A 60 kg boy and 40 kg girl stand on skateboards on a frictionless, horizontal surface. The boy pushes the girl away from him. The girl gains a speed of 0.3 m/s during the 0.50 s the boy's hands are in contact with her.
According to the law of conservation of linear momentum, the sum of intial andfinal momentum of a sytem is zero if there is no external force present.
Intially, the boy and girl are at rest. So, pi is zero.
After the boy pushes the girl away, the momentum of the girl becomes pg= 40*0.3 =12Kgm/s
Now according to the law of conservation of linear momentum,
pi=0 and pf=pb+pg
So, the speed with which the boy goes is 0.2m/s.The negative sign is just for the reverse direction as from the girl.
It won't matter who pushes whom, if the girl gets the same speed of 0.3m/s then the boy gets the speed of 0.2m/s.
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