S E C T I O N 1 0 . 2

SIMPLE MACHINES

SIMPLE MACHINES

• How do I define a simple machine?

• How do I differentiate between an ideal and a real machine?

• How do I define and calculate efficiency?

MACHINES

• Machine: a device that changes the magnitude or direction of a force

• Should make things easier

• Simple machines: bottle openers, screwdrivers, …

• Complex machines: bikes, cars, …

BOTTLE OPENER

• How does a bottle opener work?

• A person applies a force up

• This is called the effort force, Fe

• The opener applies a force to the cap

• This is called the resistance force, Fr

BOTTLE OPENER

• Different sized arrows?

• A small effort creates a larger resistance force

• This is called the mechanical advantage

SINGLE PULLEY

• Fe = Fr

• MA is 1

• Why have a single pulley?

DOUBLE PULLEY

• If we add a pulley, MA goes over 1

• What happens to direction?

WORK

• Work is Fd

• There is input work and output work

• A machine can increase force but not energy so, …

• Wo = Wi

• Frdr = Fede

IDEAL MECHANICAL ADVANTAGE

• For an ideal machine, no effort would be lost

• The difference between the ideal mechanical advantage and actual mechanical advantage tells us the efficiency of the machine

MACHINES

• Maximum efficiency is 100%

• Rare

• Most machines are made up of a combination of• Levers• Pulleys• Wheel and axle• Incline plane • Wedge• screw

BASIC MACHINES

STEERING WHEEL

• Example of a wheel and axel

• Compound machine: A machine with two or more simple machines linked such that the resistance force of one machine becomes the effort force of the second machine

COMPOUND MACHINES

• Compound machine: A machine with two or more simple machines linked such that the resistance force of one machine becomes the effort force of the second machine

COMPOUND MACHINES

• The pedal and front gear are wheel and axel

• Effort force is rider on the pedal

• Turns into resistance force of front gear on chain

• This force turns into the effort force of the chain

• That turns into the resistance force of the wheel on the road

BICYCLE

• MA = MAmachine 1 x MAmachine 2

• By same logic

• Since both gears use the same chain and have the same teeth size:

• Shifting gears adjusts the gear radii to obtain different IMAs

EXAMPLE

• The rear wheel on a bicycle is 35.6 cm and a gear with a radius of 4.00 cm. When the chain is pulled with a force of 155 N, the wheel rim moves 14.0 cm. The efficiency of this part of the bike is 95.0 %. Find

a. IMA of the wheel and gear (for wheel and axis we can use the ratio of the radii)

b. MA of the wheel and gearc. Resistance forced. How far was the chain

pulled to move the rim 14.0 cm?

HOMEWORK

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