A 1500 kg car traveling at 15m/s to the south collides with a 4500 kg truck that is initially at rest at a stoplight. the car and truck stick together and move together after the collision, what is the final velocity of the two vehicles

3.75 m/s

Explanation:

Here's what you know: The car has mass 1500 kg, and initial velocity 15.0 m/s.  The truck has mass 4500 kg and initial velocity 0 m/s.  The final mass is 6000 kg (4500 kg plus 1500 kg, because they stick together) and we are looking for the final velocity.  Using the formula p = mv, and the concept that p before equals p after, the p of the car plus p of the truck equals the p of them together. So, 1500 x 15 + 4500 x 0 = 6000 x v final.  22500 = 6000 v, and v = 3.75 m/s.  The velocity is positive, and the initial velocity of the car was positive, so the two vehicles together are traveling to the south just like the car was initially.