golgiapparatus31
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#1
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#1
https://imgur.com/a/ujYZVWl

Failed to get the correct answer of 9/100 mu^2. Got 3/100 mu^2 instead. I reattempted this question but I'm not able to locate my error

Please see working below:

Conservation of momentum along line joining centres of P and Q
musin30 = mVp + 2mVq
u/2 = Vp + 2Vq

Newton's experimental law: Vq - Vp = 4/5(1/2u) = 2u/5
so Vq = Vp + 2u/5

replacing in first eqn
u/2 = Vp + 2Vp + 4u/5
giving Vp = -u/10, so Vq = 3u/10

Initial KE = 1/2 mu^2
Final KE = 1/2 m(ucos30)^2 + 1/2 m(u/10)^2 + 1/2 (2m) (3u/10)^2 = 47/100 mu^2

giving the loss as 3/100 mu^2
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Interea
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#2
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#2
(Original post by golgiapparatus31)
https://imgur.com/a/ujYZVWl

Failed to get the correct answer of 9/100 mu^2. Got 3/100 mu^2 instead. I reattempted this question but I'm not able to locate my error

Please see working below:

Conservation of momentum along line joining centres of P and Q
musin30 = mVp + 2mVq
u/2 = Vp + 2Vq

Newton's experimental law: Vq - Vp = 4/5(1/2u) = 2u/5
so Vq = Vp + 2u/5

replacing in first eqn
u/2 = Vp + 2Vp + 4u/5
giving Vp = -u/10, so Vq = 3u/10

Initial KE = 1/2 mu^2
Final KE = 1/2 m(ucos30)^2 + 1/2 m(u/10)^2 + 1/2 (2m) (3u/10)^2 = 47/100 mu^2

giving the loss as 3/100 mu^2
I don't know the theory for this topic, but your working out all follows through correctly as far as I can tell - if you know that the method is correct then maybe the mark scheme just forgot to cube the 3 as well in 3u/10? :confused:
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golgiapparatus31
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Report Thread starter 4 weeks ago
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(Original post by Interea)
I don't know the theory for this topic, but your working out all follows through correctly as far as I can tell - if you know that the method is correct then maybe the mark scheme just forgot to cube the 3 as well in 3u/10? :confused:
Thank you! Unfortunately there is no mark scheme, only the answer which is given as 9/100 mu^2

Thanks for checking!
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Justvisited
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#4
(Original post by golgiapparatus31)
https://imgur.com/a/ujYZVWl

Failed to get the correct answer of 9/100 mu^2. Got 3/100 mu^2 instead. I reattempted this question but I'm not able to locate my error

Please see working below:

Conservation of momentum along line joining centres of P and Q
musin30 = mVp + 2mVq
Shouldn't it be cos30? Have I mis-imagined the situation....
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golgiapparatus31
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#5
Report Thread starter 4 weeks ago
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(Original post by Justvisited)
Shouldn't it be cos30? Have I mis-imagined the situation....
Thanks!! That was it. Marked angle wrong on diagram

Got answer now! Thank you
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