# Lots of forces F1,F2,F3

1. Jan 17, 2009

### atavistic

Suppose we have lots of forces F1,F2,F3 etc acting at a point. Then pressure would be the magnitude of vector sum divided by area , limit of area tending to zero or just sum of magnitude of individual forces divided by area, limit of area tending to zero.

Also if we are given a surface and pressure on it is P, then we say force on the surface is FA (but keep in mind my above doubt), what is the direction of this force?

2. Jan 17, 2009

### Staff: Mentor

Re: Pressure

The first part doesn't make a lot of sense - you can't really have more than one force at a point and a point has no area. It is also not usual to add three forces, assign an area to them, and call it "pressure", though it could make sense in some cases, such as where you have a distributed load on one side of an object and point loads on the other (such as a fish tank on a four-point stand).

For the second question - I don't see where you display a "doubt" in the first, so I'm not sure what you are asking. But pressure at a point in space goes in all directions. On a surface, it is measured/acts perpendicular.

3. Jan 17, 2009

### tiny-tim

Hi atavistic!
Not quite …

pressure is the magnitude of the component of the vector sum in the normal direction, divided by area …
(you mean PA, i assume? )

The normal direction

though obviously, if the surface is curved, you'll have to do an integration of these normal forces to get a net force on the surface.

(but in most fluids, the pressure is the same in every direction )