Forces and Levers

Science background provides teachers with more in-depth information on the phenomena students explore in this unit about energy transfers. Below is an excerpt of this section from the lesson on forces and levers.

Simple machines are all around because they help us do work. Work is any change in position, speed, or state of matter due to force. Remember that a force is a push or pull that acts on an object, changing its speed, direction, or shape, and that force is always necessary to transfer energy into or out of a system.

We can use an inclined plane to explore this relationship between force, energy, and simple machines. An inclined plane is a slanting surface that connects a lower level to a higher level. Inclined planes make it easier to raise or lower a load than just lifting or lowering the load straight up or down. They do this by increasing the distance the load travels.

Objects at the top of the ramp have a form of potential energy, which is stored energy, called gravitational energy. Gravitational potential energy is stored energy related to an object’s height above the ground. To get an object from the ground to the top of the ramp requires kinetic energy, which is the energy of motion. Just lifting the object vertically up requires more force than moving it up the ramp. The ramp requires less force because it increases the distance the object has to travel. It’s important to point out here that the inclined plane doesn’t increase or decrease the amount of energy used because energy cannot be created or destroyed. Instead it redirects the force, extending it over a longer distance. This decreases the amount of force needed to move the object, but you have to move a greater distance. The steeper the inclined plane, the more force will be needed to move the object because it is moving over a shorter distance.

The wheel and axle is another simple machine that makes it easier to do work. An axle is a rod that attaches to the center of a wheel. As the wheel turns, it turns the axle. If the axle is attached to an object like a suitcase or a cart, the wheel and axle can make it easier to move the object.

The wheel and axle reduces the amount of friction that occurs between the ground and the object being pulled. Remember that friction is a force that occurs whenever two objects rub against one another, and it slows motion by transferring the kinetic energy away from moving objects, changing it into heat. One of the ways that wheels reduce friction is by reducing the amount of surface area that comes into contact with the ground.

Imagine trying to push the shopping cart without wheels. The entire bottom of the cart would scrape against the ground. All of the friction would transfer much of the kinetic energy out of the cart, meaning you would have to push the cart with a lot more force to move it the same distance as you would have to push the same cart with wheels.

The shopping cart and the moving truck’s ramp are both made up of different parts, and these parts are often held together with a third kind of simple machine: a screw. A screw is an inclined plane wrapped around a pole that holds things together or lifts materials.