# Water displacment question.

1. Jul 5, 2009

### bobcat19

well today we were in the pool together and i had just finished a drink that id had(bottle of pepsi) and somehow we got talking about displacement, see what i want to know is how much force is that 600ml bottle is creating when its under then water(remembering that its empty)

i dont know anything about physics so nice simple terms would be nice but dont worry to much cause im sure i can work it out

any help much appreciated

Alex

2. Jul 5, 2009

### Staff: Mentor

The upward buoyant force that the water exerts on the bottle is equal to the weight of the displaced water. (That's Archimedes's principle.)

3. Jul 5, 2009

### bobcat19

ok so because its about 600 cubic centimetres it should be exerting 600 grams or something like that.

4. Jul 5, 2009

### Staff: Mentor

Sounds right.

5. Jul 5, 2009

### bobcat19

but hang on, what if the bottle was filled with sand, it would still displace the same amount of water but of course it would sink. how does that work?

6. Jul 5, 2009

### Staff: Mentor

The buoyant force that the water exerts on the bottle will be the same, but that's not the only force acting on the bottle. You must also consider the weight of the bottle (and contents), which acts downward. When filled with sand, the weight will be greater than the buoyant force and the bottle will sink.

7. Jul 5, 2009

### bobcat19

ok i get it. thanks you've helped me a lot :)

8. Jul 5, 2009

### Naty1

Anytime the density of the contents is greater than the density of water, the bottle will sink...but if weighed underwater it weighs less than on land by the weight of water displaced.

9. Jul 5, 2009

### HallsofIvy

Well, no. 600g dynes, the force of gravity on a mass of 600 grams.

10. Jul 5, 2009

### Staff: Mentor

600 grams weighs about 5.89 Newtons or 5.89 x 105 dynes.