9.2 SOME BASIC CONCEPTS

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Mass and force - A mass can exert forces through gravity and other effects. Forces can also be exerted by other phenomenon, such as magnetism.

We can emphasize the relationship between mass as an absolute and gravity as a local. The effects of gravity are dealt with as forces in most statics problems.

Force has magnitude and direction. Therefore it is well suited to vectors.

many forces can also operate on the same object, we can replace these with equivalent forces, called RESULTANTS.

We have both action and reaction forces as well. As we apply ACTION forces, there are forces that will resist, these are called REACTIONS.

Some approximations,

- we are pretending the forces are applied at points, but in reality a force must be distributed,

- we generally assume there are no deflections. This is known as the RIGID BODY assumption.

- we often use PARTICLE approximations that assume bodies have no size. This simplifies calculations significantly.

- TRANSMISSIBILITY - a force can be moved along a line of action.

- Parallelogram law - a method for adding two forces to get a resultant vector.

- Forces that are the result of mechanical contact are called surface forces, and are transmitted by pressure on the surface of an object.

- Body forces are the result of Physics (read magic) and will pull on all of the mass of the object, such as gravity, electrostatic, inertia, magnetic, etc.