Done by the person– Think “I” do input work

• Output force times output distance is called the output work– Done by the machine

• The output work done by the shovel can never be greater than the input work done by the gardener.

• Input work and output work can vary depending on the distance the force exerted.

• Small input force exerted over a long distance = large output force exerted over a short distance

Making Work EasierBy Changing

Force, Distance or Direction

Changing Force

Changing Distance

Changing Direction

Mechanical Advantage

• Mechanical advantage is the number of times a machine increases a force exerted on it.– Think: “How was the force changed?”

MA

OF

IF

No units! : )

Mechanical Advantage

• If MA is over 1: – Force is multiplied! (output force more than

input, output distance is less)• If MA is less than 1 – Distance is multiplied! (output force less than

input, so output distance is more)• If MA = 1– Machine only changes direction – force and

distance are the same for input and output

Efficiency of Machines

• Assuming the input work is exactly equal to the output work is ideal, this is not always the case

• The efficiency of a machine compares the output work to the input work.

• Efficiency is expressed as a percent.• The higher the percent, the more efficient the

machine is.

Efficiency of Machines

What may be the cause of having

a less efficient machine?

Consider the difference between a new pair of garden shears and an

old rusty pair of shears.

Efficiency of Machines

To calculate the efficiency of a machine, divide the output work by the input work and multiply

the result by 100 percent.

Calculating Efficiency

Calculating Efficiency

Real vs. Ideal Machines

• An ideal machine is 100% efficient.– Does not exist!

• A real machine is less than 100% efficient.