Question

# Two particles which are initially at rest, move towards each due to mutual attraction. If their speeds are v and 2v at any instant, then the speed of centre of mass of the system will be

A
2v
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B
1.5v
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C
0
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D
v
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Solution

## The correct option is D 0The figure shown below represents the velocity of particles moving under influence of attractive forces. velocity of c.o.m is given by, →vcom=m1→v1+m2→v2m1+m2 Since particles are starting from rest, in initial case: →vc.o.m=0 and →v1=→v2=0 As they starts moving towards each other under the influence of mutual attraction, taking both particles as system attractive forces become internal for the system. ⇒→Fext=0 for the system of particles, applying newton's second law: →Fext=m→acom Hence →acom=0 ⇒→vcom=constant Therefore velocity of COM will not change. ⇒vi=vf=0 Speed of COM of the system at any instant will be Zero.

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