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6: Newton's First, Second, and Third Laws
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Last time we discussed that an acceleration is caused by a push or by a pull.
Today we will express this more qualitatively in three laws which are called Newton's Laws.
The first law really goes back to the first part of the 17th century.
It was Galileo who expressed what he called the law of inertia and I will read you his law.
"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." And now I will read to you Newton's own words in his famous book, Principia. "Every body 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." Now, Newton's First Law is clearly against our daily experiences.
Things that move don't move along a straight line and don't continue to move, and the reason is, there's gravity.
And there is another reason.
Even if you remove gravity then there is friction, there's air drag.
And so things will always come to a halt.
But we believe, though, that in the absence of any forces indeed an object, if it had a certain velocity would continue along a straight line forever and ever and ever.
Now, this law, this very fundamental law does not hold in all reference frames.
For instance, it doesn't hold in a reference frame which itself is being accelerated.
Imagine that I accelerate myself right here.
Either I jump on my horse, or I take my bicycle or my motorcycle or my car and you see me being accelerated in this direction.
And you sit there and you say, "Aha, his velocity is changing.
"Therefore, according to the First Law, there must be a force on him." And you say, "Hey, there, do you feel that force?" And I said, "Yeah, I do! "I really feel that, I feel someone's pushing me." Consistent with the first law.
Perfect, the First Law works for you.
Now I'm here.
I'm being accelerated in this direction and you all come towards me being accelerated in this direction.
I say, "Aha, the First Law should work so these people should feel a push." I say, "Hey, there! Do you feel the push?" And you say, "I feel nothing.
There is no push, there is no pull." Therefore, the First Law doesn't work from my frame of reference if I'm being accelerated towards you.
So now comes the question, when does the First Law work? Well, the First Law works when the frame of reference is what we call an "inertial" frame of reference.
And an inertial frame of reference would then be a frame in which there are no accelerations of any kind.
Is that possible? Is 26.100...
is this lecture hall an inertial reference frame? For one, the earth rotates about its own axis and 26.100 goes with it.
That gives you a centripetal acceleration.
Number two, the earth goes around the sun.
That gives it a centripetal acceleration including the earth, including you, including 26.100.
The sun goes around the Milky Way, and you can go on and on.
So clearly 26.100 is not an inertial reference frame.
We can try to make an estimate on how large these accelerations are that we experience here in 26.100 and let's start with the one that is due to the earth's rotation.
So here's the earth...
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