Force and Newton's three Laws

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Force and Newton's three Laws

We have all known from a young age that an acceleration is caused by a push or a pull. Today we will express this more qualitatively in 3 laws which are called Newton's Laws.

Newton's first Law.

Newton's first law really harks back to the early 17th century. It was Galileo who expressed what he called the law of inertia, he stated:

"A body at rest remains at rest and a body in motion continues to move at constant velocity along a straight line unless acted upon by an external force."

Now you can read Newton's own words from his famous book Principia:

"Everybody 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 upon it."

The problem is that Newton's 1st law goes clearly against our daily experiences; things that move don't move along a straight line, nor do they continue to move for ever. The reason for this is gravity; and there is another reason too, even if you remove gravity there is still friction, and there is air drag. So things will always come to halt.

But we believe, though, that in the absence of any forces, that an object, if it had a velocity, would continue along in a straight line forever, and ever, and ever.

Warning advanced ideas may be found in the following ...

However Newton's first law, this profoundly fundamental law, does not hold in all reference frames. For instance, it doesn't hold in a reference frame which itself is being accelerated. Let me try to explain.

Imagine I am being accelerated whilst typing this - I'm on my Acme physics-o-computer-cycle perhaps - you, sitting in lab 6 watch me being accelerated in a known direction, say directly towards you. You immediately say to yourself, 'Aha! His velocity is changing; therefore, according to the Newton's first law, there must be a force acting on him! You can't contain yourself and shout, "Hey there Mr Dey, can you feel the force?" I would reply, "Yeah I do, I really do. I feel something pushing me through the seat." This is consistent with the first law. Perfect, the first law works for you observing from lab 6.

Now the observation switches, I'm watching you looking at me, and am still being accelerated in the same direction. You all appear to come towards me being accelerated in the opposite direction. I think 'Aha! The 1st law should work so you people should feel a push.' I'm terribly excited by this breakthrough and shout, "Hey there! Do you feel the force?" You reply "I feel nothing! There is no push, there is no pull."

Therefore the 1st Law doesn't work for my frame of reference if I'm being accelerated towards you.

So the obvious question is; when does the first law work?
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Well the first law works when the frame of reference is what we call an 'inertial frame of reference.' An inertial frame of reference would then be a frame in which there are no accelerations of any kind. Is this possible? Is the room you are sitting in, lab 6, an inertial reference frame?

Well firstly, the Earth rotates about its own axis and your room goes with it; that gives you something called a centripetal acceleration (which you'll learn about at A21).

Secondly, the Earth goes around the sun; that too gives a centripetal acceleration, ...

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