A2L Item 016
- Description: Use strobe diagrams to find average accelerations.
- Goal: Differentiate between instantaneous and average acceleration.
- Source: UMPERG
- Keywords: Mechanics, Graphing, Acceleration
The question for students:
Below is shown a strobe diagram indicating the position of four objects at successive time intervals. The objects move from left to right.
During the intervals shown, which object would you estimate has the largest average acceleration?
- Object A
- Object B
- Object C
- Object D
- Objects A, B, & D
- Cannot estimate for (A) because its acceleration is changing
- Cannot estimate average acceleration from a strobe diagram
- None of the above
- Cannot be determined
Commentary for teachers:
Answer
(3) Assuming that the question is referring to magnitude, the largest average acceleration is experienced by object (C). The other three objects appear to start and end with approximately the same velocity. For object (C) the velocity decreases in magnitude as the object moves to the right. Students who answer (5) because they realize that the average acceleration of C is negative and think zero is larger should not be considered wrong.
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.
The concept of average acceleration depends only on the initial and final velocity over some specified time interval. Some students will make their judgments on the basis of changes in the velocity at different points in the motion.
Questions to Reveal Student Reasoning
How is the average acceleration determined? What is the difference between average acceleration and instantaneous acceleration? Where is the instantaneous acceleration greatest?
Suggestions
Draw velocity vs. time graphs for the objects (A) and (B). Analyze the average acceleration (instantaneous acceleration) for different time intervals (times).