# Homework Help: By how much is each post compressed?

Tags:
1. May 3, 2017

### Wimpalot

1. The problem statement, all variables and given/known data
A large 30000 L aquarium is supported by four wood posts at the corners. Each post has a square 6.00 cm x 6.00 cm cross section and is 90.0 cm tall. Take Y=10^10 for wood

2. Relevant equations
ΔL = FL/AY
F = ma
ρ = m/V

3. The attempt at a solution
So the total area is 0.06 * 0.06 * 4 = 0.0144m2
The length of each post is 0.9m
The total mass of water is 30000kg (taking the density as 1kg/L)
Meaning the weight is 30000*9.8 = 294000N
And Y=1010

Which gives:
ΔL = (294000*0.9)/(0.0144*1010
= 0.00184

But this is apparently wrong. I have no idea what I did wrong

2. May 3, 2017

### Staff: Mentor

Were they expecting the answer to be given in particular units?

3. May 3, 2017

### Wimpalot

It has never mattered before as I can specify the units. So it shouldn't matter.

4. May 3, 2017

### Staff: Mentor

Well, I can't spot any other issues. Your calculations look okay to me. Unless of course this is a salt-water aquarium?

5. May 3, 2017

### Wimpalot

That was my second thought. In which case I get 0.0019 which was also incorrect. So yeah, no idea what the issue is maybe they do want different units so I may try that otherwise I don't know. I only get 5 attempts (this is mastering physics)

6. May 3, 2017

### Wimpalot

Turns out every other question wants two significant figures except this one which wanted exactly 3 to be correct so yeah. 0.00184 was correct. Thanks for the assistance