# Homework Help: Physics Problem (densuty, pressure etc)

1. Nov 28, 2004

### shalei

1. A block of wood weighing 105 N and of specific gravity 0.75 is tied by a string to the bottom of a tank of water in order to have the block totally immersed. What is the tension in the string?

2. A vertical test-tube has 2 cm. of oil (density= 0.80 g/cu. cm.) floating on 8 cm. of water. What is the pressure at the bottom of the tube due to the fluid in it?

3. A standing woman whose head is 35 cm. above her heart bends over so that her head is 35 cm. below her heart. What is the change (in torr) in the blood pressure in her head?

4. In which liquid will an iron ball weighing 20 N weigh more in water or in alchohol? By how much?

Oh, I'm new here but not new at lurking..

2. Nov 28, 2004

### ponjavic

4: any liquid with a higher density perhaps mercury there will be a bigger upthrust from the pressure in the liquid than in water, at a certain height the ball should be able to weigh 20N at the same height in water the upthrust would be less so the ball would weight more, just a guess though

3. Nov 28, 2004

### ceptimus

The upthrust on a body immersed, or partially immersed in a liquid is equal to the weight of the liquid displaced.

So, the first question. You need to work out how much a volume of water equal to the volume of the wood weighs. The specific gravity = 0.75 means that the wood is 3/4 as heavy as the same volume of water, so the water would weigh 4/3 as much as the wood. The upthrust pushes the wood up, and you have to subtract the weight of the wood to get the resulting string tension.

The second question. Add the weight of the water to the weight of the oil to get the total weight in the tube. Divide that by the area of the bottom of the tube to get the pressure. Now you don't know the cross sectional area of the tube, but you can just call it A, and you'll find that it will cancel out and disapear from the final answer. As the value of A doesn't make any difference to the final result, you can set A=1 to make the math easier.

Question 3. This is a bit of a dumb question, as there is the hidden assumption that the blood pressure at her heart remains constant, which it might not do. Also they don't tell you the density of blood, so I guess they want you to assume it is the same as water. What it seems they're really asking you to work out is the pressure at the bottom of a 70cm (35 + 35) column of blood (or water).

Question 4. Ask yourself whether an Iron ball weighs more in air than it does underwater. That should give you a clue on how to work out the answer.