A2L Item 038
- Description: Compare the time for two different moving blocks to stop due to kinetic friction.
- Goal: Reason qualitatively. Consider alternate solution paths.
- Source: UMPERG
- Keywords: Dynamics, Friction, Mechanics, Problem Solving, Reasoning
The question for students:
Two blocks, M2 > M1, having the same speed move from a frictionless surface onto a surface having friction coefficient μk as shown below.
Which block stops in the shorter time?
- M1
- M2
- Both blocks stop in the same time.
Commentary for teachers:
Answer
(3); both blocks have the same acceleration and the same initial velocity, so they must stop in the same length of time.
Background
This problem can be reasoned through without the use of equations. However, the problem can be solved easy enough algebraically. The item provides an opportunity for students to reflect on different approaches for solving problems.
Questions to Reveal Student Reasoning
Which block experiences the largest net force?
Which block experiences the largest acceleration?
What determines which block stops first?
Suggestions
Ask students to consider the following questions, and to determine if their answer to the problem is inconsistent with their answers to these questions:
If two blocks enter the rough region side by side and have the same mass, which one will stop first?
If the blocks are connected by a rope, will the time it takes for the blocks to stop change? Would the time it takes to stop change if the blocks were glued together?