How can a rock sink in water and a boat stays up?

Question

Like how does something over 100 pounds float?

Answer 1

The boat is effectively hollow, the rock is not...it's solid.

The size of the boat displaces a volume of water, as long as the volume of water displaced is not greater than the volume of the boat then it will float.

The space inside the boat occupied by air counts.

Answer 2

How does something over a 100 pound float?

Well, "bouyancy" is the key factor here, a rock has no "bouyancy? - but it could have, if it was shaped correctly.

You see, water has weight... in fact, it has lots of weight.  Air has very little weight (none that we can feel anyway)... So, water weighs much - much more than air... in fact, a gallon of milk and a gallon of sea water are about the same... each weighs about 8 pounds a gallon, or 64 pounds a cubic foot.

So, if you take 2 cubic feet of air... and put it in a box container that weighs 8 pounds you could then put "up to" 120 lbs inside the container, and it would float... obviously, the more weight you put in the container, the more it would sink into the water... but if we put (for example) 60 lbs of weight (or cargo) in the container, and the box would float about half in and half out of the water...

So, as 2 cubic feet of air x 64 pounds = 128 lbs of "bouyancy".  Our container weighs 8 lbs, our "cargo" is 60 lbs, and therefore our box (or boat) floats about half in and half out of the water.

    This "bouyancy" is caused by the upward force of the water, that is "displaced" by the downward force of our box (or boat)  Our box which is now taking up space in the water, but is lighter than the space it is taking up - therefore floats.

     So, the 2 cubic foot box or boat is pushing downward with all its 68 lbs... but, the water in that space is pushing up at 128 lbs... so, the box remains about half in - but still on top of the water...

So, if you take that 8 lb rock for example, melt it, and shape it so that it looks like bowl that would hold a gallon of water, or "air" - the rock would float.  (This is exactly how steel boats float.)

Got it!

Answer 3

bouyancy, displacement, read about it above.

what makes it all work is surface tension.

the ability of water molecules to stick together.

so the boat doesn't slip right through 'em the way it cuts through air.

The water pushes back, with a force equal to the weight of the water that is displaced.  An object that weighs more than the water it pushes out of the way sinks.

So rocks sink.  Even tiny little rocks.  But ducks float.

Answer 4

displacement.  If the boat were to displace more weight of water than the boat were to weigh, it will float, once the boat weighs more (by adding weight into the boat) than the water it can displace, it will sink.

In your case, a 100 pound weight, if its volume is greater than 100 pounds of water, it will float.  Water weighs 8 pounds per gallon or about 62.5 pounds per cubic foot.

Answer 5

It is the density that matters. A rock is denser than water and sink, but boat is a thin shell of wood or rubber or even steel but has mostly air inside so the average density is lower than water, so it floats.

Answer 6

something floats when it displaces an amount of water equal or more than its mass.  Since a boat has hollow walls, it displaces a bigger volume of water than itself, so it floats.