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Machines - Machines - Ch. 7Ch. 7
Machines - Machines - Ch. 7Ch. 7
II. The Simple Machines (Chapter 8)
Introduction to
MachinesMechanical
Advantage
Lever Pulley Wheel & Axle
Machines
2 kinds of work involved when a machine is used
force applied to the machine“what you do”
force applied by the machine“what the machine does”
MachinesWork Input (Win)
Work done on a machineYou apply a force, called the
input force, to the machine and move it through a distance
Win = F × d
MachinesWork Output (Wout)
work done by a machine The machine applies a force,
called the output force, through a distance
Wout = F × d
Machinescan never get more work out
than you put inWork output can never be
greater than work input
Win = Wout
F × d = F × d
MachinesMachines make work easier
because they change the size or direction of the input force.
Using a machine does not mean that you do less work….a machine allows a smaller force to be applied over a longer distance
MachinesForce – Distance Trade – Off
Force or distance can increase, but not together. When one increases, the other must decrease.
ie. When force increases distance decreases
Mechanical AdvantageMechanical Advantage (MA)
number of times a machine increases the force
output
input
FMA
F
Mechanical Advantage A worker use a crowbar to apply an
input force of 20 N to open a window with a output force of 500 N. What is the crowbar’s MA?
GIVEN:
Finput = 20 N
Foutput = 500 N
MA = ?
WORK:
MA = Foutput ÷ Finput
MA = (500 N) ÷ (20 N)
MA = 25
MA
Foutput
Finput
LeverLever
a bar that pivots at a fixed point, called a fulcrum
“Give me a place to stand and I will move the Earth.”
– Archimedes
Engraving from Mechanics Magazine, London, 1824
Input force
Output force
Fulcrum
Lever
Levers are used to apply a force to a load
There are three classes of levers, based on locations of the fulcrum, the load and the input force
PulleyPulley
grooved wheel that holds a rope or a cable
A load is attached to one end of the rope and an input force is applied to the other end
Le
Lr
F
Wheel and AxleWheel and Axle
two wheels of different sizes that rotate together
a pair of “rotatinglevers”
Wheel
Axle
Problems You use a 140 cm plank to lift a large
rock. If the rock is 20 cm from the fulcrum, what is the plank’s MA?
GIVEN:
Loutput = 20 cm
Linput = 140 cm
MA = ?
WORK:
MA = Linput ÷ Loutput
MA = (140 cm) ÷ (20 cm)
MA = 7
MA
Linput
Loutput
20cm
140cm
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