# Homework Help: Pressure difference

Tags:
1. Dec 31, 2017

### Kolika28

1. The problem statement, all variables and given/known data

A ceiling can blow off due to pressure difference between the inside and outside of the house. In a magazine, it said: "Let's have a ceiling of 100 square meters. A pressure difference of 1% of the air pressure between the two sides of the ceiling is equivalent to a lift of 10 tonnes. We see that the ceiling should be properly fixed against being lifted up.

a) On which side of the ceiling is the air pressure the greatest when it blows heavily outside?
b) What can be meant by the phrase "a lift of 10 tonnes"?
c) Perform calculations that confirm the information in the text.
d) When it's windy out, it's a good idea to open air lanterns in the house. Why?

2. Relevant equations
p=F/A

3. The attempt at a solution
b) What can be meant by the phrase "a lift of 10 tonnes"?
F=ma=10 000kg*9,81m/s^2=98 100N≈100kN

Question b is the only one I get right. I don't know how to solve the rest because my book only tells me the equation p=F/A and does not have any more information about pressure.
Really appreciate some help :)

2. Dec 31, 2017

### CWatters

I have some issues with the wording of the question but...

a) if the ceiling goes up/lifts is the pressure greater on the upper or lower surface of ceiling?

c) what is normal air pressure and what would 1% of that be?

3. Dec 31, 2017

### kuruman

Hi Kolika28 and welcome to PF.

You have not answered (a). It does not require an equation, just an understanding of pressure, and the answer is helpful to understanding how to use p = F/A and to answering the rest of the questions.

4. Dec 31, 2017

### Kolika28

CWatters and Kuruman, thank you for your help! I think I understand it now. I am sorry for the wording of the question by the way if it caused some confusion!
a) If the ceiling goes up/lifts the pressure must be greater on the inside of the house, right?
c) Have to find 1% of 101, 3 kPa, which is 1013 Pa. Do I then have to add 1013Pa to 101,3kPa? And then calculate the force by putting the number in the equation like this:
101,3kPa+1013Pa=F/(100m^2)?
d) One should open the laterns to equalize the pressure?

5. Dec 31, 2017

### kuruman

(a) and (d) are correct. For (c) consider that you have a force pushing down and another force from the inside pushing up. If the two are equal, nothing happens. It's the net force that counts. What is the net force in this case?

6. Dec 31, 2017

### Kolika28

Ohhh, of course. The net force is 1013Pa.
p=F/A
F=pA
F=1013Pa*100m^2

Thank you so much!!!