# B Archimedies principle

1. Sep 12, 2016

### Deebu R

I was just a bit confused about Archimedes principle. Say an object is partially immersed in water. The weight of water water displaced will be equal to weight of the entire object or just the part immersed in water?

2. Sep 12, 2016

### Orodruin

Staff Emeritus
Archimedes' principle states that the buoyant force on an object is equal in magnitude to the weight of the displaced liquid.

A corollary is that for a freely floating object on the surface, the buoyant force must exactly cancel the weight of the object. Since Archimedes' principle states that the buoyant force is equal in magnitude to the weight of the displaced liquid, the weight of the displaced liquid must be equal to the weight of the entire object or the object will not be floating.

Note that freely floating in not the same as partially immersed. For a partially immersed object, there may be other forces at work as well in addition to gravity and the buoyant force. In that situation, those forces must also be taken into account.

3. Sep 12, 2016

### BvU

It depends: on whether there is equilibrium ! If you have to push down to keep a cork partially under water, the weight of the water is more than the weight of the cork. And if you have to pull up to keep a block of concrete halfway in the water, the weight of the water is less than the weight of the concrete.

But if a boat floats peacefully on the surface, then yes: the volume of water displaced weighs as much as the wole boat

4. Sep 12, 2016

### Deebu R

Ah.ok. I understand. Thank you orodruin and BvU.

5. Sep 12, 2016

### Deebu R

One more thing. Is there a case were the weight of water displaced equal the weight of the part of the body which is below water level?

6. Sep 12, 2016

### Orodruin

Staff Emeritus
Yes, when there is nothing above the surface and the object has the same mean density as water.

Or when the remaining force to hold the object in place is provided by another external force.

7. Sep 12, 2016

### Deebu R

I understand. Thank you