5.1 Machines help people do work.

Important questions:

1) How do machines help us do work?

2) How do we calculate a machine’s efficiency?

 

Machines change the way force is applied.

·       Machine - Any device that helps people do work

·       Does not decrease the amount of work that is done, just changes it

·       Example: If you have to lift a heavy box, you can use a ramp to make the work easier.  The ramp is a machine.

·       Machines make work easier in 2 ways:

1)    By changing the force needed to do the work and the distance over which the force is applied

2)    By changing the direction in which the force is exerted

·       Machines are powered by different types of energy like electric and mechanical energy

 

CHANGING SIZE AND DISTANCE

·       A doorknob allows you to apply a smaller force over a greater distance

·       A rake allows you to apply a greater force over a smaller distance

·       Input force- The force exerted on a machine

·       Output force- The force that a machine exerts on an object

·       As you rake leaves, you apply an input force on the rake.  As a result, the rake exerts an output force on the leaves

 

CHANGING DIRECTION

·       Machines also help us do work by changing the direction of a force

·       When you raise a flag on a flagpole you pull the rope down and the flag goes up 

·       The force doesn’t change, the direction does

 

MECHANICAL ADVANTAGE

·       When machines help you do work, there is an advantage (benefit) to using them.

·       Mechanical Advantage- The number of times a machine multiplies the input force

·       Mechanical Advantage = Output Force/Input Force

 

Work transfers energy.

·       When you lift an object, you transfer energy to it in the form of gravitational potential energy

·       The higher you lift something, the more work you do and the more energy you give to the object

·       When you use a machine to do work, there is always an exchange, or tradeoff, between the force you use to do the work and the distance over which you apply that force

·        You apply less force over longer distances and more force over shorter distances

 

Output work is always less than input work.

·       Efficiency- The ratio of a machine’s output work to the input work

·       An ideal machine would be 100% efficient

·       All machines, though, lose some input work to friction

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

·       The more mechanical energy lost in the transfer to other forms of energy, the less efficient the machine

·       Machines lose some energy in the form of heat due to friction.

·       A car engine has an efficiency of only 25%

·       What are some common ways to increase a machine’s efficiency?

Oil in car engines, greasing parts of a bicycle