# How to calculate pressure inside an internal combustion cannon

1. Aug 12, 2013

### Spartan-1801

Hi. Im needing help on this...

For a Year 12 physics assignment (australia), Im testing a Spud gun/lemon cannon. I need to work out the pressure inside the chamber just before the projectile begins to move, without using a pressure gauge. Tomorrow i'll be measuring the muzzle velocity using a high speed camera, but i have absolutely no idea on the pressure calculations. My physics teacher wont tell me either, not sure wether its because he doesnt know or not, but he keeps telling me to look online and i cant find anything comprehensive.

Also, i apologize for posting this in the wrong forum... only just realized

Last edited: Aug 12, 2013
2. Aug 12, 2013

### voko

Perhaps you could explain what your particular setup looks like and how it operates.

Fundamentally, when you burn some gas mixture in a closed chamber, it produces more gas and some heat. The heat increases the temperature of the gas, and with the chamber volume known, you should be able to estimate the pressure using the gas laws.

3. Aug 12, 2013

### A.T.

Use a well lubricated projectile to minimize the friction in the barrel. Assuming negligible friction the only horizontal force on the gun is the pressure on the back wall of the barrel. If you have a high speed cam you could hang the gun on strings, and measure the initial recoil acceleration. From the acceleration and the mass of the gun (without projectile) you get that force. Or use a fast force sensor if possible.

Divide the force by the cross sectional area to get to pressure.

Last edited: Aug 12, 2013