How do machines do work? 12-2

What is a machine? A device that allows you to do work in a way that is easier By changing Amount of force you exert Distance over which you exert your force Direction you exert your force

Ex. Shovel to move dirt If you use a shovel to move dirt compared to using just your hands, does this make less work for you?

Input and output forces Input force Force you exert on the machine Moves the machine a certain distance, called the input distance Output force Force that the machine exerts on the object The work the machine does by exerting a force over another distance, output distance

Input and Output Work Input work Output work Input distance times input force Output work Output distance times output force When using a machine the amount of output work can never be greater than the input work Don’t get more out of the machine that you put in

Making work easier Changing force With some machines output force is greater than input force This means there is an increase in the distance over which the force is applied

Ramp Length is greater than height, input force is exerted over a greater distance The longer the ramp, the less of an incline there is, and the easier it is to push something up the ramp

Faucet knob Handle of faucet has greater diameter than pipe it is turning, so force is exerted over a greater distance. It is much easier to turn something that has a large diameter than something with a small diameter

Change force Some machines have an output force that is less than the input force Allows you to exert your input force over a shorter distance Must have a greater input force

Hockey Stick Your hands don’t move much, but you have put in a strong force. This causes the stick to move a greater distance than your hands, hitting the hockey puck

Chopsticks Your fingers don’t move very much, but this causes a larger movement in the chopsticks. The force you put in is larger than the output force

Changing direction Some machines don’t change either the force or distance They change the direction in which the force is exerted

Weight machine Allows you to pull the weight at an angle, making it easier for you to move the weights

Mechanical Advantage Number of times a machine increases a force exerted on it Ratio of output force to input force gives you mechanical advantage Mechanical advantage = output force / input force

Changing Force Output force is greater than input force, mechanical advantage is greater than 1 Exert force of 10N on hand held opener, opener exerts force of 30 N Calculate the MA for the can opener:

Changing Distance Output force is less than input force, so mechanical advantage is less than 1 Input force is 20N and machine’s output force is 10N Calculate the MA for this machine:

Changing Direction If only the direction changes, the mechanical advantage will always be 1 Input and output forces are equal, only the angle of force changes

Efficiency of Machines Efficiency of a machine compares the output work to the input work. Expressed as a percent Higher percent, the more efficient the machine is Efficiency = output work / input work x 100

What is more efficient, a pair new scissors or rusty scissors? Why?

Real and ideal Machines A machine with an efficiency of 100% would be an ideal machine Do you think these kinds of machines exist? Why?