In this topic you will learn about how motion can be described.

An object's position is its location compared with things around it. A distance and a direction are often used to describe where an object is located.

Motion is a change in an object's position compared to fixed objects around it. Motion in a moving car is described in relationship to trees, lamp posts, and other objects that sweep past view. Without other objects as references, it would be hard to describe motion.

Motion can be described by giving an object's position at different times. The length between two points is called the distance. A position is measured as the distance from a reference point.

When you describe how fast an object is moving, you are describing its speed. Speed is how fast an object's position changes with time at any given moment. A car traveling 60 miles per hour is traveling faster than a car with a speed of 45 miles per hour. The first car's change in position is greater over the same period of time, so its speed is greater.

Speed can change from moment to moment. If you are at rest, you have zero speed. If you begin by walking, and then move to a jog and then a fast run, your speed is increasing.

Instead of looking at your speed from moment to moment, you might be interested only in the total distance you moved and the time it took you to reach your destination. If you divide the total distance traveled by the amount of time, you get the average speed. Remember that traveling with an average speed of 5 km/h does not mean that you actually moved at 5 km/h for the whole trip. You may have traveled faster or slower at some points. On average, your speed was 5 km/h.

The description of both speed and direction of a moving object is its velocity. Two objects can have the same speed but different velocities if they are moving in different directions. Just as the speed of an object can change with time, so can an object's direction of motion.

When a moving object's velocity increases with time, the object is accelerating. When an object's velocity decreases, the object is decelerating. Acceleration also changes when the direction of movement changes.

The acceleration of an object falling freely near Earth's surface due to gravity is called a G.