Force on the bottom of a swimming pool

confusedbyphysics

I don't understand WHY my Webassign says I have this wrong. Here is the problem:

" What are the total force and the absolute pressure on the bottom of a swimming pool 2.6 m by 1.6 m whose uniform depth is 2.0 m?"

My book says the force = density X Area X depth below the surface X g

so f = (1X10^3) X (2.6 X 1.6) X (2) X (9.8) = 81536...with 3 sig figs 81500. Tried entering both 81500 N and -81500 N and both were wrong. It seems straightforward so what did I do wrong? Thanks!

FredGarvin

Perhaps becuase the question asked for two quantities? It asks for both absolute pressure and total force. Also, I get 81619 N when I run the numbers. Perhaps that may be the issue as well.

confusedbyphysics

Yea, there are two slots in my Webassign, Im looking for the force first and then absolute pressure. Sorry I should have clarified that.

you got 81619? I keep getting 81536..am I missing a number??

FredGarvin

Well, I noticed I used 9.81 for g in stead on just 9.8...

acherentia

Webassign sometimes gives problems with sig figs. Try to be as accurate as possible with constants. Solve the problem with the numbers given in the textbook first. Check with the back of the book. If you get the same answer proceed to webassign using the same number of sig figs for g. Use one additional "sig fig" in all answers.

Eurika

I just had a very similar problem on my webassignments.
I finally figured it out.
the answer is wrong for total force because you are not taking into account the force of the air on the water. you need to calculate the pressure of the water in the pool then add 1.013 X 10^5 Pa (the pressure of air) to find the absolute pressure and then use that value to find the force. this way you are taking into account the force of the air also and then you will get the total force correct.