# Chapter #3 ~ Work and Simple Machines

Work is done on an object when you apply a force to it and it moves in the direction of that force.

• When work is done on an object, its energy changes.

• If an object is pushed or pulled but doesn’t move, no work is done on it.

• In the metric system, work is measured in joules (J).

When you do work on an object, you change its energy.  This is an important idea known as the Work-Energy Theorem. (The work is always EQUAL to the energy)

• By pushing or pulling an object, you give it kinetic energy.

• By lifting an object, you give it potential energy.  You also give it kinetic energy while you’re lifting it.

Power is the rate at which work is done.  As we said in class, it’s a measure of how quickly (or slowly) you do work on an object.

• In the metric system, power is measured in watts (W).

• The more work is done on an object, the more power will be used to do that work.

• The more time it takes to do work, the less power will be used to do that work.

A machine is any device that makes work easier.

• Machines don’t decrease the amount of work done.  They make doing the work easier by changing how the work is done.

• With any machine, there are always two forces involved:  The input force (which you use on the machine), and the output force (which the machine uses on something else).

• With any machine, there are always two distances involved:  The input distance (which you use on the machine), and the output distance (which the machine uses on something else).

There are three (3) ways machines can make work easier to do:

1. They can increase the size of the input force, making your force stronger.

2. They can increase the distance over which your force acts, making your force last longer.

3. They can change the direction in which a force is applied.

With any machine, there is always a tradeoff.  The tradeoff in a machine is that it can increase the strength of your input force OR increase the distance over which it acts, but it CAN NOT do both at the same time.

Mechanical advantage is a measure of how much a machine increases your input force.  It also tells us which of the three ways a machine is making work easier:

• If the mechanical advantage is > 1, then the machine increases your input force, making it stronger.

• If the mechanical advantage is < 1, then the machine weakens your input force but makes it last a longer distance.

• If the mechanical advantage = 1, then the machine changes the direction of your input force.

The efficiency of a machine is a percentage of how much useful work the machine can actually do.  It’s always less than 100%.

No machine can ever be 100% efficient, because friction is always present.

Simple Machines do work by using only ONE MOVEMENT.

There are six (6) kinds of simple machinesLevers, wheel & axle, screws, wedges, pulleys, and inclined planes.

With any simple machine, there is always a TRADEOFF- either a machine can increase the amount of our input force, or make our input force last a longer distance, but it CAN NOT DO BOTH.

A lever is a simple machine that pivots, or rotates, around a fixed point.

• The pivot point of the lever is known as the fulcrum.

• The input force you use to operate the lever is known as the effort.

• The output force that the lever lifts/lowers is known as the load.

The mnemonic “Frogs Lay Eggs” can help you remember the three classes of levers:

• A 1st class lever has the Fulcrum in the middle.

• A 2nd class lever has the Load in the middle.

• A 3rd class lever has the Effort in the middle.

A good example of a 1st class lever is a seesaw.

A good example of a 2nd class lever is a wheelbarrow.

A good example of a 3rd class lever is a broom.

With a 1st class lever, the mechanical advantage could be greater than, equal to, or less than 1.  This means that a 1st class lever could increase our input force, change the direction of our input force, or make our input force last a longer distance.

With a 2nd class lever, the mechanical advantage is always greater than 1.  So a 2nd class lever will always increase the amount of our input force.

With a 3rd class lever, the mechanical advantage is always less than 1.  So a 3rd class lever will always increase the distance our input force acts, making it last longer.

A wheel and axle is an axle attached to the center of a wheel.  Both rotate together.  Wheel/Axles always increase the amount of our input force.

Ex: Door Knob

An inclined plane is a flat, sloped surface that makes lifting loads easier.  It also increases our input force but makes us apply the force a longer distance (the tradeoff).

Ex: Ramp

A wedge helps us by cutting or separating two objects.  It’s a sloped surface that moves.

Ex: Teeth

A screw is an inclined plane that is wrapped around a cylinder.

Ex: Threads on a light bulb

A pulley is a grooved wheel with a rope/chain/cable wrapped around it.  It helps us lift loads and change the direction of force applied.

Ex:

A compound machine is two or more simple machines that are put together to help us do work.  Compound machines are less efficient than simple machines at doing work.

Ex: Scissor