Question

A wagon 8kg travelling due east with a velocity of 10m/s collides with a second wagon of mass 12kg travelling due west at 2m/s. After collision the wagon moves off at 3m/s due east. Determine the magnitude and direction of the velocity of the 12kg wagon after collision.

Answer 1

M1*V1 + M2*V2 = M1*V3 + M2*V4.
8*10 - 12*2 = 8*3 + 12*V4.
80-24 = 24 + 12V4.
12V4 = 32, V4 = 2.67 m/s, East.

Answer 2

How come there is a negative 8*10-12*2=8*3+12*V4

instead 8*10+12*2=8*3+12*V4