Newton’s First Law: no Force, no Change in MotionLaw 1
Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.
He immediately adds, tying this in precisely with Galileo’s work:
Projectiles persevere in their motions, so far as they are not retarded by the resistance of the air, or impelled downwards by the force of gravity.
Notice that here “persevere in their motions” must mean in steady speed straight line motions, because he is adding the gravitational acceleration on to this.
This is sometimes called “The Law of Inertia”: in the absence of an external force, a body in motion will continue to move at constant speed and direction, that is, at constant velocity.
So any acceleration, or change in speed (or direction of motion), of a body signals that it is being acted on by some force.
Let's study the "skater" to understand this a little better.
This law is the same reason why you should always wear your seatbelt.
