https://youtu.be/gfV-ZMAEZKQ
PHYS 1101: Lecture Fourteen, Part Five
For this next exercise, I want you to watch this little movie, because now we’re going to look at circular motion inside of a tube. So the motion’s very complicated as the ball rolls around the inside of the tube, but for a brief second near the end there it’s going to rattle around. I want us to consider a ball rolling around right at the bottom here along the inside of a tube.
So our scope is really — let’s wait till this ball settles down. It’s in slow motion. Right now we see it circling around the inside of that hoop.
That’s the scope. Let’s think about that motion. Here I’ve got a snapshot already set for you. Right at this instant, the object was, the ball was going around on the inside of this ring, this tube. At that instant, what’s the direction of the acceleration vector? That’s question 16.
Here’s a larger picture of this. I tried to find a better animation. This is the best I could find, but in real life you can imagine giving this ball a push and having it follow around inside this ring. So follow this circular path.
So in either of these scenarios, when the ball gets right here, picture a side view and what direction is the acceleration vector at that instant? That’s question 16.
Question 17 is what would be the free body diagram for that scenario?
Let me make this smaller here for you so you can see it on the same page. As this ball is spinning around inside this ring, what would be the right free body diagram to describe that motion? What’s different? It’s undergoing very similar motion to a hockey puck attached to a string that’s going around in circular motion, but obviously the nature of what’s in contact with the ball or this object is different now. There’s no longer a rope pulling it, but I do have the edge of this ring now that it’s riding against. What’s the right free body diagram?
Question 18 is this acceleration that you’ve identified the direction of, this what some people call the centripetal acceleration, a, the force that’s responsible for it — what is it? So from your free body diagram, what’s the force that’s consistent with the direction of the acceleration?
And how would you describe that force and what it exerts on the ball? Is it a push now as compared to the pull that it was before? Or is it a pull and a push? How do they compare? The string when the string was acting on the ball compared to now, when it’s a ring?