1. Please refer to the first image, the yellow star question. No answer is provided at the back so I'm not very sure whether my reasoning is correct. Is it because the spring exerted an impulse on that trolley, causing its momentum to change? But all the spring's energy was provided by the trolley, how can it increase the trolley's momentum when it does not have any initial energy itself? 2. (This is a supplementary question ) The yellow star question in image 2. My workings in image 3. Is there any shorter solution?