Forces

A lift is moving upward at constant speed. Which statement about the lift is correct?

1. A

   The upward force is greater than its weight.

2. B

   Its weight is greater than the upward force.

3. C

   The upward force equals its weight.

4. D

   No forces act because its speed is constant.

1. a

   Select and explain Select the correct statement and connect it to the lift's motion.

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   Correct choiceC

   1. 1

      Selects option C: the upward force on the lift equals its weight.

   2. 2

      States that constant velocity means zero acceleration.

   3. 3

      Uses zero resultant force to conclude that the upward force balances the downward weight.

A cart is pulled along a horizontal track. The same resistive force acts in every run.

1. a

   Determine Use the change across the data to determine the cart's mass. Show how the units support your answer.

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   1. 1

      Finds slope ΔF ÷ Δa = (5.0 − 1.0) ÷ (1.80 − 0.20).

   2. 2

      Obtains mass = 2.5 kg from F = ma.

   3. 3

      Recognises N per m s⁻² is equivalent to kg.

2. b

   Calculate Use one data row to calculate the resistive force.

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   1. 1

      Uses applied force − resistive force = ma.

   2. 2

      For example, at 3.0 N calculates ma = 2.5 × 1.00 = 2.5 N.

   3. 3

      Obtains resistive force = 0.50 N, opposing the motion.

3. c

   Explain Explain why a graph of applied force against acceleration would not pass through the origin.

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   1. 1

      The applied force must first balance the approximately constant resistive force.

   2. 2

      Therefore zero acceleration occurs at a non-zero applied force of about 0.50 N.

A school is choosing wheelchair tyres. Tyre P needs an average push of 18 N on a smooth floor and 31 N on carpet; it stops in 1.8 m on a wet ramp. Tyre Q needs 23 N on the floor and 38 N on carpet; it stops in 1.2 m on the same wet ramp. Both meet the load rating.

1. a

   Interpret Explain what the push-force and stopping-distance evidence suggests about the two tyres.

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   1. 1

      Explains that P has lower rolling resistance because it requires less force on both surfaces.

   2. 2

      Explains that Q provides greater wet-ramp grip because its stopping distance is shorter.

   3. 3

      Recognises that the evidence shows a trade-off rather than one tyre being best in every respect.

2. b

   Recommend Recommend a tyre for a route that includes carpet and a short wet ramp. Justify the choice using physics and user needs.

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   1. 1

      Makes a clear recommendation that is consistent with a stated priority.

   2. 2

      Uses at least two relevant numerical comparisons from the evidence.

   3. 3

      Balances effort or fatigue against control and stopping safety for the user.

3. c

   Propose Propose one further test needed before a final purchasing decision and explain why it matters.

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   1. 1

      Proposes a relevant controlled test such as durability, side-slope grip, different user loads, or repeated wet braking.

   2. 2

      Connects the test to a limitation in the current evidence or a real user requirement.