A $2$ kg trolley moving at $6$ m s$^{-1}$ collides with a stationary $4$ kg trolley and the two move off together. Find their common velocity.
A$1.5$ m s$^{-1}$
B$2$ m s$^{-1}$CORRECT
C$3$ m s$^{-1}$
D$12$ m s$^{-1}$
JUPEBPhysicsKinematics and Dynamics
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Why the answer is B, and why the others tempt you.
By conservation of momentum, $2 \times 6 + 4 \times 0 = (2 + 4)v$, so $12 = 6v$ and $v = 2$ m s$^{-1}$. Dividing the momentum by the mass of the second trolley only gives 3 m s$^{-1}$.
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