Why is the net force acting on a massless body zero? I know that massless bodies can accelerate (in theory) even with the net force equaling to zero. But, why cannot there be a net force on a massless object? Why does it always have to be zero as a resultant in the end?
I'm talking about object whose mass is assumed to be zero, i.e. $m\to0$
 A: If we start from Newton's Second Law
$$F=ma,$$
then we can see that if the mass is zero, then the total force must be as well:
$$F = ma = (0)a = 0.$$
Then again, this leaves acceleration completely undefined since $a = F/m = 0/0.$ This is why every massless object in a physics problem--whether rope, spring, or pulley--is attached to something with mass. The combined object has a non-zero total mass and so can have a sensible acceleration. The massless object simply follows the massive object it is attached to. Newtonian physics doesn't work with massless objects. You can try taking limits as mass goes to zero, as was attempted when the deflection of light by gravity was predicted according to Newtonian gravity theory, but this resulted in an incorrect prediction.
A: Massless objects cannot be accelerated. You cannot apply force on them. Massless object ALWAYS move on the shortest path between two points. The only two things that you can do to a massless particle are 1) in presence of gravity the shortest path between two points is not a straight line (i.e. Euclidian straight line) so the path may look curved to an observer (like you and me)
2) it can be absorbed and emitted by massive particles. For example. Photons in an optic wire seems to be moving on a curved path but what actually happens is that the photon is absorbed and re emited by the atoms.
A: In truth, the answer to your question is that no massless objects exist.
Sometimes an object's mass is negligible compared to something else - such as strings carrying weights in a pully system, or such as a human being on the surface of the earth - and then we can safely ignore it in our calculation with no significant change in our results. Other times that same mass is not negligible when compared to other more similar-scaled objects - such as a heavy chain carrying those weights, or such as a human being compared to another human being.
In short: Massless is another way of saying so comparatively small that it can be ignored.
Such an object can still accelerate, though, and that similarly takes comparatively negligible force. We often indicate that as $F\approx 0$ force. But in reality, there is a non-zero force, albite small, when a massless object accelerates because that object actually does have a mass, albite small.
