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# M1 Question - Momentum watch

1. Q. A block of mass 3kg travelling at 5ms-1 catches up another block of mass 7kg travelling at 2ms-1 along the same line and in the same direction.
a) If after the collision the second block has increased its speed to 3ms-1, what is the speed of the first block?
b) You are given that the difference between the speeds of the two blocks after the collision cannot be greater than the difference of the speeds before the collision. Deduce that the speed, v ms-1, of the first block after the collision must satisfy the inequalities 0.8 <=v<=2.9.

I have done a) - just a simple standard momentum question - but how do I go about doing part b) ...?

If given that the difference in speeds of the two blocks cannot be greater than that of the two blocks before the collision then we can generate the inequality that:

mod(V(2)-V(1)) <= 3 but I am not sure if this helps at all. Can someone help?..
2. anyone...?
3. (Original post by Mathematicus65)
anyone...?
The lower bound follows from the conservation of momentum.
I'm heading out now, and haven't had a chance to find the upper bound, hopefully that'll get you started.
4. For A i got a result of 8/3 which is roughly 2.6
so difference before>difference after
5-2=3 and 3-(8/3)=0.3 roughly

so far putting that together i get this 0.3≤v≤3
but i'm not sure how to reach that inequality they've given
5. I got the lower bound, attached my working if that helps. I basically wrote an equation for the velocity of V2 using conservation of momentum and subbed that ijnto your inequality

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6. (Original post by pineneedles)
I got the lower bound, attached my working if that helps. I basically wrote an equation for the velocity of V2 using conservation of momentum and subbed that ijnto your inequality

Posted from TSR Mobile
where's your working? xD
7. (Original post by thefatone)
where's your working? xD
Lol, im using a tablet and struggling a bit
Here it is, i did ot in a rush and im tired so i may have buggered aomething up

Um, in hindsight, this isnt very helpful as I did some of the steps in my head.
8. (Original post by pineneedles)
Lol, im using a tablet and struggling a bit
Here it is, i did ot in a rush and im tired so i may have buggered aomething up

Um, in hindsight, this isnt very helpful as I did some of the steps in my head.
i think you use this working, but consider when the other blocks speed becomes 0?
9. (Original post by drandy76)
i think you use this working, but consider when the other blocks speed becomes 0?
Apologies for hijacking your post, I wanted to update my working because it wasn't clear at all.

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