https://youtu.be/Fc9CoR5im_4
PHYS 1101: Lecture Ten, Part Three
Chapter 4 is the step beyond just describing motion. And it’s what we call dynamics. It’s the categorization effectively asking the question, “How or why do velocities change?” You’ll see why that’s the fundamental question behind dynamics here in a minute.
I’d like to begin this discussion with the first step of asking you to consider what’s your intuition? What do you think is the natural state of an object? And by natural state, I mean, if you left this object alone, if nothing is touching it, nothing is done to it . . . What’s its natural state? What would happen to the motion? What’s the natural state of the motion that would result?
For question 4, I’ll give everyone 3 points for just giving me your opinion? What does your intuition, coming into this class, tell you the answer to that question is? Is the natural state, in terms of the motion for an object, to A) come to rest, to keep meeting, or do you think it just depends on the situation?
To try to ease ourselves into thinking about this more critically, I’d like you to do this demonstration. Likely you have something in front of you that you could use to do this. Imagine, or let’s picture you have a book in front of you on the table that you’re sitting at. Try giving this book a push. Give it a quick push, let go. Give it enough of a push that you actually watch it slide for a bit before it comes to a stop.
With that kind of motion, and then thinking about this question: What’s the natural state of an object?
For 3 points, give me your opinion for question 5. Ask yourself, after the push is done, after you’ve let go, why does the book keep moving forward? Is that A) because of the force behind it coming from the push? Is that B) because it was given an initial speed from the push? Or C) Neither sounds quite right.
This question focuses on why does the book keep moving. Let’s ask it from the other perspective, which is why does the book slow down? What’s your opinion when you think about it from that side? What causes the book to slow down? Is it A) friction eventually overcomes the force of the push? Does that sound right to you? B) Friction alone slows it down or C) Neither sounds quite right.
Enter your, your intuition, your perspective. I’m not going to grade that. I’m just going to give everyone 3 points. Though, let me now say a few words about what the right answers are to these. But, historically, what people believed and what they struggled with coming to accept the right view.
It turns out that at the time of, say, Aristotle, or even much later, the scientists in the medieval period. They had this perception of what is called an Impetus Theory, meaning that when an object was pushed, there was some impetus that was imparted into that object. And that’s what caused it to continue to move forward. But then, different versions described what happened somewhat differently. But in essence, the picture was that this, this impetus gradually leaked away or went away, if you will, and that’s why the object came to a stop.
So in blue, I’ll highlight that many great minds in history and in the past likely would have chosen A. They would have said that that book keeps moving forward because of the force behind it coming from the push.
Newton quite revolutionized our view of motion, which is the whole heart of this class. Newton proposed that, well, the reason the book keeps moving forward is just because it was given an initial velocity from the push. That’s all it took. It no longer needs a force for it to keep moving forward.
So let me do here, and note that this is the old view. Studies show that the majority of students coming into a physics class, your typical person on the street, will, will have some form of this view. B is what Newton tells us. Newton’s view.
Okay. From the perspective of why does the book slow down, along the same lines, somebody like Aristotle would’ve answered something like, “The book slows down eventually because there’s roughness from the table, or what we now call friction, that eventually overcomes the force of the push.”
Newton, though, argued that you could just as well explain the motion by noting, or invoking, the idea that friction alone is slowing the book down. So, from the motion diagram picture, Newton might jot down, “Here starts out my book. It slows down, comes to a stop. And we now know in terms of our building blocks to describe that as a velocity that’s decreasing. And, therefore, an acceleration that points in the opposite direction.”
And Newton would argue all that book needed was an initial velocity and some single force that opposes that motion that causes this book to slow down. And translating that to our building blocks, that means a constant force causes this Delta v or the deceleration of that book. So, again. Over here, let me highlight for you. This is Newton’s view . . . And this is the old view . . . And I suspect that if many of you kept with your initial answer, your first intuition, that many of you quite naturally, and in good company, would have chosen A perhaps for both of those questions. So, much all of chapter 4 is getting you to appreciate the concepts behind these choices B.