Question

For low velocity of sliding friction proportional to velocity

Answer 1

In general, it's not. Friction of one dry object against another is fairly constant over a fairly large velocity range. For a solid object moving at a high speed through some fluid, friction is roughly proportional to the square of velocity. However, friction is roughly proportional to velocity for a solid object moving at a low speed through a fluid. Why proportional to velocity at low speeds, but the square of velocity at high speeds? One key difference is laminar versus turbulent flow.

Reference https://www.physicsforums.com/threads/why-is-friction-proportional-to-velocity.614942/