Lesson: Chapter 15 — Fluid Pressure and Its Applications

15.5. Floating and upthrust 5 of 6 Reset answers

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Question 5

The weight of an object in air is 20 N. When it is fully immersed in water its apparent weight is 5 N. What is the upthrust the water exerts on the object?

A 20 N B 15 N C 25 N D 5 N

The right answer is the one marked above.

15 N. upthrust = weight in air − apparent weight = 20 N − 5 N = 15 N.

25 N comes from adding instead of subtracting. Note too that by Archimedes' principle the weight of the water this object displaces is also 15 N.

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