A body continues in its state of motion unless a force is applied to it. But how does an object stay in motion in the first place? A force must have caused it to move right?

  • $\begingroup$ en.wikipedia.org/wiki/Inertia $\endgroup$ – Wolphram jonny Aug 9 '17 at 14:36
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    $\begingroup$ What if I am sitting in a train and see you standing still on a station platform. Who is in uniform motion? $\endgroup$ – ZeroTheHero Aug 9 '17 at 18:25
  • $\begingroup$ Functions, this is a misconception that may be common among people who haven't studied physics. Newton's first law states that an object will preserve its state of motion (either stationary or moving in a straight line at constant speed) unless acted upon by a net force. It is well worth thinking about this law long enough to internalize it, and it is important to "let go" of the misconception, if you want to advance in your understanding of Newtonian mechanics. $\endgroup$ – David White Jan 22 '19 at 18:06

While you know the statement of Newton's first law, Newton's second law can be used to answer your question. Newton's second law is stated mathematically as $$\vec{F} = m\vec{a}$$

This statement tells us that, for a given object of mass $m$, the acceleration of that object (whether it speeds up, slows down, or travels at a constant velocity), is directly proportional to the net force applied to it. Thus, if a net force is applied to an object initially at rest, the object will accelerate in the direction of the force (so yes, a force is required for an object to begin moving initially). However, if the force is then no longer applied to the object, then $\vec{a} = \frac{\vec{F}}{m} = \frac{0\text{N}}{m} = 0 \frac{\text{m}}{\text{s}^2}$. Thus, the object's speed will not change. So, in short, "staying in motion" (traveling at a constant velocity and following a straight line) is the "default" action of a given object. If the object was speeding up or slowing down, $\vec{F} \neq 0$.


The Physics term is Inertia, but I will try to explain in non-rigorous terms that I think may help you, and in the future may help you with concepts like relativity.

To take an object at rest and put it into motion, that requires a force which accelerates that object and now it has velocity. Inertia says that velocity will remain, unchanged, unless another force acts on it. How you can think of that is all objects are in an inertial frame of reference. When you apply a force and accelerate the object, what you are doing is putting that object into a new inertial frame of reference.

Where some initial concepts of relativity come into play, to the object that you now observe as having velocity, the object itself does not see that. It felt the acceleration, as force. Now, that force is gone, so it no longer feels that. To that object, in its frame of reference, it is at rest. It sees itself as not moving, but is we see it as moving because we are in a different frame of reference, while it it, we look like we are the one that is moving. This is a base concept of relativity.

Now, you will say wait a minute, I feel velocity when I am moving. Actually, no you don't. You are feeling force because we are on Earth. That means we are feeling force from gravity at all times. If we are moving relative to Earth, we also tend to feel drag from friction and some other forces. When we think we are feeling velocity, we are actually feeling these forces attempting to change our velocity, and maybe an car engine providing more force to compensate for them to keep our velocity up. If we were say in space, we would not feel those forces, so we could really experience the full effect of how inertia is described.

  • $\begingroup$ a core property of inertial reference frames is that they are not accelerating. If a force accelerates an object, a coordinate axis that is affixed to that object is no longer inertial. There are also many typos and other possible factual errors in your answer. I would recommend an edit to touch up your answer. $\endgroup$ – UniqueWorldline Aug 9 '17 at 18:10

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