CBSE Class 11-science Answered

When we try to push an object on rough surface it won't immediately move.  As we keep increasing the applied force so as to overcome the friction, at a certain point we succeed, the applied force at the onset of motion is called limiting friction, i.e. minimum necessary for a particular combination of surfaces. But then we can't just stop appling the force, we have to keep applying a force which is lightly less than limiting, and is called kinetic friction force.  It's the friction force which must be overcome in order to sustain the motion.

 

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