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A2L Item 003

Meta-Information
Description: 
Identify situations in which a body is not accelerating.
Goal: 
Hone the concept of acceleration.
Level: 
Middle School
Level: 
High School
Keywords: 
Acceleration
Keywords: 
Mechanics
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”.

  1. One
  2. Two
  3. Three
  4. Four
  5. Five
  6. Six
  7. Seven
  8. None
  9. 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.