A body of weight 20N displaces 12N of the liquid in which it is immersed. What is the upthrust of the liquid on the body?

Analyze the question: A body weighing 20 N is immersed in a liquid and displaces 12 N of that liquid. We need the upthrust (buoyant force) on the body.

Concept: Archimedes' Principle When a body is fully or partially immersed in a fluid, the upthrust on it equals the weight of the fluid it displaces.

Upthrust = weight of liquid displaced

Solution:

• Weight of body = 20 N (this is a distractor; it does not determine the upthrust)
• Weight of liquid displaced = 12 N
• Upthrust = weight of liquid displaced = 12 N

Check: The apparent weight of the body in the liquid would be 20 − 12 = 8 N. This is option E, which is the trap answer: it is the apparent weight, not the upthrust. The question asks for upthrust, so the answer is 12 N.

Tip: In upthrust questions, always look for the words "displaces" or "weight of liquid displaced". That value is the upthrust. Subtracting the upthrust from the real weight gives the apparent weight (weight in liquid), so check which of the two the question asks for.