# Newton's First Law of Motion

## Main Question or Discussion Point

Newton's first law says that when an object is at rest or moving in constant velocity, it will stay that way unless external force is applied. So, net force is zero.

What I don't get is on an object moving in constant velocity. How is the object begin moving? Is netforce applied initially before it reaches its constant velocity?

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phinds
Gold Member
2019 Award
Newton's first law says that when an object is at rest or moving in constant velocity, it will stay that way unless external force is applied. So, net force is zero.

What I don't get is on an object moving in constant velocity. How is the object begin moving? Is netforce applied initially before it reaches its constant velocity?
How it got that way is not part of the first law of motion, but yes, to get something moving on, for example, a flat surface, you have to give it a push. If the surface is frictionless it just keeps moving. If you don't give it a push it just keeps sitting there.

A.T.
How is the object begin moving?
The object itself doesn’t have to begin to do anything. You can analyze any object in a reference frame where it always has been moving.

Cruz Martinez
Consider a frictionless surface.

Is netforce applied initially before it reaches its constant velocity?
Yes, some force must have been applied to the object in it's past in order to accelerate it to the velocity which it presently has.
(In relation to the same given frame of reference)

Before Newton, people didn't know or realize that things could move on their own. He taught people that if something gets a push in one direction, it will need a push is the opposite direction to stop.

jbriggs444
Homework Helper
2019 Award
Yes, some force must have been applied to the object in it's past in order to accelerate it to the velocity which it presently has.
(In relation to the same given frame of reference)
Why would one assume that in the infinite past that the object was at rest in an arbitrarily selected frame of reference?

Why would one assume that in the infinite past that the object was at rest in an arbitrarily selected frame of reference?
Because it's unlikely that the moving object does have an infinite past.
It could of course be that the object was formed out of matter which already has momentum - like the solar system,
However the momentum in that case hasn't come from nothing, it's momentum that was previously included in a nebula, and now is included in the Sun.

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A.T.
It could of course be that the object was formed out of matter which already has momentum
What do you mean by "it could be"?. It is absolutely certain that there are inertial frames (infinitely many) where it always had momentum.

Poor wording on my part, yes I agree there certainly are frames where an object always had momentum,
In relation to the original question," How is the object begin moving , Is net force applied initially before it reaches its constant velocity?",
I was trying to explain that if the object was motionless at one stage and now it isn't, then a force must have been applied to it.
On the hand an object which comes into existence in an already moving condition, and still is moving now, doesn't 'begin to move' and no force need have been applied to it since it started existing.

A.T.