But what keeps the train moving? I know the answer to this question is inertia, but intuitively it makes sense that there must be some force that is making the object continue to move, even at a constant velocity. I guess a better question is do we know why objects with no net force can remain in motion? Like, it makes sense to me that when net force = 0 = no net movement, but not the constant velocity part.

Yes, but why does only change in motion require external force, and not the motion itself? How can an object like Earth just move in space with constant tangential velocity? Intuitively, it makes more sense to me that something is exerting force on the earth to make it move at all. If I throw a ball in space, I know my hand exerts force on the ball for it to accelerate, but when I let go, it will keep moving forever in a straight path, with no force acting on it. How is that?

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Also, when I push a box on a table at constant velocity, my applied force is equal to the friction force. The net force is 0, but how can the box move? It intuitively makes sense to me that net force = 0 = no movement. I know the answer to this question is inertia, but I don't know this property of matter confuses me. Do I just need to accept this as a fact?

>object can travel at constant velocity without any added force. It in fact it will take a force to slow it.

I understand that the forces acting on the object opposite of its motions causes it to decelerate. With your space example, I know if you give something an initial push, it will keep moving forever unless another net force acts on it. But how does it keep moving forward? I know that something must act on the object for it to change acceleration, but how can it move in the first place without any net force. It intuitively makes sense to me that when net force = 0, then that means no motion, but how can an object move at all when net force =0? Is that just inertia by definition?

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Thanks for the reply, and sorry if my original question was unclear.

thanks for the response! I get that friction and opposite forces causes objects like the skateboard to decelerate, but how is it that the object itself can keep going forward at constant velocity? I am just wondering what causes the object to keep moving if there is no net forces acting on it? Is that part of inertia something we just have to accept is true?

Another example, if I push a box on a table with a constant velocity, the friction force is equal to my applied force, but since they are both equal it makes intuitive sense that net force = 0 = no movement at all. How is it that the object can till move with no net force?