An object weighs 22 kg in water and 30kg in air. what is the upthrust exerted by the liquid on the object. [g = 10 ms2]

Analyze the question: The object has an apparent weight (mass reading) of 22 kg in water and 30 kg in air. We need the upthrust from the liquid.

Concept: Upthrust = Weight in air − Weight in liquid (apparent weight).

This is Archimedes' principle: the loss in weight when an object is immersed equals the upthrust.

Solution:

• Weight in air = 30 × 10 = 300 N
• Weight in water = 22 × 10 = 220 N
• Upthrust = 300 − 220 = 80 N

Equivalently, the loss in mass is 30 − 22 = 8 kg, and 8 × 10 = 80 N.

Checking the options:

• A (50 N): doesn't match any sensible calculation.
• B (520 N): this is 300 + 220, which wrongly adds the weights.
• C (220 N): this is just the weight in water, not the upthrust.
• D (80 N): matches our calculation.

Tip: Upthrust is always the difference between the real weight (in air) and the apparent weight (in liquid). Never add them. Also remember to convert kg to N by multiplying by g before giving your final answer. A quick shortcut is to take the mass difference and multiply by 10.