General Question

Odysseus's avatar

If gravity is the same underwater as it is on land why does a rock feel easier to lift underwater ?

Asked by Odysseus (2746points) January 17th, 2011

what is this buoyancy thing in layman’s terms? I always assumed buoyancy happened because an object contained air, but a lead pipe has no air and still feels lighter under water.

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6 Answers

leftt's avatar

It’s because there is something under the pipe or rock that prevents it from falling very fast, which is the water. In air, it is much easier to move around, and just about less dense than everything. But in water, it is slower to move, depending on depth, and also a lot denser than air. Also, displacement is in effect here.
So when you lift up the pipe or rock underwater, the water displaces around it accordingly, and in other words, helps you lift the pipe up.

Uh, this is what I think, in the very least. Didn’t look it up, just used common(?) knowledge.

Sarcasm's avatar

There’s Archimedes’ principle: “Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.”

It’s not a matter of how much air is in the object. But we as humans have easy, free access to air, and air is less dense than water, so that’s what we use in a lot of our inventions to achieve buoyancy.
It’s a matter of density. A jar of salad dressing left out for a while, the ingredients will all separate into layers, not because of how much air is in one layer and not in the other, but because some ingredients are less dense. In a room, you know that heat rises and cold falls. If you have two balloons, one filled with air, another filled with helium, the helium one is the one that’ll float away on you. There are some gases more dense than air, such as Sulfur hexafluoride, which is great at science fairs.

Silence04's avatar

In laymans terms, the water is helping holding the weight of the rock/lead pipe. Think of how things are suspended in jello…

LuckyGuy's avatar

Here’s a thought experiment for you.
Hang a rock from a spring scale. Read the number.
Now dip the rock in water and read the number. It is lower.
The amount it is lower is the amount of water the rock displaces. You can use that to determine the volume of the rock and its density.

josie's avatar

See above.
Bouyancy. It’s why an aircraft carrier floats.

ETpro's avatar

As @Sarcasm notes, any object, even a heavy one, has considerable buoyancy in the water. That means that the water its weight displaces is trying to push it back out of the water and replace its space. Imagine if you cut down a large oak tree and cut the limbs away from it leaving just its massive trunk. It would probably weigh tens of thousands of pounds and be impossible for you to budge by hand. But put it in the water, and it would float. You could hod it up without any effort. Rocks are heavier per unit volume (denser) than oak wood, so they don’t float to the surface. But just like the oak, they try to. If you dropped a rock into a liquid medium more dense than the stone, say a vat of liquid mercury, the rock would actually float.

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