6. A 3.0 kg toy truck moves freely along a track at and collides with a 1.0 kg toy car that is at rest. After the collision, the two cars stick together. What is their velocity after the collision?
   A.
   B.
   C.
   D.

To find the velocity after the collision, we use the principle of conservation of momentum. The initial momentum of the toy truck is which equals . The toy car is stationary, so its momentum is . Therefore, the total initial momentum is .

After the collision, both toys stick together, forming a combined mass of . Setting the initial momentum equal to the final momentum gives us:

Solving for :

Thus, the correct answer is A. .

You’d be surprised to know how often momentum conservation comes up outside of physics classes! Ever seen a game of pool? When the cue ball hits another ball, it transfers momentum. The underlying physics ensures that no matter how chaotic things get on the table, the total momentum before and after the collision remains constant.

To avoid mistakes in problems like this, always double-check the mass and velocities involved before doing any calculations. A common pitfall is forgetting that momentum is a vector, which means you need to keep track of the direction too. Remembering to apply the conservation principle carefully will help you strike the right answers every time!