Question

when two trucks traveling in different directions approach each an intersection at the same time , on of the trucks must change its speed or direction to avoid a collision. Two airplanes, however, can travel in different directions and cross paths without colliding. Explain how this is possible.

1. This happens because the trucks move in one-dimension, while the airplanes move in 3 dimensions.

### Why the trucks need to change direction or speed while the airplanes don’t?

Well, the trucks move along a street. Thus, we can think that this is an one-dimensional kind of motion.
Where the trucks can go forward, backward, or get out of the line.
In the case of the airplanes, they can move actually in 3 dimensions (the airplane can go upwards, downwards, left, right, etc).
Now, in the one-dimensional case of the trucks there are only two directions (which are opposite) so if the trucks travel in different directions, then the trucks travel into each other, which would cause the collision.
While in the case of the airplanes more directions exists, so there isn’t (necessarily) a collision.