## Section 1: Describing Motion

If you look around you, you will probably notice many things in motion: a classmate chewing gum, a ball rolling across the floor, or an airplane flying in the sky as you look out the window. In science, **motion** occurs when an object changes its position relative to a reference point. When describing the motion of an object, you should refer to its **distance** or how far it has moved. The SI unit of distance or length is the meter (m).

**Displacement** is the distance and direction of an object’s change in position from its starting point.

To measure the motion of an object, you must know how fast and in what direction it is moving. How fast something moves is called its **speed**. This is the distance an object travels per unit of time. The SI unit for speed is meters per second (m/s). To calculate the average speed, you divide the total distance by the total time.

There are three types of speed: instantaneous, average, and constant speed. **Instantaneous speed** is the speed at any given time. For example, the speed of a car is moving when you look down at the speedometer. The **average speed** is the total distance traveled divided by the time, which can be calculated when you take a road trip with your family. **Constant speed** is a speed that does not vary. The speed of a car driving in cruise control is an example of constant speed.

The motion of an object can be plotted on a distance-time graph. For example, to graph an object’s movement over time, you would plot the time on the x-axis and the distance on the y-axis.

**Review:**

**What is****displacement****?****Identify****the****equation****used****to****determine****the****average speed of an object.****Compare****instantaneous speed****to****constant****speed.**