# Another problem about computing pressure?

a gas contained in a vertical, frictionless piston-cylinder device. The piston has a mass of 4 kg and cross-sectional ara of 35cm^2. A compressed spring above the piston exerts a force 60N on the piston. If the atmospheric pressure is 95 kPa, determine the pressure inide the cylinder, thanks for helping me,the answer is 123.4 kPa, plz help me with the solution,thankz again

Answer:
The pressure inside the cylinder is the sum of the ambient pressure plus the pressure acting on the piston due to weight and spring force.

First step: Calculate force due to weight = 4kg * 9.81m*s^-2 = 39.24N

Second step: Add force of the spring: 39.24N + 60N = 99.24N

Third step: Calculate force per area due to force found in 2nd step. 99.24N / 0.0035m^2 = 28354Pa = 28.354kPa
Fourth step: Add ambient pressure: 28.354kPa + 95kPa = 123.354kPa Rounded to one decimal = 123.4kPa

Remark: Instead of calculating the force that is acting on the cylinder due to ambient pressure and then back-converting it to pressure again on the other side it is more straight forward to add the ambient pressure as it is in the last step.

So the ambient pressure is the most important part of the total pressure
The spring force influences the pressure in the second rank and the weight of the piston is the least significant influence.

The first answer was a bit off the mark due to conversion of units. It helps to think metric and decimal in practical everyday activities.
I am a little rusty with this type of question but here goes!
Total force acting on the piston is (4kg*9.81ms^-2)N+60N +( 95 N/M^-2 * 0.0035m^2 )=99.5725N
The pressure inside times the area of the piston must equal the force acting downward on the piston. To produce that much counter force the pressure inside has to be 99.5725N/.0035 m^2
=28449Nm^2= 28449kPa
The force on the piston is mostly supplied by the mass of the piston and the spring
the ambient pressure has a negligible effect.
I think that there is something wrong with your supplied answer 123.4 Nm^-2 * .0035 m^2 = .435N Sorry

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