1. Describing Motion
Chapter 9.1

2. Objectives 9.1.1. Determine when an object is in motion.
Distinguish between distance and displacement.

3. Motion
An object is in motion if its distance from another object is changing.
Because the distance from your chair right now is not changing, you can conclude that you are not in motion.
To decide if you are moving you use your chair as a reference point. A reference point is a place or object used for comparison to determine if something is in motion.

4. Motion
An object is in motion if it changes position relative to that reference point.
Objects that are stationary - such as a tree, a flagpole, or a building - make good reference points.
Once you have selected your reference point, you can indicate a change in position by using a plus (+) or minus (-) sign.

5. Motion
Example:

6. Relative Motion Relative to your chair, you are not moving.
Relative to the sun, you ARE moving!
You and the Earth, right now, are moving 30 km (19 miles) per second!
But since we are moving WITH Earth, we do not seem to be moving.

7. Distance and Displacement
Distance: The length of a path between two points.
Displacement: length and direction that an object has moved from a starting point.

8. Distance and Displacement
Vector: a quantity that consists of both a magnitude and a direction
Displacement is a vector, but distance is not.
Other vectors: velocity, acceleration, and force, which we will learn about later.
Vectors are shown using an arrow. The direction of the arrow indicates the direction of the vector.