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###### Ram Prasath posted an Question
October 25, 2021 • 18:35 pm 30 points
• IIT JAM
• Physics (PH)

## Someone explain this method ?

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• ##### Rishabh

Directions opposite hai velocity ki isiliye negative sign aaya

if you

• ##### Isha lohan 1

As the question says, all three masses will stick together, after collision, it will become one body moving with some velocity V, which have two components Vx and Vy. This is what first line of solution says. Now in second part of solution, they have used the law of conservation of momentum. As there are no external forces on this system, the net linear momentum will be conserved. As velocity has x and y components so, here we have conserved the momentus also in x and y direction separately. By using the conservation of momentum law, we here find out the final velocity in vector form, of one final body. Is there still any more doubt you have in any particular line or step? Please do ask.

why they use negative signs. and why did they write mv cos theta?

we know that momentum of a body is equal to mass*velocity. and x-velocity for mass2 here is 2u cos theta in negative x direction. similarly y-velocity of mass 2 is in negative y direction. but y projection of mass 3 is u sin 60, but it points in positive y direction. As velocity components have cos or sin terms, therefore we are writing mvcos theta.