Work and Machines

Machines Do Work A Machine is a device that changes a force.

Ex. A Jack used to change a tire.

Machines make work easier to do. They change the size of a force needed The direction of a force The distance over which a force acts

Machines Do Work The more Force applied, the shorter the overall

distance. Ex. Carry 5 books at the same time from Mrs.

Larose's class to Mrs. Hendee’s class.

The less force applied, the longer the overall distance. Ex. Carry one book at a time from Mrs. Larose's

class to Mrs. Hendee’s class

Machines Do Work Change in direction of applied force.

Ex. Pulling back on the handle of a oar causes its other end to move in opposite direction.

If the oar is pushed father away from the boat, how will the force needed to pull the oar through the water change?

Work Input and Work Output A rower pulls back on each oar handle

and the other end of the oar pushes against the water.

Work is done on the oar (Machine) by pulling on them, and the oars do work on the water to move the boat.

Because of friction, the work done by a machine is always less than the work done on the machine

Work Input and Work Output The force you exert on a machine is called

input force. The distance the input force acts through is

called input distance. The work done by the input force acting

through the input distance is called work input.

Formula: Work input= Input force x input distance

Work output of a Machine The force that is exerted by a machine

is called output force.

The distance the output force is exerted through is called output distance.

Formula: Work output= output force x output

distance

Work Output of a Machine If there is no change in work input, there

cannot be an increase in the work output.

You cannot get more work out of a machine than you put into it!

Mechanical Advantage The mechanical advantage of a

machine is the number of times that the machine increases an input force.

The relation of input force used to operate a machine and the output force exerted by the machine depends on the type of machine and how it is used. Ex. A nutcracker

Actual Mechanical Advantage The mechanical advantage determined by

measuring the actual forces acting on a machine is the actual mechanical advantage

Formula: Actual mechanical advantage= Output

force Input forceEx. Long incline ramp

Ideal Mechanical Advantage The ideal mechanical advantage of a machine

is the mechanical advantage in the absence of friction.

Because friction is always present, the actual mechanical advantage of a machine is always less than the ideal mechanical advantage.

Formula: Ideal mechanical advantage= Input distance Output distance

Calculating IMA A woman drives her car up onto wheel

ramps to perform some repairs. If she drives a distance of 1.8 meters along the ramp to raise the car 0.3 meter, what is the ideal mechanical advantage of the wheel ramp?

Efficiency The percentage of the work input that

becomes work output is the efficiency of a machine.

Because there is always friction, the efficiency of any machine is always less than 100%.

The work output of a machine is always less than the work input.

Formula: Efficiency= (Work input / Work output) x

100%

Calculating Efficiency You have just designed a machine that

uses 1000J of work from a motor for every 800J of useful work the machine supplies. What is the efficiency of your machine?