Chemical Forums

Chemistry Forums for Students => High School Chemistry Forum => Topic started by: shawwnns on September 22, 2021, 10:30:34 PM

Title: Bernoullis Principle on Density and Water Displacement.
Post by: shawwnns on September 22, 2021, 10:30:34 PM
Alright, this one is a bit of a doozy.

We had a lab where we had a graduated cylinder filled with water and some object. We then put the object in the water to find the volume and we weighed the objects to get the mass in order to find density.

For the lab abstract, my teacher is hellbent on using Bernoulli's principle to find density. The best thing we have right now dropping the object into the test tube causes pressure to the water inside the test tube that needs to be dealt with. Through this, the water creates velocity by which causes displacement and the water levels go up.

I know Archimedes is the better principle. Yes, my teacher knows about Archimedes. He does not care about it and doesn't solve the question.
Title: Re: Bernoullis Principle on Density and Water Displacement.
Post by: JWhite001 on November 09, 2021, 12:23:45 PM
Let's say you have a tank of water open to the air and poke a hole at the bottom of the tank, where the water is flowing out of the hole at some velocity, which we will call V2. According to Bernoulli's principle equation, P + 1/2*rho*v1^2 + rho*g*h1 = P + 1/2*rho*v2^2 + rho*g*h2, we are referring to two points involving the tank: at the top of the tank water level, where h1= some number (since h is relative to the datum set at the hole) and the point at the hole, where h1 = 0. We also assume that velocity is insignificant at the top, so v1 = 0. We then set up an equation that looks like this and then just solves for the velocity at the point of the hole, or v2.

My question is, what is v1, or the velocity at the top of the tank? Is it referring to the change in position of water molecules, as the water level drops (velocity = displacement/time)? Is it referring to the velocity of the individual water molecules at the top of the tank water level? I just need someone to explain what makes v1 approximately 0.