When you pull the plug in a bath: Does the water gather speed as it travels down the plug hole(faster toward the end)?

According to the conversion of energy from gravitational potential to kinetic (mechanic) energy, I'd say yes, water does accelerate. There is also gravitational pull, which makes the water accelerate towards the Earth, so yes, water does accelerate, according to me.

Do you mean does the water go faster with more water compared to less in the bath OR faster from the top of the water tunnel compared to lower down?

Do you think the less water there is in the bath(as it travels down the drain) the faster the water travels, the more water the slower?

Do you think the less water there is in the bath(as it travels down the drain) the faster the water travels, the more water the slower?

No, the opposite.

When there is more water in the bath, the weight of the water on top pushes on the water below, creating a pressure that makes the water drain faster. When there is less water, this pressure is less and so the water drains slower.

It's not too hard to show that under conservation of energy the velocity or water emptying out a drain at the bottom of a container is:


v = \sqrt {2gh}


where h is the height of water above the drain. This equation is an idealization that ignores factors that slow down the flow - such as the viscosity of the water and roughness of the pipe - but it makes the point that the deeper the water, the faster the water flow out the drain.

Also, compared to a lot of other liquids, water has a lot of tension or "stickiness" between individual molecules, so that the water falling down the drain can pull the water above it to an extent, like a falling rope.

(I think this is different from viscosity.)

Thanks guys!

Your question should have used a better example than a bath tub. Most of the answers you got were correct from a physics stand point but not from a plumbing stand point. Traps, vents, elbows, Tees and Wyes all alter how water drains.