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Using machines
Work and Machine
- Machine: device that makes doing work easier by multiplying the force applied to an object,
changing the direction of an applied force, or increasing the distance over which a force can
be applied.
When thinking of machine you may picture a device with an engine or battery and moving parts.
However, machines can be simple, like knives, scissors, and doorknobs.
A machine makes work easier by changing at least one of these three following factors:
1. The change in the amount of force exerted (example: screwdriver, car jack)
2. The distance over which a force is applied (example: leaf rake, ramp)
3. The direction in which the force is exerted (example: pulley, blade of an ax)
- Input force (Fin): the force applied to the machine.
- Output force (Fout): the force applied by the machine.
For example if you try to pull a nail out with a hammer, you apply
input force on the handle. The output force is the force the claw
applies to the nail.
Two kinds of work need to be considered when using a machine:
1. The work done by you on the machine
2. The work done by the machine
The law of conservation of energy states that energy cannot be created or destroyed, meaning
the energy transferred to the object from the machine cannot be greater than the amount of
energy you transfer to the machine. In fact, when a machine is used an amount of energy
transferred changes to thermal energy (heat) due to friction. Therefore always Wout < Win.
- Ideal machine: a theoretical machine where none of the input work or output work would be
converted to heat, since there would be no friction. So for an ideal machine, Win = Wout.
Mr. Saoulis
- Mechanical advantage: ratio of the input force exerted by a machine to the input force
applied to the machine.
Mechanical advantage = output force / input force (MA = Fout / Fin)
- Efficiency: ratio of the output work done by the machine to the input work done on the
machine, expressed as a percentage.
Efficiency (%) = (output work / input work) x 100 %
In an ideal machine there is no friction and the output work equals the input work. So the
efficiency of an ideal machine is 100%. In a real machine, there is friction which causes the
output work to ALWAYS be less than the input work. So the efficiency of a real machine is
always less than 100%.
Mr. Saoulis
The efficiency of a machine can be increased by reducing friction. This is done usually by
adding a lubricant, such as oil or grease, to surfaces that come in contact together.
- Lubricant: a substance introduced to reduce friction between surfaces in mutual contact,
which ultimately reduces the heat generated when the surfaces move. It may also have the
function of transmitting forces, transporting foreign particles, or heating or cooling the
The property of reducing friction is known as lubricity (or slipperiness).
Mr. Saoulis