Momentum change after explosion

[coconut62]

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?

 

[bs15benny]

where is the image?

 

[coconut62]

There.

Error just now, heh heh.

 

[Doc Al]

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?

The relevant fact is that an outside force acted on the trolleys. That produces a change in momentum. The fact that the trolley just bounces off the spring with the same energy does not change the fact that its momentum was reversed.

2. (This is a supplementary question ) The yellow star question in image 2. My workings in image 3. Is there any shorter solution?

Your solution looks good. I don't think there's a shorter way. I would have done the same.

 

[coconut62]

Thank you.