# Water pressure homework

1. May 5, 2006

### p0ink

Tank A

|..........|
|..h2o...|
|..h2o...|
|_h2o__|

Tank B

\.........../
.\..h2o../
..\.h2o./
...\h2o/

Tank C

.../.....\
../.h2o.\
./..h2o..\
/__h2o__\

The three tanks shown above are filled with water to an equal depth. All the tanks have an equal height. Tank A has the middle surface area at the bottom, tank B the least and tank C the greatest.

(Select T-True, F-False, G-Greater than, L-Less than, E-Equal to. If the first is F the second L and the rest G, enter
FLGGGG).

A) The force due to the water on the flat bottom of tank B is .... the weight of the water in the tank.
B) The force exerted by the water on the bottom of tank C is ... the force exerted by the water on the bottom of tank B
C) The water in tank B exerts a force with an upward component on the sides of the tank.
D) The pressure exerted on the bottom of tank C is greater than for the other tanks.
E) The pressure at the bottom of tank C is ... the pressure at the bottom of tank B

2. May 5, 2006

### p0ink

i dont see why my answers: LGTFG and LGTTG can't be correct?

3. May 5, 2006

### Hootenanny

Staff Emeritus
Welcome to PF p0ink,

Perhaps if you defend your answers, we could point out where you have gone wrong? It may be a more beneficial process than us simply telling you where you have gone wrong.

~H

4. May 5, 2006

### p0ink

here is my thinking for the forces on the bottom of each tank, which made me come to those answers.

A: F=m*g
B: m*g - the upward force of the sides of the tank
C: m*g + the downward force of the sides of the tank

5. May 5, 2006

### Hootenanny

Staff Emeritus
So why shoud the force on the base of the tank be less than the weight (mg) of the water?

~H

6. May 5, 2006

### p0ink

it would be less for tank B, because the sides are pushing with an upwards force.

for tank C, it would more, since the sides of the tank are pushing with a downward force.

7. May 5, 2006

### p0ink

And tank A would have an equal force on the base of the tank, since the side forces are not significant.

8. May 5, 2006

### civil_dude

Remember that pressure does not equal force and is simply a function of density and height, P=d*h, thus for the water of equal heights in this example, wouldn't the pressure be the same at the bottom of all tanks?

Also, if the volume of the tanks are equal, then the force from the weight of the water will be equal

9. May 5, 2006

### p0ink

ahhh....

i see. the pressure was what was throwing me off. i kept thinking p=f for some odd reason.

thanks!

10. May 8, 2006

### reincarnated_soul

All u need to know here is that pressure in a liquid depends upon depth and the weight of a liquid depends on the volume and ur all set!!!!

11. Feb 23, 2008

### moniquert

help!!!!!!!!!!!!

what was the final answer to that question?

12. Feb 23, 2008

### kamerling

A & B were correct. The reasoning for it was also OK. You can also see it if you see that all tanks have the same pressure on the bottom, and the Force on the bottom = pressure * (bottom area).

you should really have had C correct as well, you probably just got confused with the force of the water on the walls of the tank and that of the walls of the tank on the water.
D & E were already discussed by civil_dude

13. Feb 24, 2008

### moniquert

my questions was a little different but has the same tanks and wording, just asks stuff about different tanks.

A) The force due to the water on the bottom of tank C is... the weight of the water in the tank.
B) The water in tank C exerts an upward force on the sides of the tank.
C) The pressure at the bottom of tank C is... the pressure at the bottom of tank A.
D) The pressure exerted on the bottom of tank B is smaller then for all the other tanks.
E) The force exerted by the water on the bottom of tank C is ... THe force exerted by the water on the bottom of tank B.

For A won't the force due to water be greater then the weight of the water in the tank?

For B would'nt it be false cause the water is pushing down on the walls not up?

I know for C the pressures are going to be equal because P=(Density)(height) and there at the same height right?

For D it's going to be false right because all the tanks have the same pressure at the bottom because what i just said before.

And for E Wouldn't the force be less on tank C than Tank B

So I put GFEFL still wont come out right

14. Feb 24, 2008

### kamerling

B is wrong. The Force that the fluid exert on the walls must be perpendicular to the walls.

for E: if you have a pressure of 1 pound per square inch on the bottom, The force this will exert is one pound for EVERY inch of the bottom. So if the bottom area is larger .....

15. Feb 24, 2008

### Staff: Mentor

As kamerling says, the pressure of the water is always perpendicular to the walls. So the water pushes out and up against the walls. (Definitely not down!) So I'd say the answer they want is true.

It probably should have been written as:
B) The water in tank C exerts a force with an upward component on the sides of the tank.