Aim for FivePhysics 1

Qualitative/quantitative translation (QQT)

Walking along a supported plank

  • Unit 5
  • 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

A uniform plank of length 4.0 m and mass 20 kg rests horizontally on two supports. Support L is at the left end of the plank, and support R is 3.0 m from the left end (1.0 m from the right end). A person of mass 60 kg starts at the left end and walks slowly toward the right end. Use g = 9.8 m/s² (answers that use g = 10 m/s² are also accepted).

Suggested time: 18 minutes

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Part (a)

3 points

Describe how the upward force exerted by support L on the plank changes as the person walks from the left end to support R, and what happens as the person continues past support R. Justify your answer by considering torques about support R. Do not use equations in this part; reason in words.

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Part (b)

3 points

Derive an expression for the magnitude of the force F_L exerted by support L as a function of the person's distance x from the left end of the plank. Then calculate F_L when the person is at the left end.

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Part (c)

2 points

(i) Explain how your expression agrees with your reasoning in part (a). (ii) Use your expression to determine whether the person can reach the right end of the plank without it tipping.

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