Newton's First Law: If the forces on an object are ________, it’s either ________ or moving at ________ speed.

Answer 1

Newton's first law: if the forces on an object are balanced, it either at rest or moving at constant speed.

My comments on that definition.

First of all, if somebody wants to state the law in such a primitive form the last word should be velocity. It has a direction and magnitude. If you said just speed, the body could move not necessarily along a straight line.

Frankly speaking, this statements follows from Newton's second law (F = ma, if F = 0, then a = 0, as a result the body moves with constant velocity or not moving at all).

But Sir Isaak Newton used to say that you do not need 2 theories, if one theory explains everything. Does it mean he contradicted himself?

The reality is that Newton's first law - is a law about existence of inertial reference frames. There are many different reference frames around us. And if we observe the motion of different bodies in them, they are all in complicated motion. But there are some reference frames, in whish the motion is the simplest one. In these frames of references if the net force acting on a body is zero, then the body is at rest or moves with constant velocity along a straight line. We call those frames inertial reference frames. And why do we need them? Because in inertial reference frames the motion of the body has the simplest form!!!