A2L Item 063
- Description: Indicate the path of a rock dropped by someone on a merry-go-round.
- Goal: Understanding the first law.
- Source: UMPERG-ctqpe120
- Keywords: Mechanics, Rotational Motion, Vectors, Velocity
The question for students:
A
child is standing at the rim of a rotating disk holding a rock. The
disk rotates without friction. If the rock is dropped at the instant
shown, which of the indicated paths most nearly represents the path of
the rock as seen from above the disk?
- path (1)
- path (2)
- path (3)
- path (4)
- path (5)
- cannot be determined
Commentary for teachers:
Answer
(2) is the correct path if the rock is simply dropped. Some students selecting answer (3) may be viewing the rock from the child’s perspective. Some students indicating choice (5) may interpret this path as ‘straight down’.
Background
This question is similar to others which seek to reveal student perceptions about path persistence. It is a slightly different context from the purely horizontal case of a ball rolling on a horizontal surface around an semicircular section of hoop.
Questions to Reveal Student Reasoning
What path would the child see?
What is the velocity of the rock just before it is dropped? just after?
What would the path of the rock have been if the child continued to hold it?
Suggestions
There are a variety of demonstrations that can be done as followup to this question. It is important that students perceive the similarity between the demonstration context and the problem situation.