Physics, asked by Anonymous, 1 year ago

Two particles A and B initially at rest, move towards each other mutual force of attraction. At the instant when teh speed of A is V and speed of B is 2V, the speed of centre of mass is

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Answered by TPS

543

Initially both A and B were at rest. So momentum of centre of mass is 0. And it is given that there is no external force. Hence no change in momentum of centre of mass. And it will remain 0. As momentum(product of mass and velocity) is zero ans mass is not zero, velocity must be 0.

TPS: Often it is mistaken that when the system of bodies are moving, the CM should move. That is not correct.

TPS: For example, let a bomb was at rest. Let it explode into 2 parts of equal mass. One flies towards positive x direction and the other flies towards negative x direction. As their masses are same, they must fly at same velocity and hence "the sum of their momenta"(=momentum of CM) will be 0.

Anonymous: so whenever I find momentum to be 0 then CM will be at rest

Anonymous: isn't it

TPS: Only if the system contains one particle. If the system has more than one particles, then sum of their momenta should be 0 for the CM to be at rest.

Anonymous: how can there be any system with one particle?

TPS: When you run alone, you form a system with only one person.

Anonymous: yeah. thanks a lot you prove to be a helping hang

Anonymous: *hand

TPS: Thanks. I try my best.

Answered by ha7rs8hi6la

149

We know that,

A and B initially at rest

So, Pi=0

And no external force act on the system so,

Pi=Pf

That's why velocity of centre of mass is 0

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