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Formative assessment questions for teaching physics

A2L Item 093

  • Description: Find the initial potential energy stored in a spring between two known masses given the final velocity of one of the masses.
  • Goal: Problem solving
  • Source: UMPERG-ctqpe88
  • Keywords: Springs, Mechanics, Energy

The question for students:

Item093_fig1.gif

Two blocks, M=2m, sit on a horizontal frictionless surface with a compressed massless spring between them. After the spring is released M has velocity v. The total energy initially stored in the spring was:

  1. mv2
  2. 2mv2
  3. 3mv2
  4. 4mv2
  5. 5mv2
  6. None of the above.
  7. Cannot be determined

Commentary for teachers:

Answer

(3) The big mass has kinetic energy mv2 and the small mass has energy 2mv2. Some students may answer (6) because they have confused M and m. It is important to determine the reasons that any student might select (7). They might be unwilling to assume that the system is initially at rest. Students taking this perspective should not be disconfirmed but congratulated for making a critical interpretation of the wording.