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# Mechanics momentum question watch

1. Hello. I've been struggling with a question. Any suggestions would be appreciated.

Arnold (mass 50kg) is standing on a trolley (mass 20kg) which is stationary. Bernice is standing behind the trolley. She throws Arnold a package of mass 5kg, which is catches while it is travelling horizontally at a speed of 4 m/s, causing the trolley to start moving forwards. He immediately throws the package horizontally back towards Bernice at a speed of 5 m/s, Find the speed of the trolley at the end of the process.

I've tried to get SUVAT equations of the trolley in terms of the distance between Bernice and the trolley, which I called 'd',

So I got v = 10d/7
u =2/7
t = d/5
a = (50d - 10)/7d

However then when I use v = u + at I come out with:

350d - 70 = 350d - 70

I think I'm either missing something vital, or making an assumption I shouldn't be. Thanks for any suggestions in advance.
2. (Original post by tommy_lees)
I think I'm either missing something vital, or making an assumption I shouldn't be. Thanks for any suggestions in advance.
suvat isn't the appropriate method here. It's simply conservation of momentum.
3. (Original post by ghostwalker)
suvat isn't the appropriate method here. It's simply conservation of momentum.
Ah thank you.

Yes I see, he gained momentum from the throw and the catch.

2/7 from the catch and 5/14 from the throw.

So the total momentum is 9/14.
4. (Original post by tommy_lees)
Ah thank you.

Yes I see, he gained momentum from the throw and the catch.

2/7 from the catch and 5/14 from the throw.

So the total momentum is 9/14.
Agreed - assuming the velocity of the ball when thrown the second time is relative to the ground, rather than the trolley.

When answering the question, don't forget your units, m/s - though I appreciate you may not have bothered on the forum.

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