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

Machinedevice that makes work easierchanges the size and/or

direction of the force

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

Problems A crank on a pasta maker has a radius

of 20 cm. The turning shaft has a radius of 5 cm. What is the MA of this wheel and axle?

GIVEN:

rinput = 20 cm

routput = 5 cm

MA = ?

WORK:

MA = rinput ÷ routput

MA = (20 cm) ÷ (5 cm)

MA = 4

MA

rinput

routput

5 cm 20 cm