MODERN TECHNOLOGY USES COMPOUND

MACHINES•What simple machines do you recognize in a tow truck?•A machine that is made up of two or more simple machines is a compound machine. •How are simple machines related to compound machines?

Gears

Gears are based on the wheel and axle.

The gear that turns another gear applies the input force and the gear that is turned applies the output force.

Mechanical Advantage of Compound Machines

The mechanical advantage of any compound machine is equal to the product of the mechanical advantage of all the simple machines that make up the compound machine.

For example , the ideal machine advantage of a pair of scissors would be the product of the ideal mechanical advantage of its two levers and two wedges.

Compound Machines and Friction Compound machines

typically must overcome more friction than simple machines, because they have more moving parts.

Scissors for example have a lower efficiency than one lever because they have more moving parts.

Mechanical Advantage

When a machine helps you work, there is an advantage to them. The number of times a machine multiples the input force is called the machine’s mechanical advantage.

To find a machine’s mechanical advantage, divide the output force by the input force.

Mechanical advantage= Output force/Input force

Mechanical advantage of a machine

For machines such as a rake the output force is less than the input force.

If the input force of the rake is 10 newtons and the output force is 5 Newtons mechanical advantage is .5

Output force

Input force

Output Work is always less than input work

The work you do on a machine is called the input work, and the work the machine does in turn is called the output work.

A machines efficiency is the ratio of its output work to the input work.

An ideal machine would be 100 % efficient.

Actual machines lose input work to friction.

Calculating Efficiency

You can calculate a machines efficiency by dividing the machines output work by it’s input work and multiplying that number by 100.

Efficiency (%) =Output work/Input work x 100

Calculating Efficiency

Work is measure in Joules, suppose you do 600 J of work using a rope system to lift a box. The work done is 540J.

Efficiency= 540 J/600 J x 100=90%

Increasing Efficiency

All machines lose input force to friction, one way to improve efficiency is by reducing friction.

Oil is used to reduce friction between the moving parts of a car engine and a bicycle.