Can projectile motion under gravitational fields be considered an example of collision? It is given in my book that the scientific definition of collision is that two or more bodies are said to collide when their motion is affected by the force they exert upon each other. For Eg. When two protons are moving towards each other, they change their direction of motion away from each other due to electrostatic repulsion.
So can this definition be extended to state that when a projectile changes its expected linear trajectory to a parabola due to the gravitational force, it is colliding with the earth??
 A: Although in a very broad sense this could indeed be viewed as a collision, this is not what is usually meant when this term is applied in physics. Collision usually implies that the two objects interact only within a short span of time and/or within a restricted region of space. Even when they interact via potentials/interactions that cannot be considered localized in space and/or time, one usually takes the limit of the two objects incident from infinitely far away infinitely long time ago, so that their initial behavior could be treated as that of non-interacting object. This is, e.g., the case when one talks about a collision of two protons.
Thus, the projectile motion cannot be meaningfully considered as a collision, since the projectile has always been too strongly influenced by Earth to consider it independent. On the other hand, one could discuss the impact of the projectile as a collision, since the impact happens within a short period of time and specific location in space, even though the projectile was never really in a free flight.
What one could discuss as a gravitational collision is, e.g., a comet passing near the Earth, and being deflected, even without direct impact.
A: I believe in physics a collision is defined as direct physical contact between objects. In that context it would appear that the interaction of an object with a field (electrical or gravitational) wouldn’t constitute a collision.
Hope this helps
A: That's why no one in the very history of mankind have ever really touched another human being, or anything at all: if that happens, oh boy what a catastrophic thing it would be!
You are right.
There are some people that work really hard trying to come up with good theories and good experiments to put your proton collision on equal footing to your "gravity collision". As for the protons, we understand that some virtual exchange of photons is (partially) responsible for this change of paths.
We don't always bother to have our paths changed, unless we are very close to other stuff (when you try to touch someone, for instance). That is due (again, partially) to the fact that ordinary-day-to-day matter is neutral in a macroscopic sense: you only feel the charges if you are much nearer to them, otherwise it is "screened" by their surrounding mates.
There is positive and negative: lack and presence of charge.
But gravity... It doesn't seem to come in more than one flavor: all it takes is inertia (as long as General Relativity is concerned). Therefore, we can't screen those effects in a daily basis, and so gravity cannot but "collide" with us all the time.
But please, be sure to understand that classical collision such as billiard-like are emergent phenomena, and that - again, as far as we know - the fundamental thing that goes is an electromagnetic interaction.
Any questions are welcome, and so is any critique/correction.
A: If you go by that definition in your book, then literally every interaction in the universe is a collision. But then, why have two words: "interaction" and "collision" for the same thing?
Don't follow that book definition. Collision is whatever it means in plain english, i.e. two bodies straight up bumping into each other. It is usually two bodies approaching each other until they get too close when repulsive forces take over. A ball falling toward ground is not any undergoing collision until it actualy gets too close to the ground when repulsive forces take over.
