A piece of stone weights 180 N in air and 150 N in a beaker containing 160 N of methylated spirit. What is the up thrust exerted on the stone?

Analyze the question: A stone weighs 180 N in air. When it is immersed in methylated spirit, its apparent weight is 150 N. We need the upthrust on the stone.

Concept: When an object is immersed in a fluid, the fluid pushes up on it. This force is called upthrust.

Upthrust = Weight in air − Apparent weight in the fluid

(This is Archimedes' principle: the upthrust equals the weight of fluid displaced.)

Solution:

• Weight in air = 180 N
• Weight in methylated spirit = 150 N

Upthrust = 180 N − 150 N = 30 N

Note on the distractor: The 160 N is the weight of the methylated spirit in the beaker. It is not needed for this calculation, so do not be tempted to use it. It is included to test whether you know which values matter.

Answer: C. 30 N

Tip: In upthrust questions, find the "loss in weight". Upthrust is always the difference between the weight in air and the apparent weight in the liquid. Ignore any extra numbers, such as the weight of the liquid or the beaker, unless the question asks for them.