A body of mass 5 kg moving with a speed of 3ms−1 collides head on with a body of mass 3 kg moving in the opposite direction at a speed of 2ms−1. The first body stops after the collision. Find the final velocity of the second body.
A
3ms−1
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B
5ms−1
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C
−9ms−1
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D
30ms−1
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Solution
The correct option is D3ms−1
Given:––––––––
m1=5kg
m2=3kg
u1=3ms−1
u2=−2ms−1
Before collision, the 2nd body is moving backwards. Hence its velocity is negative.
v1=0ms−1
After collision, the 1st body stops moving. So its velocity is zero.
Tofind:v2=?
Solution:–––––––––––
m1u1+m2u2=m1v1+m2v2
Substituting the values, we get,
(5×3)+(3×(−2))=5×0+3×v2
∴15−6=3×v2
∴9=3v2
∴v2=9÷3=3
∴v2=3ms−1
Since we got a positive value for v2, it means that the 2nd body moves forwards after collision.
Velocity being a vector quantity, the direction is also important.
After collision, the 2nd body moves with a velocity 3ms−1 in the forward direction.