Hydrostatics:change in weight of a system if a foreign body is introduced

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harjyot
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I have a beaker with a liquid of density
rho, and weight = W
now I throw into it s Bob say, of density rho' and weight=w.
can anyone explain what will be total new weight,with all forces explained for the cases:
body floats.
body sinks
 
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What do you think the new weight will be? Why not try to describe all the forces acting on that bob in both cases?
 
the force on the Bob will be
weight - upthrust.
by total weight I mean the weight of the whole system
 
harjyot said:
the force on the Bob will be
weight - upthrust.
OK.

When the bob is floating, the forces on the bob are weight (down) and the buoyant force (up). They must balance if it floats.

When the bob sinks, the buoyant force is not enough to support the weight of the bob. When the bob settles on the bottom of the beaker, the bottom of the beaker will exert an upward force on it.
by total weight I mean the weight of the whole system
Right. So what do you think happens to the total weight of the system?
 
in the first case, when it floats, the Bob pushes the liquid down with It's weight. so the total weight is W+weight of Bob

when it sinks, the net force on Bob is upthrust + reaction force = weight.
here too the weight of Bob gets added?