
(a)
Prove that ξlab=(π−θcm)2
(a)

Answer to Problem 14.28P
The angle is ξlab=(π−θcm)2.
Explanation of Solution
Consider particle 1 with initial momentum plab 1 and particle 2 with zero initial momentum. Final momentum of particle 1 is p'lab 1 and final momentum of the particle 2 is p'lab 2.
According to law of conservation of momentum
plab 1+0=p'lab 1+p'lab 2p'lab 2=plab 1−p'lab 1
From the figure, AC denotes plab 1, AD represents p'lab 1 and p'lab 2 is represented by AC−AD
From the triangular law of vectors, AC−AD=DC
BD and BC is the radius of the circle. Thus the angles opposite to these sides are equal.
From the triangle ΔBCD,
∠BCD=∠BDC=ξlab
And,
∠DBC=θcm
The sum of all three angles of the triangle is equal to π rad.
Therefore for ΔBCD
ξlab+ξlab+θcm=π2ξlab+θcm=πξlab=π−θcm2
Conclusion:
The angle is ξlab=(π−θcm)2.
(b)
Show that in the case of equal masses, the angle of these outgoing particles in elastic collision is 90°.
(b)

Answer to Problem 14.28P
For the collision of equal masses, ξlab+θlab=π2
Explanation of Solution
Write down the expression connecting θlab and θcm
tanθlab=sinθcmλ+cosθcm
λ=1 for m1=m2.
Then,
tanθlab=sinθcm1+cosθcm=2sinθcm2cosθcm22cos2θcm2=tanθcm2
That is,
θlab=θcm2θcm=2θlab
Substitute π−2ξlab for θcm
ξlab=π−θcm2ξlab=π2−θlabπ2=ξlab+θlab
Hence proved.
Conclusion:
For the collision of equal masses, ξlab+θlab=π2
(c)
Prove ξlab+θlab=π2 using law of conservation of momentum.
(c)

Answer to Problem 14.28P
The expression ξlab+θlab=π2 has been proven using the law of conservation of momentum.
Explanation of Solution
Apply law of conservation of momentum in horizontal direction
plab1=p'lab1cosθlab+p'lab2cosξlab
Along vertical direction,
0=p'lab1sinθlab−p'labsinξlabp'lab1sinθlab=−p'lab2sinξlab
Apply the law of conservation of energy
p2lab12m1=p'2lab12m1+p'2lab22m2
Substitute m1=m2,
p2lab1=p'2lab1+p'2lab2
Then,
p'2lab1+p'2lab2=(p'lab1cosθlab+p'lab2cosξlab)2p'2lab1+p'2lab2=p'2lab1cos2θlab+p'2lab2cos2ξlab+2p'lab1cosθlabp'lab2cosξlab0=p'2lab1(1−cos2θlab)+p'2lab2(1−cos2ξlab)−2p'lab1p'lab2cosθlabcosξlab0=p'2lab1sin2θlab+p'2lab2sin2cos2ξlab−p'lab1p'lab2cosθlabcosξlab
On further simplification,
p'2lab1sin2θlab+p'2lab2sin2cos2ξlab=p'lab1p'lab2cosθlabcosξlab(p'2lab1sin2θlab+p'2lab2sin2cos2ξlab)2=2p'lab1p'lab2(cosξlabcosθlab+cosξlabcosθlab)(p'2lab1sin2θlab−p'2lab2sin2cos2ξlab)2=2p'lab1p'lab2(cosξlabcosθlab+cosξlabcosθlab)0=2p'lab1p'lab2(cosξlab+cosθlab)
Here 2p'lab1p'lab2≠0. Therefore cosξlab+cosθlab=0
Take cosine inverse on both sides
ξlab+θlab=cos−10=π2
Conclusion:
The expression ξlab+θlab=π2 has been proven using the law of conservation of momentum.
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