When calculating total force on a piston, what pressure is used?

In a steam engine or any hydraulic system that involves the movement of pistons, the total force exerted on a piston is directly related to the pressure of the fluid acting on it. The correct choice emphasizes that the fluid pressure inside the cylinder is the determining factor for calculating that force.

When calculating total force on a piston, the fundamental relationship you rely on is given by the formula:

Total Force = Pressure × Area

In this case, the pressure referred to is the fluid pressure within the cylinder, which acts uniformly on the surface area of the piston. This pressure can originate from a variety of sources within the system, such as steam in a boiler or pressurized hydraulic fluid.

Using the fluid pressure inside the cylinder is essential because it represents the actual operational conditions affecting the piston. The weight of the piston itself does contribute to the overall dynamics of the system but does not provide a measure of the force exerted by the fluid; atmospheric pressure and fluid density are also not relevant for the calculation of total force in this case because they do not affect the internal pressure directly acting on the piston.

Thus, the fluid pressure inside the cylinder is the appropriate measure to use when calculating the total force on a piston, making this answer correct.