Tutoring high school physics, you get the privilege of retelling Newton’s three Laws.  They can’t be discussed too often.

Newton’s First Law:

If no unbalanced force acts on an object, it either continues moving in a straight line at constant speed, or else remains in its state of rest.

Like his other laws, Newton’s First Law contains some surprises if you examine it closely.  First of all, how could he have predicted it, when nothing on Earth does continue moving at a constant rate when left alone?

Back in high school, one of my textbooks (I can’t remember which one; it was over twenty years ago!) explained that Newton understood the tendency to retain constant velocity by comparing a ball to a brick.  He noticed that when you throw a brick, it stops almost immediately when it hits the ground.  However, a ball might continue moving for a long time after it contacts the ground.

Newton realized that the difference between the ball and the brick is that friction acts more emphatically on the brick.  Given its shape, the brick catches the edges of the ground’s surface.  The ground grips the brick, stopping it.  The ball, with its round shape, makes smooth contact with the ground rather than rubbing along it.  Friction is, of course, resistance to rubbing, so the brick’s movement across the ground is much more affected by friction than is the ball’s.

Newton realized that air resistance is just another case of friction.  Therefore, everything moving on Earth – including the ball – is eventually arrested by friction.  Friction is unbalanced unless you apply force to counteract it.  Hence, a cyclist can continue at a constant speed if he or she is willing to pedal against friction.  However, when that cyclist stops pedalling, the air gradually halts him/her.

In space, there is no air resistance, so objects can (and do) continue their straight line motion forever.  Gravity can make things change direction, but gravity is then an unbalanced force.  Unbalanced means that there is not an equivalent force opposing it.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.