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 exertthe distance over which you exert your force

the direction in which you exert your force

INPUT AND OUTPUT FORCES

Input force

Output force

Input force = the force exerted on the machine.

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

INPUT AND OUTPUT FORCES

Input distance

Output distance

Input force = the force exerted on the machine.

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

INPUT AND OUTPUT FORCES

Output force

Output force

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

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

INPUT AND OUTPUT WORK Input work = input force x input distance

Output work = output force x output distance

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

CHANGING FORCEHow can the output force be greater than the input force?Work = Force x Distance

For work to stay the same, a decrease in force must mean and increase in distance.For the input force to be smaller, you must apply the force over a longer distance.Example = ramp work is easier because you exert a smaller input force

WHEN A MACHINE INCREASES FORCE, YOU MUST EXERT THE INPUT FORCE OVER A GREATER DISTANCE

CHANGING DISTANCE Why would you use a machine where the

input force is greater than the output force?

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.

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

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

WHEN A MACHINE INCREASES DISTANCE, YOU MUST APPLY A GREAT INPUT FORCE

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

A single pulley changes the direction of the lifting force. For example, if you are lifting a heavy object with a single pulley anchored to the ceiling, you can 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

MECHANICAL ADVANTAGE It is useful to think about a machine in

terms of the input force (the force you apply) and the output force (force which is applied to the task).

When a machine takes a small input force and increases the magnitude of the output force, a mechanical advantage has been produced.

Mechanical Advantage = Output force

Input force

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MECHANICAL ADVANTAGE Mechanical advantage is the ratio of output

force divided by input force. If the output force is bigger than the input force, a machine has a mechanical advantage greater than one.

In machines that increase distance instead of force, the MA is the ratio of the output distance and input distance.

INCREASING FORCEWhen 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 newtons on the can. mechanical = output force = 30 N = 3advantage input force 10 N

INCREASING DISTANCEFor 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 = .5advantage input force 20 NThe 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.

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 frictionThe 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 EFFICIENCYEfficiency = 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 EXAMPLEYou 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 JOutput 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 MACHINESIf 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%.