lets say we have some sphere under some pressure and I want to find what is the force pressing one-half of the sphere toward the opposite half?,lets take one piece of area then the pressure force exerted in this area is:

$$p\Delta A$$

and the horizontal component is: $$p\Delta A cos\theta$$

book says the following:

A cos θ is the projection of the area ΔA on a vertical plane, i.e., it is the fraction of the area facing to the right, in silhouette. For the complete half sphere, the net area facing to the right is πR^2 and the net force toward the left is therefore pπR^2

I dont understand that statment.why the area is not onehalf of that, can someome offer me an intuitive explanation of what happen, why the net force to the left is pπR^2?

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1 Answer 1


The force is being exerted onto the flat, circular face of the hemisphere, which has area $\pi R^2$. Since that face is flat, the force at each point on that face points to the left, so that the net force on that face of the hemisphere is $p\pi R^2$ to the left. We know that the hemisphere as a whole is at rest, so the net force on the curved bit of the hemisphere must be $p\pi R^2$ to the right.

  • $\begingroup$ why is that deltaA cos is a projection into a plane? $\endgroup$ Apr 12, 2023 at 21:08
  • $\begingroup$ @PulsarPlasma For the entire left plane, $\theta=0$ (or $\pi$ depending on your sign convention). $\endgroup$
    – DanDan面
    Apr 12, 2023 at 22:09

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