# Homework Help: Equation help - water pressure at bottom of a pond

1. Dec 15, 2011

### elchoi

equation help -- water pressure at bottom of a pond

I'm trying to solve this problem "A car is flipped upside down in a pond, .50m below the surface, how much force is needed for the driver to push open the door." I got this wrong because the formula that was chose was F = (P - (P+rho x g x y)) So my question is when do include rho x g x y when solving problems?

2. Dec 15, 2011

### Spinnor

Re: equation help -- water pressure at bottom of a pond

Similar problem here,

Found by Googling "physics force of water on swimming pool wall physics examples"

You have not given enough information to solve the problem, important details were left out. Also as the door is hinged there is a subtle reduction in force required to move the door, if one simply used the total force on the door the answer would be off?

Good luck!

3. Dec 16, 2011

### ehild

Re: equation help -- water pressure at bottom of a pond

The pressure is the force on unit area. To get the force exerted on the door, you need to know the area of of the door.
The difference of the outside and inside pressures at depth y is rho g y. As the door is on the side the car, the pressure varies along the height. You can use the average height, but it is not clear what is at 0.5 m depth from the surface. Taking 0.5 m the average depth, calculating the average pressure, and multiplying it by the area (what is the typical area of a car door?) you can get an estimate of the force and you will see that
it is very big .

ehild

Last edited: Dec 16, 2011
4. Dec 16, 2011

### elchoi

Re: equation help -- water pressure at bottom of a pond

Thank you both

5. Dec 17, 2011

### Jobrag

Re: equation help -- water pressure at bottom of a pond

If the driver can hold his breath till the inside and outside pressures equalise then the answer is close to zero.