How Machines Do Work

What is a machine, Mechanical advantage & efficiency.

What is a machine?Device that allows you to do work in a way that is easier

change at least one of the three factors

machine1. Change the amount of

force you exert2. Change the distance over

which you exert the force3. Change the direction in

which you exert the forceMachines make work easier by

changing either the force, distance or direction.

Input & output forcesInput force = force you

exert on the machine

Output force = the force the machine exerts on an object

Input and output workInput work = input force x

input distance

Output work = output force x output distance

No machine produces more work than is input.

The amount of input work done by the gardener equals the amount of output work done by the shovel.

Mechanical Advantage

The number of times the machine increases the force exerted on it.

MA = output force/input force

Increasing force: MA > 1You exert an input

force of 10 NCan opener exerts

an output force of 30 N

Mechanical advantage =

30N/10N = 3

Increasing Distance: MA < 1 You exert an Input

force of 20 N on the chopsticks

Chopsticks exert an Output force of 10 N

Mechanical advantage?

10 N/ 20 N = 0.5 Input distance Output distance

Changing direction If you only change the direction, what will be the mechanical

advantage?Mechanical advantage = 1

Input force Output force

EfficiencyEfficiency =output work/input work x 100% Friction reduces the efficiency of objects

(like glue on scissors)

Real and ideal machinesEfficiency of 100 % ideal machine100 % efficiency perpetual motion

machineUnfortunately no such machine exists

All machines lose some work due to friction

1. Machine increases force, you must exert input force over a greater distanceExample: faucet

2. Machine increases distance, you must apply greater input forceExample: chopsticks

3. Machine changed direction, the amount of force and distance remain the same.Example: tricep presses