You are correct that there is more to it! When you push the air-hockey puck along the surface while riding in the truck, you will observe the puck's motion relative to your position inside the truck. Since there is negligible friction between the puck and the surface, and the truck is moving at a constant velocity, the puck will glide smoothly across the surface in the direction you pushed it.
This observation can be explained through the concept of Galilean relativity, which states that the laws of motion are the same in all inertial frames (frames of reference that are not accelerating). Since both you and the air-hockey puck are inside the truck, which is an inertial frame in this case, the puck will move in a straight line with a constant velocity, relative to the truck and you, until it is stopped by an external force (like hitting the edge of the air-hockey table).
If someone were to observe the situation from outside the truck, however, they would see the air-hockey puck moving forward with the combined velocities of the truck and the velocity it gained from your push. This is because, in Earth's frame of reference, the puck's motion will be influenced by the truck's motion as well.
You push an air-hockey puck along a surface with negligible friction while riding in a truck as it moves at a constant velocity in Earth's frame of reference. What do you observe? Why?
I'm tempted to say that the puck moves forward because that's all I can think of, but I really don't think that's right; there should be more to it... :(
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