Friction is not itself a fundamental force but arises from fundamental electromagnetic forces between the charged particles constituting the two contacting surfaces. sliding friction is less than static fiction because the irregularities on each surface do not get time to get interlocked.means they find less time to get interlocked against each others irregularities
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When something is static, a greater force is required to break the interlocking between two surfaces. When something is in motion, there is no need to break the interlocking and a smaller force is required the keep the object in motion. Hence, sliding friction is less than static friction.