# Diagram pressure forces over a body resting on an inclined surface

1. Jul 9, 2009

### orlan2r

I've not found information in web about diagram pressure forces over a body resting on an inclined surface.
For example, if we have a prism in rest on an inclined surface, the Diagram Pressure is always a straight line?

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2. Jul 9, 2009

### rock.freak667

...well yes essentially pressure acts in a straight line, normal to a surface. In the diagram, only the component of the force reaction surface (the force normal reaction) would exert a pressure.

3. Jul 9, 2009

### Bob S

You might want to consider a torque about the CM, because the friction force does not act on CM, while the gravitational force does.

4. Jul 9, 2009

### orlan2r

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5. Jul 10, 2009

### tiny-tim

Hi orlan2r!

You're asking about the line formed by the ends of the vectors representing the reaction force at each point, as shown in the diagrams.

So far as I know, it is assumed that the ends lie along a line, but there is no mathematical way to prove this.

Consider a beam supported at two points … there are two independent equations, from which we can find the two reaction forces.

But if the beam is supported at three points, there are still only two independent equations, but there are three reaction forces.

We can only find those three forces if we include a third equation, which in that case will describe the elasticity of the beam.

Similarly, we usually assume that a table supported on four legs has four equal reaction forces. But that need not be so, as is easily seen by cutting one of the legs … the table can still stand on three legs!

We cannot say what the "diagram pressure" will be for your prism, unless we know the internal structure of the prism.