# Im having trouble with a simple question (simple thermodynamics)

1. Apr 22, 2006

### nick727kcin

HI! i was wondering if someone could please help me with this physics problem. it looks very simple, and im only missing one value. thanks

gas pressure is: p= F/A = (mg)/A(of the cylinder)

my problem is: how can i get the mass of the piston? its really annoying me. please help. thanks

2. Apr 22, 2006

### Andrew Mason

What is the volume and density of the piston? (ie. what is the density of copper?).

AM

3. Apr 22, 2006

### nick727kcin

thanks for responding!!!

the density is: 8.96 g/cm^3

the volume of the piston is: [(3.14) x (.03)^2].04 = .000113 m^3 = 113.0973 cm^3

so then, i need to multiply : M= 113.0973 x 8.96 = 1013.35212g

The area of the container is: A= [(3.14 x .03^2)] = .002827

So then: p = Mg/A = 3512858.979

this is wrong though.... can you plesae help me find what i did wrong :grumpy:

Last edited: Apr 22, 2006
4. Apr 22, 2006

### Andrew Mason

First of all, work out the solution algebraically and then plug in numbers. Second, you should stick to significant figures.

Mass of piston = $\rho V = \rho\pi r^2h$
Pressure = Mg/A = $\rho\pi r^2hg/\pi r^2 = \rho hg$

g = 9.80 m/sec^2
$\rho = 8.96 g/cm^3 = 8.96 x 10^-3 kg/10^{-6} m^3=8.96 x 10^3 kg/m^3$

Pressure = 8960 x .04 (9.80) = 3512 N/m^2 = 3512 Pa. = 3.512 kpa

In significant figures, this would be 3.5 kpa

AM