Sir Isaac Newton’s First Law of Motion states that an object in motion will remain in motion until an external force is applied to it.
As an example, we could use a ball thrown at a wall. The ball is in motion, travelling towards the wall until it hits it. The wall is an external force applied to the ball, which stops it.
However, if the ball missed the wall it wouldn’t carry on travelling indefinitely. It would eventually fall to earth under the force of gravity. Gravity, although invisible, is also an external force. It draws the ball downwards, which scientists call a ‘turning moment’.
Now, let’s try to give an example of Newton’s First Law working in an everyday setting. Take a man walking down a straight road. For argument’s sake we’ll call this man ‘the husband’. The husband is on his way from his house to the Pig and Whistle public house, to take part in a darts match. Unless some external force is applied, the husband should carry on unimpeded to the pub.
But at that very moment he sees, rounding the far corner, another person. For argument’s sake we shall call that person ‘the wife’, and she’s carrying some rather heavy shopping bags.
Now, although he has not been stopped by an external force, the husband does execute a turning moment, ducks down a side alley, and carries on to the pub by a series of further turning moments. This seems to contradict Newton’s First Law, but you must consider that it’s just an aberration of the main rule.
Later, in the early hours of the following morning, the person we’ve called the husband is returning home on as straight a course as he can manage after drinking 15 pints of Scrutocks Real Ale. However, he does execute a number of turning moments in all directions, including backwards and lying down.
Eventually, as he reaches his front door, singing loudly but rather off key “For he’s a jolly good fellow,” he’s stopped dead in his tracks by an external force. In this case, it’s a frying pan to the fizzog, wielded by the person we called, for the sake of argument (even though in this instance there really was no argument) the wife.
So, even in this complicated example, Newton’s First Law is proved correct. The motion of the object (husband) was halted by an external force (the frying pan).