Aim for FivePhysics 1

Qualitative/quantitative translation (QQT)

Motor pulling a crate up two ramps

  • Unit 3
  • 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 motor pulls a crate of mass m up a frictionless ramp at a constant speed v, from the ground to a loading platform at height H. The motor's cable is parallel to the ramp. Two ramps are available: ramp A is steeper than ramp B, but both reach the same platform. The motor pulls at the same constant speed v on either ramp.

Suggested time: 18 minutes

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Part (a)

3 points

Compare (i) the total work the motor does on the crate and (ii) the power the motor delivers, for ramp A and ramp B. For each, state whether it is greater for ramp A, greater for ramp B, or the same, and justify your answer. Do not use equations in this part; reason in words.

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Part (b)

3 points

For a ramp at angle θ above the horizontal, derive expressions for (i) the power P the motor delivers and (ii) the total work W the motor does, in terms of m, g, v, H, and θ, as appropriate.

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Part (c)

2 points

(i) Explain how your expressions from part (b) agree with your reasoning in part (a). (ii) Calculate the power if m = 50 kg, v = 0.40 m/s, and θ = 30°. Use g = 9.8 m/s² (answers that use g = 10 m/s² are also accepted).

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