Conservation of momentum
In all isolated systems, total momentum is conserved:
Car A (2.5 kg) begins at 1.5 m/s [fwd], while Car B (1.5 kg) beings at rest. Car A collides into Car B, and they stick together. What is the velocity of AB after the collision?
We begin with the conservation of momentum,
into which we substitute the individual momenta:
Now we can substitute the masses and velocities with
and solve for the final velocity:
Substituting the known values where [fwd] is positive, we have
therefore the final velocity is 0.94 m/s [fwd].