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## Homework Statement

Block A in the figure hangs from a spring balance D and is submerged in a liquid C contained in a beaker B. The weight of beaker is 1 kg, and the weight of the liquid is 1.5kg. The balance D reads 2.5 kg and balance E reads 7.5 kg. The volume of the block is 0.003 m^3.

What is the density of the liquid?

Ans: 1667.7kg/m^3

## Homework Equations

## The Attempt at a Solution

I know how to do the problem, and I got the right answer, but I don't understand what's going on.

So here's what I did,

Reading of E = Weight of container + Weight of liquid + Force of Buoyancy

75 = 25 + (0.003)(10)P

P= 1667.7

I don't get why reading of E isn't just (Weight of container + Weight of liquid + Weight of block A)? Why are we considering the buoyancy? If you say that the liquid exerts a buoyant force on the block, and due to Newton's third law, the block also exerts a buoyant force on the liquid (and so we consider buoyancy), aren't both these forces internal (and action- reaction pairs), so the net effect of buoyant force is zero?