Physics

An Explanation of Newtons three Laws of Motion



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Have you ever tripped over a paving slab? Exhausted yourself trying to push start your car? Been whacked by a swing door that you pushed open a little too enthusiastically? If so, you'll have had a perfect demonstration of Newton's Laws of Motion. Although it may seem that inanimate objects are conspiring against us, they are, of course, simply obeying the underlying mechanical processes of the universe. Newton outlined these fundamental processes in the Philosophi Naturalis Principia Mathematica of 1687.

1. THE FIRST LAW OF MOTION - The Law of Inertia

As Newton put it, "every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed". In other words, stationary objects remain stationary and objects in motion remain in motion unless an external force compels them to do otherwise.

The ornaments on your shelf do not spontaneously move, unless you knock them over whilst dusting, or there's an earth tremor, or the screws holding the shelf in place rust to such an extent that they break and the shelf falls off the wall. If the shelf does fall off, it won't fall halfway and then stop. It will keep going until it encounters another force - the floor.

Likewise, if you're lying in bed, it takes your own muscular force to overcome your inertia and send you out for a walk, and you'll keep walking until you reach your destination, when the same muscular force will cause you to stop, or you trip over a paving slab.

So why doesn't a ball keep going forever once you've thrown it? Even if there's nothing in its way, it eventually stops moving because it encounters air friction and gravity. If you were to throw it in deep space, it would keep going forever, assuming it didn't meet the gravity of a star or planet.

2. THE SECOND LAW OF MOTION - F=MA (Force equals Mass x Acceleration)

Closely related to the first law is the observation that acceleration is the product of force acting on a mass. The greater the mass of an object, the more force is required to get it moving. Knocking an ornament of a shelf requires relatively little force, but ripping the whole shelf off the wall requires quite a lot of it.

In the same way, trying to push start your car may well require more muscular force than you're able to give it. You might need somebody else's muscular force alongside yours, or if you're lucky, you'll be on a downward slope, so gravity can lend a hand. Once the car starts moving, of course, the first law takes over.

3. THE THIRD LAW OF MOTION - For Every Action, There's An Equal And Opposite Reaction

If you push something, it pushes you right back, and just as hard. This is the classic swing door scenario. That little bit of slapstick actually illustrates all three laws quite nicely. The swing door is perfectly stationary (the first law) until you push it open. The acceleration of the door is proportionate to how enthusiastically you pushed it (the second law). It keeps moving until it encounters another force - the limit of its hinges, or the wall behind it - whereupon it swings back and catches you painfully in the face (the third law). This physics lesson may be no consolation if it actually happens to you, of course.

These three laws may seem like obvious common sense. Indeed, they were known and talked about long before Newton. Chinese and Arabic philosophers had discussed the law of inertia, for instance, as had Galileo, as Newton himself acknowledged. Newton's great genius was to clearly perceive the commonplace, and express it so concisely and elegantly in these three laws.

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