Newton’s\u00a0First Law:<\/p>\n
If no unbalanced force acts on an object, it either continues\u00a0moving\u00a0in a straight line at\u00a0constant speed,\u00a0or else remains in its state of rest.<\/em><\/p>\n Like his other laws, Newton’s First Law contains some surprises if you examine it closely.\u00a0 First of all, how could he have predicted it, when\u00a0nothing on Earth does continue moving at a constant rate when left alone?<\/p>\n Back in high school, one of my textbooks (I can’t remember which one; it was over twenty years ago!) explained\u00a0that Newton\u00a0understood the tendency to retain constant velocity by comparing a ball to a brick.\u00a0 He noticed that when you throw\u00a0a brick, it stops almost immediately when it hits the ground.\u00a0 However, a ball might continue moving for a long time after it contacts the ground.<\/p>\n Newton realized that the difference between the ball and the brick is that friction acts more emphatically on the brick.\u00a0 Given its shape, the brick catches the edges of the ground’s surface.\u00a0 The ground grips the brick, stopping it.\u00a0 The ball, with its round shape, makes smooth contact with the ground rather than rubbing along it.\u00a0 Friction is, of course, resistance to rubbing, so the brick’s movement across\u00a0the ground is much more affected by friction than is the ball’s.<\/p>\n Newton realized that air resistance is just another case of friction.\u00a0 Therefore, everything moving on Earth – including the ball – is eventually arrested by friction.\u00a0 Friction is unbalanced unless you apply force to counteract it.\u00a0 Hence, a cyclist can continue at a constant speed if he or she is willing to pedal against friction.\u00a0 However, when that cyclist stops pedalling, the air gradually halts him\/her.<\/p>\n In space, there is no air resistance, so objects can (and do) continue their straight line motion forever.\u00a0 Gravity can make things change direction, but gravity is then an unbalanced force.\u00a0 Unbalanced means that there is not an equivalent force opposing it.<\/p>\n