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Formative assessment questions for teaching physics

A2L Item 042

  • Description: Determine the motion of a pulley sustem at rest when the support position is moved.
  • Goal: Reasoning
  • Source: UMPERG
  • Keywords: Dynamics, Mechanics, Reasoning, Tension

The question for students:

Consider the arrangement of pulleys and masses shown below. The masses of the pulleys are small. Ignore friction.

Item042_fig1.gif

This system is initially at rest. What will happen if the ring R is moved to the right?

  1. Nothing will happen.
  2. Mass M will go up and m will go down.
  3. Mass M will go down and m will go up.
  4. Both masses will go down.
  5. Both masses will go up.
  6. Cannot be determined because what happens depends on the masses.

Commentary for teachers:

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.