Q.Write the laws of static friction and kinetic friction.

Both static and kinetic friction are proportional to the normal reaction (f = μN) and are independent of the contact area; static friction is self-adjusting up to a maximum (μs N), while kinetic friction is roughly constant (μk N) and slightly smaller than the maximum static value.

Laws of static friction:

1. Static friction is a self-adjusting force — it automatically adjusts in magnitude (from 0 up to a maximum) to exactly balance the applied force, as long as the body remains at rest.
2. The maximum value of static friction (called limiting friction), fs(max), is directly proportional to the normal reaction N between the two surfaces: fs(max) = μs N, where μs is the coefficient of static friction, a constant depending on the nature of the two surfaces in contact.
3. The limiting friction is independent of the apparent area of contact between the surfaces (for a given normal reaction).

Laws of kinetic friction:

1. Once relative sliding begins, kinetic (or dynamic) friction opposes the relative motion between the surfaces, acting along the direction opposite to the relative velocity.
2. The kinetic friction force is also directly proportional to the normal reaction: fk = μk N, where μk is the coefficient of kinetic friction. …