Forces & Motion
How do objects move and how do machines help?
Forces
What is a force?
As we consider the motion of objects we must ensure that students understand that a force is a push or a pull that causes an object to start moving, stop moving, or change direction. Students are familiar with forces in their everyday life such as pulling a wagon forward or hitting a baseball with a bat to change the direction of the ball. In a soccer game there are many examples of forces that act on the ball such as starting the ball in motion, stopping it, or changing the direction of the ball as a result of a force that is applied to the ball.
It is also important to know that more force is needed to make an object go faster. Think about what it is like to bump into a person who is running down the hall at a fast speed versus running into someone moving at a slow speed. The force is much greater for the two people running into each other at a faster speed than a slower speed. You can also think about the different types of waves in the ocean and their forces. The larger waves have a stronger force than a smaller wave.
What is gravity?
Gravity is the force that keeps us on earth; it pulls objects toward the center of the earth. Because of gravity objects fall to the ground and we don’t float into the air. It also keeps or moon in orbit and the sun’s gravity keeps the planets in orbit. Gravity is the force that causes a roller coaster to move along the track. As the roller coaster begins the chain pulls the cars to the top of the hill and then the roller coaster is pulled by gravity and momentum through the rest of the ride. On roller coasters the first hill is your tallest and steepest hill and then the roller coaster goes faster as it accelerates toward the ground.
Simple Machines
What is a simple machine?
Simple machines are “simple” because they have one or no moving parts. A machine is a device that makes work easier. In order for work to occur something must move in the direction the force is being applied. A simple machine reduces the effort (force) need to move the object by moving the object a greater distance.
How do simple machines help?
The simplest machine is the inclined plane or ramp. It works by allowing you to lift an object to a higher level by traveling a greater distance up the ramp. A wedge is an inclined plane turned on its side. A wedge pushes two objects apart; examples are a knife, shovel, nail or an ax. A screw is also made up of an inclined plane wrapped around a cylinder. The spacing of the threads determines how hard it will be to turn the screw. Another simple machine is a lever. The point where the lever moves is called the fulcrum. When you change the position of the fulcrum you are able to do work with less effort. Examples of levers include a seesaw, rake, and hockey stick. A wheel and axle allows you to roll an object by reducing the amount of friction on the object. The larger the wheel the easier it is to move an object. The final simple machine is a pulley. It is made of a wheel and axle with a rope or chain or attached. A pulley allows you to change the direction of the motion so that you can lift an object off of the ground.