since we are talking of uncharged and non- magnetic bodies in mechanics. At the microscopic level, all bodies are made of charged constituents (nuclei and electrons) and the various contact forces arising due to elasticity of bodies, molecular collisions and impacts, etc. can ultimately be traced to the electrical forces between the charged constituents of different bodies. The detailed microscopic origin of these forces is, however, complex and not useful for handling problems in mechanics at the macroscopic scale.
This is why they are treated as different types of forces with their characteristic properties determined empirically. . . Friction Let us return to the example of a body of mass m at rest on a horizontal table.
The force of gravity ( mg ) is cancelled by the normal reaction force ( N ) of the table. Now suppose a force F is applied horizontally to the body. We know from experience that a small applied force may not be enough to move the body. But if the applied force F were the only external force on the body, it must move with acceleration F/m , however small.
Clearly, the body remains at rest because some other force comes into play in the horizontal direction and opposes the applied force F , resulting in zero net force on the body. This force f s parallel to the surface of the body in contact with the table is known as frictional force, or simply friction (Fig. . (a)).
The subscript stands for static friction to distinguish it from kinetic friction f k that we consider later (Fig. . (b)). Note that static friction does not Fig.
. Static and sliding friction: (a) Impending motion of the body is opposed by static friction. When external force exceeds the maximum limit of static friction, the body begins to move. (b) Once the body is in motion, it is subject to sliding or kinetic friction which opposes relative motion between the two surfaces in contact.
Kinetic friction is usually less than the maximum value of static exist by itself. When there is no applied force, there is