1. The problem statement, all variables and given/known data A rectangular block of length L is travelling on a smooth surface at velocity u. When it encounters rough ground with coefficient of friction mu, it decelerates to a stop. What is the time taken for the rectangular block to stop? 2. Relevant equations F= (mu)mg 3. The attempt at a solution Deceleration of the block is given by a = (mu)gx/L if only part of the block is on the rough ground. If the entire block is on the rough ground, the maximum deceleration is given by a = (mu)g. This means i have to split the question into two cases. 1. time taken when x is less than or equal to L 2. time taken when x is greater than L But I am stuck here. I don't know how to relate x to t and u.