Initial speed of body on the horizontal rough surfaces, u=10ms−1 Final velocity, v=4ms−1 and t=2s. If a be the retardation due to friction, then by using equation, v‌=u−at ⇒‌‌4=10−a×2 ⇒‌‌2a=6⇒a=3ms−2 According to given situation, friction force =ma ‌⇒‌‌µkmg=ma ‌⇒‌‌µk=‌