# Forces affecting a body at the bottom of water

1. Oct 20, 2011

### Misr

Consider a body at the bottom of water whose density is certainly more than that of water,so it settles under water:
[PLAIN]http://img710.imageshack.us/img710/1778/unled123copy.jpg [Broken]
I'm trying to explain how the body is under equilibruim although the buoyancy is less than gravity
That's because of the normal force of the bottom of the container,or the reaction of the container to the body pushing on it due to its weight,thus we have

Fg=Fb+Fnorm.

Is that true?

Last edited by a moderator: May 5, 2017
2. Oct 21, 2011

### jetwaterluffy

Pretty much.

3. Oct 21, 2011

### Misr

What do you mean by "Pretty much" :D ?
Is this true or not?

4. Oct 21, 2011

### vkash

Is this true or not?[/QUOTE]I think you are correct.
His pretty much means if the system is in non inertial frame then your equations may(depend on direction of acceleration) wrong. In that case you need to add pseudo forces.

5. Oct 21, 2011

### sophiecentaur

Fb (the buoyancy force?) may be affected by the details of how the object contacts with the bottom of the tank. If there were a lip / seal around the outer bottom edge, not allowing water in underneath, then there might be no buoyancy force (only a small amount of air pressure in the space. The upwards force could be largely from the forces through the lip / seal and not due to hydrostatic pressure (upthrust).

6. Oct 23, 2011

### Misr

Okay,but this is basically not wrong..
In which case can we say that this equation works?

7. Oct 23, 2011

### Naty1

it works.

The buoyant force is equal to the weight of the volume of water displaced and at the same
time gravity is pulling the object in the opposite direction.

So the resultant force holding the object in equilibrium is your normal force....a force less
than the weight of the object in air,

8. Oct 24, 2011

### Misr

Yeah,Okay thanks very much :)