Kinetic Friction
Definition
The steady resisting force that opposes motion while an object is already sliding across a surface.
Symbol: · Unit: N
Technical definition
Friction during relative sliding, with magnitude f_k = μ_k·N, where μ_k is the coefficient of kinetic friction. Unlike static friction it does not self-adjust — it stays roughly constant regardless of speed at everyday speeds.
Example
Once you get the heavy box sliding, it becomes easier to keep pushing — kinetic friction (μ_k) is smaller than the maximum static friction (μ_s) you had to overcome.
Common confusion
Kinetic friction is usually less than maximum static friction — that is why it is harder to start pushing something than to keep it moving. But μ_k is not zero; ignoring it makes energy calculations wrong.