What is a machine ?

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# What is a machine ? - PowerPoint PPT Presentation

What is a machine ?. A machine is a device that allows you to do work in a way that is easier . . How can a machine make work easier ?. …..by changing at least one of three factors. the amount of force you exert the distance over which you exert your force

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Presentation Transcript
What is a machine?
• A machine is a device that allows you to do work in a way that is easier.
How can a machine make work easier?
• …..by changing at least one of three factors.
• the amount of force you exert
• the distance over which you exert your force
• the direction in which you exert your force
Input and Output Forces

Input force = the force exerted on the machine.

Input force

Output force

Output force = the force the machine exerts on the object.

Input and Output distances

Input distance

The input force moves the machine a certain distance = input distance

Output distance

The machine does work by exerting the a force over another distance = output distance.

Input Work – Output Work

Output Work = Foutput x doutput

Input Work = Finput x d input

Input Work = Output Work

Input Work = Finput x d input

Output Work = Foutput x doutput

The amount of output work can never be greater than the amount of input work.

You put in a large force (input) and move your hand a small distance (input)……………

the shovel moves a big distance (output) with a small force (output)

1. Changing Force

How can the output force be greater than the input force? Finput< Foutput

For work to stay the same, the input force to be smaller and the input distance must be longer.

Example = ramp work is easier because you exert a smaller input force but move a longer distance

Input Work = Finput x dinput

When a machine increases force, you must exert the input force over a greater distance

Finput< Foutput

dinput< doutput

2. Changing Distance

Why would you use a machine where the input force is greater than the output force?Finput< Foutput

The benefit of a hockey stick is that it increases the distance of the output.

dinput< doutput

The end of the stick travels faster and farther than the player’s hands do.

This means that the puck travels faster and a greater distance than it would have if the player hit the ball with his or her hand or foot.

Finput< Foutput

dinput< doutput

Changing Direction

Why would you use a machine that changed neither force not distance?

A single pulley changes the direction of the lifting force.

You pull down on the rope to lift the object instead of pushing up.

The same amount of effort is needed as without a pulley, but it feels easier because you are pulling down.

When a machine changes the direction of the input force, the amount of force and the distance remain the same
• When a machine takes a small input force and increases the magnitude of the output force, a mechanical advantagehas been produced.
• Mechanical Advantage = Output force

Input force

Increasing Force

When the output force is greater than the input force the mechanical advantage is greater than 1.

Example: If the input force of 10 newtons on a hand-held can opener. The opener exerts a force of 30 newtonson the can.

mechanical = output force = 30 N = 3

Increasing Distance

For a machine that increases distance, the output force is less than the input force. The mechanical advantage is, therefore, is less than 1.

Example: If your input force is 20 newton and the machine’s output force is 10 newtons,

mechanical = output force = 10 N = .5

The output force is half the input force, but the machine exerts the force over a longer distance.

Changing Direction
• What can you predict about the mechanical advantage of a machine that changes the direction of the force?
• If only the direction changes, the input force will be the same and the output force.

mechanical = output force = 15 N = 1

• The mechanical advantage will always be 1.
Efficiency of Machines
• In an ideal situation the work you put into a machine is exactly equal to the work done by the machine.
• In real situations, the output work is always less than the input work.

In every machine, some work is wasted overcoming friction

The less friction there is the closer the output work is to the input work.

Efficiency
• A rusty pair of shears is less efficient than a new pair of shears.
• 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.
Calculating Efficiency

Efficiency = output work x 100

input work

If the rusty shears have an efficiency of 60%, only a little more than half the work you do goes into cutting the bushes.

Calculating Efficiency Example

You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the hand mower is 200,000 J, what is the efficiency of the lawn mower?

Efficiency = output work x 100

input work

Input work = 250, 000 J

Output work = 200,000 J

Efficiency = 200,000 J x 100 = 80%

250,000 J

The efficiency of the lawn mower is 80%

Real and Ideal Machines

If you could find a machine with an efficiency of 100%, it would be an ideal machine.

Such a machine does not exist…In all machines some work is wasted due to friction.

An ideal machine transfers all the energy, so the output work equals the input work

All real machines have efficiencies less than 100%.