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A2 Daily A Level Physics question

2026-02-20 OCR A Archimedes principle OCR-A 3.1.3 Forces in action: density, pressure in fluids, upthrust (Archimedes’ principle) OCR-A 2.1.1 Physical quantities and units: ratios, proportional reasoning, percentage change

An irregular metal object is hung from a Newton meter. In air, the reading is 50 N. When it is fully submerged in fresh water, the reading is 30 N. The object is then fully submerged in a sugar solution of density 1.25 times that of fresh water. Which statement must be true about the new meter reading, and why?

  1. A About 35 N, because the extra upthrust is 25% of 30 N.
  2. B About 55 N, because the denser fluid exerts a greater force on the object, increasing the reading.
  3. C About 25 N, because the upthrust scales with fluid density, so a 25% increase makes the water upthrust (50 − 30 = 20 N) become 25 N, reducing the reading to 50 − 25. (correct)
  4. D Approximately 0 N, because a denser liquid makes the object effectively weightless.

Answer

The correct answer is C.

Correct: C — About 25 N, because the upthrust scales with fluid density, so a 25% increase makes the water upthrust (50 − 30 = 20 N) become 25 N, reducing the reading to 50 − 25. A misapplies the 25% to the water reading (30 N) instead of to the upthrust (20 N), so it underestimates the extra upthrust. B has the wrong sign: greater upthrust reduces the meter reading, it does not increase it. C correctly finds the upthrust in water (20 N), scales it by 1.25 to 25 N, and subtracts from the true weight (50 N). D confuses a denser liquid with matching the object’s density; the reading would only approach 0 N if the fluid density equalled the object’s density.

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