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Formative assessment questions for teaching physics

A2L Item 211

  • Description: Identify the acceleration of a rotating mass after having to determine the velocity.
  • Goal: Problem solving with rotational kinematics
  • Source: CT151.2S02-39
  • Keywords: Acceleration, Mechanics, Problem Solving, Rotational Motion

The question for students:

Two masses, attached to the ends of a rigid massless rod, are rotating about pivot P as shown in the picture below. The mass two meters from P has speed 0.5m/s. What is the acceleration of the mass one meter from P?

Item211_fig1.gif

  1. 0.05 m/s2
  2. 0.0625 m/s2
  3. 0.125 m/s2
  4. 0.250 m/s2
  5. 0.5 m/s2
  6. 1 m/s2
  7. None of the above
  8. Cannot be determined

Commentary for teachers:

Answer

(2) Every one of the possible wrong responses indicates a common error that students make. After the problem has been discussed it is useful to have students find the acceleration of the mass at 2m and see that the accelerations are in the same ratio as the velocities. Drawing vector diagrams showing the Δv for each mass is useful for explaining this relationship.