Basic Thermodynamics problem I can't see.

[saucebandit]

Ive just started thermo after being out of the physics game for awhile. The professor is less than stellar and bascially we have to learn out of the book. The problem is easy, I know, but I just can't figure it out. Its as follows:
Cylinder containing gas has piston with cross-sectional area of .029m^2. Atomospheric pressure is .1035MPa and accel. due to gravity is 30.1ft/s^2. To produce absolute pressure of .1517MPa, what mass (kg) of piston is required?
The equation I believe I need to use is
Pabs= (Mp*g) + Patm* Ap
The only problem is I wrote it down quickly and I am not sure its correct, and cannot find it in the book, also can't figure out where area comes into play. Can anyone confirm this eqaution?
Thanks

 

[Andrew Mason]

Ive just started thermo after being out of the physics game for awhile. The professor is less than stellar and bascially we have to learn out of the book. The problem is easy, I know, but I just can't figure it out. Its as follows:
Cylinder containing gas has piston with cross-sectional area of .029m^2. Atomospheric pressure is .1035MPa and accel. due to gravity is 30.1ft/s^2. To produce absolute pressure of .1517MPa, what mass (kg) of piston is required?
The equation I believe I need to use is
Pabs= (Mp*g) + Patm* Ap
The only problem is I wrote it down quickly and I am not sure its correct, and cannot find it in the book, also can't figure out where area comes into play. Can anyone confirm this eqaution?
Thanks

You are confusing force and pressure. Of course, P = F/A. The weight of the piston (force) divided by the piston area will give you the pressure due to the piston weight. Add the atmospheric pressure and you will have the pressure in the cylinder. You don't need to know the density of air. Also, g should be 32.1 f/sec^2.

AM

 

[saucebandit]

So I just subtract atmospheric pressure from absolute pressure, then multiply that by area? Thats not getting the answer I need ao I think I am still doinf it wrong.

 

[Andrew Mason]

So I just subtract atmospheric pressure from absolute pressure, then multiply that by area? Thats not getting the answer I need ao I think I am still doinf it wrong.

Subtract atmospheric pressure from the absolute pressure that you are trying to achieve. That is the pressure that must come from the weight. If you multiply that by the piston area, you will get the Force that the piston must provide: weight not mass. Weight is mass x g:

$$P_{abs} - P_{atm} = P_{pist} = mg/A$$

AM