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

1. Dec 20, 2012

### harjyot

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

2. Dec 20, 2012

### Staff: Mentor

What do you think the new weight will be? Why not try to describe all the forces acting on that bob in both cases?

3. Dec 21, 2012

### harjyot

the force on the Bob will be
weight - upthrust.
by total weight I mean the weight of the whole system

4. Dec 21, 2012

### Staff: Mentor

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.
Right. So what do you think happens to the total weight of the system?

5. Dec 22, 2012

### harjyot

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?