Aim for FivePhysics 1

Qualitative/quantitative translation (QQT)

Pushing two blocks in contact

  • Unit 2
  • 8 points
  • About 18 minutes

You can use a calculator on this question, just like on exam day.

A shorter multipart problem that connects reasoning in words with an equation. You make and justify a claim without equations, derive an equation from a physics principle, and then explain whether the two agree or use them to predict what happens in a changed situation. On the exam: Question 4 of 4. Section II has 4 questions in 95 minutes (50% of the exam score), one of each type in this order; calculator allowed. The CED suggests 15–20 minutes.

The question

Block 1 has mass m and block 2 has mass 3m. The blocks sit side by side, touching, on a horizontal frictionless surface. In Case 1, a constant horizontal force of magnitude F pushes on block 1, so block 1 pushes on block 2. In Case 2, the same force F instead pushes on the other side, on block 2, so block 2 pushes on block 1. In both cases the blocks move together.

Suggested time: 18 minutes

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Part (a)

3 points

(i) In Case 1, is the magnitude of the force that block 1 exerts on block 2 greater than, less than, or equal to F? (ii) Is the magnitude of the contact force between the blocks greater in Case 1, greater in Case 2, or the same in both cases? Justify your answers. Do not use equations in this part; reason in words.

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Part (b)

3 points

Derive expressions for the magnitude of the contact force between the blocks in Case 1 and in Case 2, in terms of F.

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Part (c)

2 points

Explain how your expressions from part (b) agree with your reasoning in part (a).

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