CHAPTER 14: MACHINES

14.1: Machines help people do work Machine:

Any device that helps people do work. Work is the use of force to move an object

Does not decrease the amount of work that is done

Only changes the way the work is done Example:

Ramp pulley

How do machines help? Machines make the work easier by

changing: The size of the force needed to do the work the distance over which the force is applied The direction in which the force is exerted

Can be powered by different types of energy depending on the type of machine: Electronic machines use electrical energy Mechanical machines use mechanical energy

Mechanical energy is usually supplied by the person using the machine

Changing size and direction Some machines help by changing the

size of the force needed If a machine allows you to exert less

force, you must apply that force over a greater distance: Total amount of work remains the same

whether a machine is used or not Work = force x distance

Because a machine does not decrease the amount of work to be done, less force must mean a greater distance

Some machines allow you to apply a greater input force over a shorter distance (rake) You will move your hand a shorter distance

to move the end of the rake a longer distance

Input force: The force exerted on a machine When using a rake, the input force is the

force from the boy on the rake Output force:

The force that the machine exerts on an object

This is the force that the rake exerts on the leaves

Machines can help you do work by changing the direction of a force:

Flagpole: When raising the flag up the flagpole, you

pull down on the rope to raise the flag up the pole

The rope system does not change the size of the force, only the direction. The force pulling the flag up is equal to the

force you apply in your downward pull

Shovel: Once you have the shovel in the ground,

you push the handle down to lift the dirt up. A shovel also changes the size of the force

you apply-you need less force to lift the dirt

Mechanical advantage of a machine Mechanical advantage

MA The number of times a machine

multiplies the input force Formulas to know:

MA = output force ÷ input force Fout = MA x Fin

Fin = Fout ÷ MA

If your machine allows you to apply less force over a greater distance (doorknob) the output force is greater than the input force; MA is greater than 1

For machines that allow you to apply greater force over a shorter distance (rake) the output force is less than the input force; MA is less than 1

For machines that change only the direction of a force (rope system of a flagpole) the input and output forces are the same; MA is equal to 1

The output force of a machine is 600N and the input force is 200N. What is the MA of the machine?

A machine has an input force of 150N and a MA of 0.5. What is the output force?

The output force of a machine is 135N and the MA is 2.5. What is the input force?

Work transfers energy Machines transfer energy to objects

on which they do work. If the machine lifts an object it gives

off potential energy The higher you lift an object, the more

work you do and the more energy you give the object

A machine that causes an object to move gives the object kinetic energy

Output work is always less than input work Efficiency:

The ratio of a machine’s output work to the input work

An ideal machine would be 100% efficient so all of the input work would be converted to output work (not possible due to friction)

Calculate efficiency: Efficiency (%) = (output work ÷ input

work) x 100

If someone does 500J of work on a pair of pliers and the pliers do 300J of work on a wire, what is the efficiency of the pliers?

E = Output ÷ Input x 100Output =Input =

The more efficient the machine, the less mechanical energy is lost

Some energy is lost to heat (friction) The more moving parts the machine has,

the more energy is lost to friction Car engine:

Efficiency is only about 25% due to the heat generated

Typical electric motors are about 80% efficient

Increase efficiency: Decrease friction Decrease air resistance

14.2:Six simple machines

There are six machines on which all other mechanical machines are based: Inclined plane Lever Wheel and axle Pulley Wedge screw

Lever: A solid bar that rotates, or turns,

around a fixed point (fulcrum) Bar can be straight or curved Can multiply the input force Can also change the direction of the

input force 3 classes of levers all with different

arrangements of the fulcrum, input force (effort), and output force (resistance)

First-class lever: Fulcrum is located between the input force

and the output force Used to change the direction and size of the

force Second-class lever:

Output force is located between the input force and the fulcrum

Used when a greater output force is needed Third-class lever:

Input force is between the output force and the fulcrum

Used to reduce the distance over which you apply the input force OR increase the speed of the end of the lever

1st class lever system

2nd class lever system

3rd class lever system

Wheel and Axle

Made of a wheel attached to a shaft or axle

Act as a rotating collection of levers Axle at the wheel’s center is like a

fulcrum Screwdrivers, steering wheels,

doorknobs, electric fans

Pulley A wheel with a grooved rim and a

rope or cable that rides in the groove As you pull the rope, the wheel moves

Fixed pulley: Pulley that is attached to something that

holds it steady Makes work easier by changing the

direction of the force You must apply enough force to

overcome the weight of the load and any friction

Distance you pull the rope is the same distance that the object is lifted

Movable pulley: One end of the rope is fixed but the

wheel can also move. Load is attached to the wheel Person pulling the rope provides the

output force that lifts the load Single movable pulley does not change

the direction of the force-it multiplies the force You would need only half the force which

means you need twice the distance

Block and tackle system Contains both fixed and movable pulleys Used to haul and lift very heavy objects

Inclined plane A simple machine made of a sloped

surface (ramps) Makes work easier by supporting the

weight of the object over the distance it travels

The less steep the incline, the less force you need Which means the distance will increase

Wedge

Simple machine with one thick edge and one thin edge

Used to cut, split, or pierce objects; also to hold objects together

Can be as simple as a doorstop, a chisel, or an ice scraper, blade of an ax

Angle of cutting edge determines the input force needed (thick wedge with large angle needs more force to cut)

Thin edges provide a smaller surface area for the force to act upon

Screw Inclined plane wrapped around a

cylinder or cone to form a spiral Used to raise and lower weights and

to fasten objects Distance between the threads of the

screw determine the amount of force needed: Threads close together = less force over

greater distance

Calculating MA of specific machines Inclined plane:

Ideal MA = length of incline ÷ height of incline

Wheel and Axle: Ideal MA = Radius of input ÷ Radius of

output

Lever: Ideal MA = distance from input force to

fulcrum distance from output

force to fulcrum