Work, Power, and Simple Machines for Middle School Science

Work, power, and simple machines are all related to force. If you haven't checked out our force page yet, you may want to read it before focusing on work, power, and simple machines.

You can learn more about work, power, and simple machines by checking out the Google Slides below. If you would like a copy of the presentation, you can get it at Teachers Pay Teachers.

Work, Power, and Simple Machines

A force is a push or a pull. There are many types of force, including friction, gravitational, electric, buoyant, spring, tensional, magnetic, and applied. 

When a force moves an object across a distance, work is done. For example, you are doing work when you push a box across a room.

You can exert a force without moving anything. For example, you can pull on a rope during a game of tug-o-war. If nothing moves, you are not doing work, but if the other team falls forward, then you have done work.

The Basics

Work is a force moving an object. Unlike force, work is a scalar quantity, meaning it can be represented by only a number instead of a number and a direction. However, work can be either a positive or negative number. If work is positive, the object is moving in the same direction as the force. If work is negative, the object is moving in the opposite direction of the force.

Work is measured in Joules (J). Joules are named after James Prescott Joule, an English physicist from the 1800s who worked with energy. Work and energy have the same unit because work is a change in energy.

Work

Work is only done in the direction the object is moving. Force may be applied at an angle, but on the part of the force is the direction of motion counts towards work.

Older students can use trigonometry to solve problems when only part of the force counts towards work.

Power

Power is a measure of the rate of work. In other words, power is how fast work is done. 

We solve for power by dividing work by time in seconds. The units for power are watts.

Watts are named after James Watt, a Scottish engineer who invented the first steam engine in 1776.

Two machines can do the same amount of work, but the machine that does the work faster has more power.

Simple Machines

Simple machines make work easier. Simple machines do not change the amount of work it takes to move an object, but they decrease the amount of force necessary to do the work by increasing the distance the object travels.

Remember, work is a product of force and distance, so increasing the distance decreases the force.

Simple machines do work in a single motion. Multiple simple machines can be combined into a compound machine to perform more complex tasks. Scientists have fun putting many simple machines together to complete a task. These compound machines are called Rube Goldberg Machines after the American cartoonist who first drew the needlessly complicated machines.

Inclined Plane

The inclined plane makes lifting objects easier by increasing the distance the object travels. Instead of lifting a wheelbarrow straight up, a person can push the wheelbarrow up an inclined plane to raise it the same distance.

The wheelbarrow ends up in the same position, so the same amount of work is done, but the distance has increased, and the force has decreased. The inclined plane makes it easier for people to lift heavy objects.

Wedge

The wedge is a portable inclined plane. A wedge can be used to split something, like an ax splitting a log. A wedge can also be used to stop an object, like a doorstop. Finally, a wedge can be used to lift an object, such as a plow lifting soil off the ground.

Like other simple machines, a wedge decreases the force needed to do work by increasing the distance over which the work happens. 

A wedge can also change the direction of force, making it easier to do the work. When an ax splits a log, you are applying a downward force, but the log's sides are moving outward. Pushing down on the wood is easier than pulling it apart.

Screw

A screw is an inclined plane wrapped around a pole. Turning the screw can move objects up the inclined plane. Screws are also used to hold objects together.

Examples of screws include jar lids, bolts, drill bits, light bulbs, water faucets, and the ends of hoses.

Like other simple machines, screws decrease the amount of force needed to do work by increasing distance. A screw takes less force to push into a wall than a nail because the edges of the screw are moving in a circle and traveling a longer distance than the nail.

Screws are also helpful because the inclined plane's edges hold the screw in place and can hold things together.

Lever

A lever is a rigid bar and a fulcrum. The fulcrum is also called the pivot point. You can apply force to one side of the lever to move an object on the other side of the lever. 

There are three parts of a lever, the effort, the load, and the fulcrum. The effort is the place you apply force, the load is the object you are moving, and the fulcrum is the pivot point between the effort and the load. Levers are classified based on the location of the three parts. 

First class levers have the fulcrum between the effort and the load. A teeter-totter is an example of a first class lever.

Second class levers have the load between the effort and the fulcrum. A wheelbarrow is a second class lever. The wheels of the wheelbarrow are the fulcrum of the lever.

Pulley

The pulley is a rope looped over a wheel to make lifting an object easier. Pulleys make lifting easier in two ways. First, they change the direction of the force. Instead of pulling up on an object, you can pull down on the rope to lift it. Second, by adding wheels to the pulley, you can increase the rope's distance and decrease the force needed to lift the object. 

Lifting an object with a pulley with one wheel will change the direction of the force, but it won't change the amount of force needed to lift the object. Using a pulley with two wheels both changes the direction of the force and cuts the amount of force needed to lift the object in half. Instead of taking 500 N to lift an object, it will take 250 N. As you add wheels to the pulley, the force continues to decrease.

Wheel and Axle

The wheel and axle are a large wheel attached to a smaller rod called an axle. The larger wheel takes less force to turn because the wheel travels around a longer distance than the axle. 

A screwdriver is an example of a wheel and axle. The screwdriver handle is larger than the end of the screwdriver, so it is easier to turn.

The Work-Energy Theorem

Work is a force moving an object over a distance, but it is also a change in kinetic energy. 

When an object is moving, it has kinetic energy. Work that speeds up the object increases its kinetic energy. Work that slows an object down decreases its kinetic energy.

The work-energy theorem says that the amount of work done on an object equals the object's change in kinetic energy. Work and energy are so closely related that they have the same units, Joules. 

In this picture, a child is doing work on the car. The car started at a velocity. As the child pushed it, the car accelerated, and its kinetic energy increased. The increase in kinetic energy was equal to the work done by the child (if we ignore friction).

Force

Middle School Science