Types of Force
Forces have an origin and an effect
Origins
| Type | Common name | Local effect | Notes |
| Gravitational | Weight | All bodies are attracted downwards to the earth. | All bodies possessing the property of mass attract each other, although usually with extremely small forces. But the earth is big, so it is able to exert a substantial force on, say, a pencil. The pencil, being small, doesn't usually exert significant forces on other bodies, but the earth is so big that the pencil is able to exert a substantial force on it: in fact a force exactly equal to the force the earth exerts on the pencil. So the pencil and earth attract each other with equal forces. |
| Tension/Compression | Changes the shape of the specimen by elongating or shortening it. |
These forces arise when you try to stretch (for
tensions) or squash (for compression forces) a material. The forces
result from the interatomic forces in the material, which are highly
dependent on how close the atoms are to each other. The forces are
caused by the positive and negative charges present in the atoms. These forces are particularly associated with elastic bands, springs and Hooke's Law (F = k x) |
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| Electrical |
All bodies possessing the property of being
electrically charged (perhaps by being rubbed with a cloth or piece of
silk), and especially insulators so rubbed, will experience electrical
forces of attraction or repulsion. Like charges repel. Unlike charges attract. Charges are not affected by magnetic fields unless they are moving (in which case, they constitute currents) |
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| Magnetic |
Magnetic fields exist in the vicinity of permanent
magnets or electric currents. Anything producing a magnetic field of its
own can also feel a force when placed in the magnetic field produced by
something else. Like poles repel (we don't use the term 'charge' in connection with magnetism) Unlike poles attract. Current-carrying wires experience sideways forces in magnetic fields, according to Flemming's Left Hand Motor Rule. (First finger - Field, seCond finger - Current, thuMb - Motion) Like currents attract. Unlike currents repel. |
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| Friction | Gives rise to a dissipation of heat if there is movement (calculated from frictional force ´ distance moved) | Nature creates frictional forces, as and when required, for the sole purpose of opposing externally-applied forces. The maximum possible force that can be created in this way is (coefficient of friction ´ normal contact force) | |
| Drag | Air resistance, water resistance | Acts on a body that is moving through a fluid (gas or liquid). The faster the body is moving, the bigger the drag is - often proportional to v2 |