5.1 Machines help people do work.
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
· 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
· 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