4-1 The mass of block A and B are 5 kg and 10 kg, respectively. A and B both have a friction coefficient of 0.25 with the ground. Now A collides into B when B was at rest. After the impact, A and B moves along with the same speed. 4 seconds after the impact, A and B stop due to the friction. Find the initial velocity of A before the impact. 4-2 Following the previous question: if block A bounced back (to the left) and stopped after 3 seconds of sliding alone, find the traveling time (from impact to stop) for block B.

4-1 The mass of block A and B are 5 kg and 10 kg, respectively. A and B both have a friction coefficient of 0.25 with the ground. Now A collides into B when B was at rest. After the impact, A and B moves along with the same speed. 4 seconds after the impact, A and B stop due to the friction. Find the initial velocity of A before the impact. 4-2 Following the previous question: if block A bounced back (to the left) and stopped after 3 seconds of sliding alone, find the traveling time (from impact to stop) for block B.