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A2L Item 013

Meta-Information
Description: 
Use a strobe diagram to tell when something is accelerating.
Goal: 
Perceiving acceleration from changes in position.
Level: 
High School
Keywords: 
Acceleration
Keywords: 
Graphing
Keywords: 
Mechanics
Source: 
UMPERG

Below is shown a strobe diagram indicating the position of four objects at successive (equal) time intervals. The objects move from left to right.

During the intervals shown, which of the objects are accelerating?

  1. Object A only
  2. Object B only
  3. Object C only
  4. Object D only
  5. Objects A and B
  6. Objects B and C
  7. Objects A and C
  8. None of the above
  9. Cannot be determined
Commentary

Answer

(7) (A) and (C) are clearly accelerating since the displacement is different for different time intervals (implying different average velocities). For (B) and (D) the average velocity is the same for each time interval.

If there is something quirky about the motions of (B) and (D), it is possible that these objects are accelerating even though their average velocity is always the same for the time intervals observed. Therefore students could be justified in selecting (9). Students should realize that (A) and (C) are accelerating.

Background

It is important for students to develop multiple ways of interpreting concepts. This ensures that students are not just following rote procedures to answer questions. Once an idea is understood students should be able to use their understanding in a variety of contexts and with a variety of representations.

Questions to Reveal Student Thinking

Which objects have a non-zero velocity? How do you know? How can you determine from an object’s position at several times whether it is accelerating? What features of a strobe diagram indicate that an object has a non-zero velocity? a non-zero acceleration?

What are some physical situations that correspond to the different motions in the strobe diagram.

Suggestions

Draw position vs. time graphs and velocity vs. time graphs for the motion of objects that are difficult for students to analyze.