### Concept Items

## 2.1 Relative Motion, Distance, and Displacement

In which example would you be correct in describing an object in motion while your friend would also be correct in describing that same object as being at rest?

- You are driving a car toward the east and your friend drives past you in the opposite direction with the same speed. In your frame of reference, you will be in motion. In your friend’s frame of reference, you will be at rest.
- You are driving a car toward the east and your friend is standing at the bus stop. In your frame of reference, you will be in motion. In your friend’s frame of reference, you will be at rest.
- You are driving a car toward the east and your friend is standing at the bus stop. In your frame of reference, your friend will be moving toward the west. In your friend’s frame of reference, he will be at rest.
- You are driving a car toward the east and your friend is standing at the bus stop. In your frame of reference, your friend will be moving toward the east. In your friend’s frame of reference, he will be at rest.

## 2.3 Position vs. Time Graphs

Use the graph to describe what the runner’s motion looks like.

How are average velocity for only the first four seconds and instantaneous velocity related? What is the runner's net displacement over the time shown?

- The net displacement is 12 m and the average velocity is equal to the instantaneous velocity.
- The net displacement is 12 m and the average velocity is two times the instantaneous velocity.
- The net displacement is 10 + 12 = 22 m and the average velocity is equal to the instantaneous velocity.
- The net displacement is 10 + 12 = 22 m and the average velocity is two times the instantaneous velocity.

## 2.4 Velocity vs. Time Graphs

A graph of velocity vs. time of a ship coming into a harbor is shown.

Describe the acceleration of the ship based on the graph.

- The ship is moving in the forward direction at a steady rate. Then it accelerates in the forward direction and then decelerates.
- The ship is moving in the forward direction at a steady rate. Then it turns around and starts decelerating, while traveling in the reverse direction. It then accelerates, but slowly.
- The ship is moving in the forward direction at a steady rate. Then it decelerates in the forward direction, and then continues to slow down in the forward direction, but with more deceleration.
- The ship is moving in the forward direction at a steady rate. Then it decelerates in the forward direction, and then continues to slow down in the forward direction, but with less deceleration.