Understanding the meaning of mass in Newtonian mechanics

Question

So, I was reading a book on mechanics and there I found the following notion about mass. It says that "Experience shows that every body resists any effort to change its velocity,both in magnitude and direction.This property expressing the degree of unsusceptibility of a body to any change in its velocity is inertness and mass is the measure of this inertness."

Now, that is how it describes mass. But the problem which arises is that I'm unable to comprehend what does it mean to resist. Often an example is given wherein a bus suddenly starts moving or stops and we resist any change in velocity by bending in front and something like that. But, the thing here is that we do so because there is friction between our feet and bus floor, and our body is not rigid.

But considering a situation wherein a mass is moving uniformly in space and a force is acted upon it, it doesn't shows any resistance and simply accelerates.

Thinking about it, I've reached a conclusion,which I want you to tell me if its true or not. The conclusion I've drawn is that the body resists any change in its velocity by itself exerting a force (equal and opposite) on the cause of force on it. That is, resistance is basically a manifestation of Newton's third law.

I don't know whether I'm correct or not. So, I request your help. A thing I'd like to mention is that I'm only talking about classical mechanics and want you to be adhered to that as I don't have the required knowledge of quantum mechanics or Einstein's relativity.

Answer 1

Take two bodies and apply the same force. The one with the greater mass will accelerate less: It has resisted your attempt to change its state of motion to a greater degree. In the (obviously non-physical) limit of infinite mass, the body would become immovable (or more precisely, unaccelerateable).

Answer 2

Here the resistance is not an actual force but just an English word used to describe the behaviour of the body. I would say that the body tends to remain at rest or constant velocity unless an external force is applied.

"But, the thing here is that we do so because there is friction between our feet and bus floor, and our body is not rigid."

Try thinking what would happen if you were a floating inside a moving bus and the bus stops. And do ride an actual bus before that carefully observing what happens when the bus suddenly starts or stops.

"But considering a situation wherein a mass is moving uniformly in space and a force is acted upon it, it doesn't show any resistance and simply accelerates."

If this was true we would have been able to accelerate any mass with just a little force up to infinite speeds.

(And even here the body shows no actual force as resistance it just tends to remain as it is.)

And to conclude your conclusion is certainly wrong.