Goal: Reasoning
Source: UMPERG
Consider the arrangement of pulleys and masses shown below. The masses
of the pulleys are small. Ignore friction.
This system is initially at rest. What will happen if the ring R is
moved to the right?
- Nothing will happen.
- Mass M will go up and m will go down.
- Mass M will go down and m will go up.
- Both masses will go down.
- Both masses will go up.
- Cannot be determined because what happens depends on the masses.
Commentary:
Answer
(3) If the ring is moved to the right the upward force on M is
decreased,so M will accelerate downward. Initially the tension is Mg/2.
When the strings are at an angle the tension is insufficient to support
M.
Background
Answers are not as important as approach. What did students do to
understand the physical situation? Did they draw pictures? Did they draw
a free-body diagram?
Questions to Reveal Student Reasoning
Does the tension stay the same? …increase? …decrease? After moving
the ring, would I need a smaller or larger mass M to keep the system
from moving?
Suggestions
After students make predictions and discuss their reasoning have
students vote a second time. Then demonstrate what happens.