A particle of mass m1=4 kg moving at 6ˆi ms−1 collides perfectly elastically with a particle of mass m2=2kg moving at 3ˆi ms−1
A
velocity of centre of mass (CM) is 5ˆi ms−1
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B
the velocities of the particle relative to the centre of mass have same magnitude.
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C
speed of individual particle before and after collision remains same.
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D
the velocity of particle relative to CM after collision are →v1f/cm=−ˆims−1,→v2f/cm=2ˆims−1
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Solution
The correct options are A velocity of centre of mass (CM) is 5ˆi ms−1 D the velocity of particle relative to CM after collision are →v1f/cm=−ˆims−1,→v2f/cm=2ˆims−1 Vcom=m1v1+m2v2m1+m2=4×6^i+2×3^i4+2=5^i→v1in/com=→v1−→vcom=6^i−5^i=^i→v2in/com=→v2−→vcom=3^i−5^i=−2^i
From the frame of COM the velocities in elastic collision are just reversed.
So, →v1f/com=→v1−→vcom=6^i−5^i=^i→v2f/com=→v2−→vcom=3^i−5^i=−2^i