Question

A weightless plastic bag is filled with 5kg of water and then weighed in water. The reading of spring balance is? Why?

Answer 1

Spring balance reading will increase.

Suppose you are immersing the weight into the water with thread in your hand. You will feel reduced weight equal to the weight of water displaced. That reduced weight is supported by upward buoyant force of water which is supported by bucket and that much increase will be showed on spring balance.

Alternatively think like this,

There are two objects(bucket of water, weight) and two supports(spring balance, hand) taking the system as whole. The two supports must together carry the total weight of two objects. Weight supported by hand is, weight of immersed object-weight of water displaced. Then, weight supported by spring balance should be weight of bucket of water+weight of water displaced.