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Work, Energy And Power

Question
CBSEENPH11017294

Discuss two-dimensional elastic collision.

Solution
Consider two bodies A and B of masses m1 and m2 moving along the same straight line along X-axis with velocities and u2 respectively as shown in the figure.


Let, after collision two bodies move with velocities v1 and v2 making angles θ1 and θ2 with initial direction of motion i.e. with X-axis.
Since collision is an elastic collision, therefore, their momentum and kinetic energy remain conserved.
Momentum is a vector quantity, therefore the momentum along X-axes as well as along Y-axes will conserve individually.
By conservation of momentum along X-axis, 
space space space space space space space space space space space space space straight m subscript 1 straight u subscript 1 straight x end subscript plus straight m subscript 2 straight u subscript 2 straight x end subscript equals straight m subscript 1 straight v subscript 1 straight x end subscript plus straight m subscript 2 straight v subscript 2 straight x end subscript 
rightwards double arrow   straight m subscript 1 straight u subscript 1 plus straight m subscript 2 straight u subscript 2 equals straight m subscript 1 straight v subscript 1 cosθ subscript 1 plus straight m subscript 2 straight v subscript 2 cosθ subscript 2  ...(1)
By conservation of momentum along Y-axis,
       straight m subscript 1 straight u subscript 1 straight y end subscript plus straight m subscript 2 straight u subscript 2 straight y end subscript equals straight m subscript 1 straight v subscript 1 straight y end subscript plus straight m subscript 2 straight v subscript 2 straight y end subscript
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rightwards double arrow           straight m subscript 1 straight v subscript 1 sinθ subscript 1 equals straight m subscript 2 straight v subscript 2 sinθ subscript 2                 ...(2)
By conservation of kinetic energy,
  1 half straight m subscript 1 straight u subscript 1 superscript 2 plus 1 half straight m subscript 2 straight u subscript 2 superscript 2 equals 1 half straight m subscript 1 straight v subscript 1 superscript 2 plus 1 half straight m subscript 2 straight v subscript 2 superscript 2       ...(3)
Here, in two-dimension collision, there are four variables v1 v2, θ1 and θ2 to solve, but we have only three equations from this analytical study.
Hence, we cannot find the values of all the variables by analytical method.
To solve the problem, one variable is measured experimentally and the remaining three are calculated from the above three equations.