# Physics ab law of thermodynamics help please

1. Apr 3, 2009

### silntassassin

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

Steam moves into the cylinder of a steam engine at a constant pressure and does 0.79
J of work on a piston. The diameter of the piston is 1.7 cm, and the piston travels 2.2 cm
in one stroke. What is the pressure of the steam? Answer in units of Pa.

2. Relevant equations
w=f*r
a=pi*r^2
P=f/a
100 cm= 1m

3. The attempt at a solution
2.2cm=.022m
1.7cm=.017m

w=f*r
.79=.022*x
x=35.9090909091

now find area: a=pi*r^2

1.7=d
.85=r
pi*.85^2=2.269800692=a

p=f/a
p=35.9090909091/2.269800692
p=15.82

now this seems to be wrong but I do not understand what I am doing wrong. please help me.

2. Apr 3, 2009

### Redbelly98

Staff Emeritus
Try expressing the radius in m rather than cm.

3. Apr 3, 2009

### silntassassin

ohh right i forgot to do that the second time i did the problem because i initally divided .022/.79 when it should have been the other way giving me the wrong answer to plug in to get the pressure. But after I tried the radius in m I get a number like 39550.929555 and idk if thats right?

4. Apr 3, 2009

### Redbelly98

Staff Emeritus
Uh, not quite. The first answer, 15.82 Pa, was correct except for not converting cm into m.

The correct answer will have the same digits 1582, but the decimal point will be in a different place.

5. Apr 3, 2009

158.2?

6. Apr 3, 2009

### Redbelly98

Staff Emeritus
No.

35.9 N is the correct force.

What is the area in m^2, given r=0.85cm?