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# M2 Impulse Watch

1. Attachment 517345517347
I'm stuck on the bottom bit of question 11- where you need to find the speed after the second collision where the KE is 4/5mu2?
I've included my attempt but it is in terms of q (the speed of ball B) which is incorrect?

Thanks
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2. (Original post by AvM4N)
Attachment 517345517347
I'm stuck on the bottom bit of question 11- where you need to find the speed after the second collision where the KE is 4/5mu2?
I've included my attempt but it is in terms of q (the speed of ball B) which is incorrect?

Thanks
Use conservation of linear momentum to get another equation in p and q.
3. (Original post by Zacken)
Use conservation of linear momentum to get another equation in p and q.
So 4MU - 2MU= -4Mp+2Mq ???
4. (Original post by AvM4N)
So 4MU - 2MU= -4Mp+2Mq ???
Something like that, yeah.
5. (Original post by Zacken)
Something like that, yeah.
But when I do that i get-
q= u - 2p
so p= √10/5u - 1/2(u - 2p)= √10/5u - 1/2u + p, so the P cancels out
what have i done wrong??
6. (Original post by AvM4N)
But when I do that i get-
q= u - 2p
so p= √10/5u - 1/2(u - 2p)= √10/5u - 1/2u + p, so the P cancels out
what have i done wrong??
Oh dear.

Do you think that ?
7. (Original post by Zacken)
Oh dear.

Do you think that ?
Where have i done that? I thought in order to square root the RHS I had to square root everything??
8. (Original post by AvM4N)
Where have i done that? I thought in order to square root the RHS I had to square root everything??
Yes, you square root everything. But the square root everything does not mean you square root each individual term individually.

It's in your second to last line moving into your last line.

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