I have problems with a(ii). (please view the first image.) Now I have found that w = (13-8e)u/7 and v = (20e+13)u/7 What I did is like this: (Using NEL) v = 4ue Where the LHS is actually v-0 because I assume Q = 0. (20e+13)u/7 = 4ue e = 33/28 >1 ∴ e cannot exist. 1. Is this way correct? 2. My teacher used another way. (please view second image.) I don't understand the reason behind his workings. Actually he like to use the formula like this : V1- V2 = -e(U1-U2) ,even when V2 is faster than V1. Because he said in his brain, 2 always comes after 1, so he doesn't want to use V2-V1. I don't follow him, I follow the book, using speed of separation = fast - slow. But I still don't understand why he used V-W = -e(4u) to assume Q is brought to rest. (or did he even make that assumption at all?) My interpretation is like this: Since he uses "slower-faster = negative e (speed of approach)" Then in his workings, W should be assumed faster than V. And in the situation, both P and Q are moving to the left after collision, which makes their velocities negative. So if you want to assume Q is brought to rest, then W = 0. Since 0 is larger than negative, then his assumption is valid. This is just simply a hypothesis. I only have a vague picture of the concept but I cannot fully grasp it. Someone please explain it to me. 3. The second part of a(ii), determining the least possible value of w. I just substituted e=1 into v-w = 4ue and got the answer. But after thinking about it, I suddenly don't understand why it should be e=1 which is substituted inside. I tried e=0.9, and the resulting w is smaller than the answer when e=1. So why is the least value obtained when e=1?