Motion refers to the way in which an object changes its position or location. If an object is not changing its position, it is not moving. It has no motion. An object that is changing its position is in motion. For instance, when someone walks from a hallway into a room, he/she is in motion. Objects in motion can change in different ways. They can slow down, speed up, or change directions complete.

A force is necessary to change the motion of an object. When you walk down the hallway, your feet push away from the floor as you walk, and this helps move you forward. (You would say that the force acts, or is exerted, upon the object.) A force is a push or a pull. An object will not change its motion unless a force acts upon it. When you pull or push a chair, you exert a force on that chair. The force causes the chair to move. Every force has a direction and a magnitude, or strength. If two forces act on an object in opposite directions, the forces are called opposing forces.

When opposing forces have the same magnitude, they are balanced. If all the forces on an object are balanced, there is no net force on the object.

When opposing forces have different magnitudes, the forces are unbalanced. If any forces on an object are unbalanced, there is a net force on the object.

Speed is one way you can describe the motion of an object. Speed is how fast an object moves over a period of time. To compare the speed of one object to the speed of a second object, measure the distance both objects travel in a given period of time. The object that travels the greater distance is moving at a greater speed. If one runner travels 10 km in 1 hour and a second runner travels 15 km in 1 hour, the second runner is moving at a greater speed. Another way to compare speed is to see which object moves a given distance in the smallest amount of time. Imagine two cars racing a distance of 1,000 meters. The car that finishes in less time is faster. It has the greater speed.

Speed is defined as distance per unit of time. We often see speed in units of miles per hour. For instance, a car that travels 50 miles per hour is faster than a car that travels 30 miles per hour.