A2 Daily A Level Physics question
A 30 N metal block hangs from a Newton meter. When fully submerged in fresh water (density 1000 kg m^-3) the meter reads 24 N. The block is then moved, still fully submerged, into a salt solution of density 1200 kg m^-3. What will the meter read?
Answer
The correct answer is B.
Correct: B — 22.8 N. The upthrust scales with fluid density; in fresh water the upthrust is 30 − 24 = 6.0 N, so in the 1200 kg m^-3 liquid it is 1.2 × 6.0 = 7.2 N and the meter reads 30 − 7.2 = 22.8 N. A assumes the upthrust is unchanged by switching to a denser fluid, so keeps the same apparent weight. B is correct because the denser fluid increases upthrust by 20%, reducing the reading by 1.2 N. C incorrectly reduces the existing meter reading by 20% rather than increasing the upthrust by 20%. D has the wrong sign, as a denser fluid gives greater upthrust and thus a smaller, not larger, meter reading.