**Goal:** Hone the concept of acceleration.

**Source:** UMPERG

HOW MANY of the identified objects are NOT accelerating?

- A race
**car**going around a circular track at 150 MPH - A
**sky diver**falling at a constant speed - A heavy
**box**sliding across the floor, after being released - A bowling
**ball**colliding with a pin - A vibrating guitar
**string** - A
**baseball**flying through the air - A
**child**swinging on a swing

Enter the number of objects, or 8 for “none” and 9 for “cannot be determined”.

- One
- Two
- Three
- Four
- Five
- Six
- Seven
- None
- Cannot be determined

## Commentary:

## Answer

Only the skydiver, who has reached terminal velocity, has zero acceleration. In each of the other situations, the acceleration is nonzero because either the speed or the direction of motion is changing. Answer (2) is the best choice.

## Background

Context for Use: Give after introducing the concept of acceleration. Intended focus: What factors/criteria do students use to determine whether an object is accelerating? The goal is to focus students on changes in the speed/direction of an object’s motion.

## Questions to Reveal Student Reasoning

How do you know whether an object is accelerating? What are some examples of objects undergoing acceleration? If an object is falling is it necessarily accelerating?

In some cases did you need to make assumptions before deciding whether the object was accelerating? (Ask students to provide examples.)

## Suggestions

Have students write out how they determine whether an object is accelerating. After discussing the different methods, have students vote on which one they think is best.

Play a “challenge game” with the class. Two teams of students think of situations in which an object undergoes some motion. The teams then take turns challenging each other to determine whether or not the objects are accelerating.