- #1
unscientific
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- 13
Homework Statement
Homework Equations
The Attempt at a Solution
The change in velocity = a * change in ω
I think that makes sense since for pure rolling v = aω
16 marks like that?
haruspex said:You've assumed the impulse is horizontal, which I don't think is right. However, as far as I can see, correcting that does not change the answer.
I don't know straight off, I just don't think you can assume it's horizontal. The ball strikes the cush obliquely. If there were no friction between the two you should take the impulse as perpendicular to the surface of the ball. Note that that would imply another impulse vertically from the table. But there's no slip between the ball and cush so it's not immediately clear which way the impulse will act. Because of the spin of the ball it could be above the horizontal, causing the ball to lift off the table slightly.unscientific said:which direction should J be pointing then?
You're taking moments about the point of contact of ball with table, right? And you're taking h as the distance from there to the line of the impulse. So you need the component of the impulse orthogonal to that h. I believe that will be the horizontal component of it.For change in angular momentum, it's the component of J that is tangential to surface that contributes.
Again, you only care about the horizontal component for your equation. That's why I concluded your answer turns out to be right anyway.For change in linear momentum, it's the component of J that is perpendicular to the surface that contributes.
haruspex said:You're taking moments about the point of contact of ball with table, right? And you're taking h as the distance from there to the line of the impulse. So you need the component of the impulse orthogonal to that h. I believe that will be the horizontal component of it.
haruspex said:Again, you only care about the horizontal component for your equation. That's why I concluded your answer turns out to be right anyway.
Angular momentum is always with regard to some point of reference. Only if the mass centre is stationary is it the same wrt all reference points.unscientific said:Since the ball is in pure-roll, the angular momentum L = Iω and that is with respect to the centre of the ball. So r x p, only the tangential component "spins" the ball and contributes to angular momentum.
Yes, it dawned on me yesterday after posting that that was wrong, but for some reason I've not been able to reach the site since then until just now.I'm not sure why we can just consider the horizontal component..
unscientific said:which direction should J be pointing then?
For change in angular momentum, it's the component of J that is tangential to surface that contributes.
For change in linear momentum, it's the component of J that is perpendicular to the surface that contributes.
But the ball is initlially rolling, so as far as the cushion is concerned it is spinning. This will definitely change the angle of the impulse. It can't be just anywhere. It must be in a direction dictated by the known facts, and I see no alternative but to suppose it directly opposes the relative motion of the contacting surfaces.tiny-tim said:could be anywhere
if the table wasn't there, the ball would rotate down about the cushion,
haruspex said:… I see no alternative but to suppose it directly opposes the relative motion of the contacting surfaces.
I'm not looking for a shortcut. I'm looking for a logical basis for determining the direction of the impulse. What you suggest doesn't provide one. There's no way to determine the changes in momentum and angular momentum without knowing which way the impulse acts.tiny-tim said:stop looking for a shortcut!
You're saying that just before the collision with the cushion, the point of the ball that makes contact with the cushion has a velocity ##\textbf{v}_0## that is perpendicular to the line connecting the point of contact with the cushion and the point of contact of the ball with the table. That makes sense, since the ball can be thought of as instantaneously rotating about the point of contact with the table.haruspex said:Thanks TSny. What do you think of my argument that for no slippage on contact with the cushion the impulse must directly oppose the relative motion at point of contact?
If I think of it as an elastic collision over time, with the cushion becoming compressed, it seems to me that that will be the direction of the elastic force.
In my youth, there was a popular toy called a superball. Are these still around? One trick with them was to throw them forwards with backspin. The ball would bounce back towards you, of course, but then bounce away again, then towards you again... The spin was reversed by each bounce, implying a large horizontal component to the impulse.
TSny said:Does tiny-tim have something up his sleave?
At the point of contact of ball with cushion, you can think of it as linear motion. It might as well be a rectangular block landing flat (but traveling obliquely) on the ground. The surfaces compress, without slipping, until instantaneously at rest. The force therefore directly opposes the motion; if there were any lateral force rest would not be achieved. Decompression should be like time reversal.TSny said:I don't see why there couldn't be a component of impulse along the line connecting the two points of contact.
haruspex said:At the point of contact of ball with cushion, you can think of it as linear motion. It might as well be a rectangular block landing flat (but traveling obliquely) on the ground. The surfaces compress, without slipping, until instantaneously at rest. The force therefore directly opposes the motion; if there were any lateral force rest would not be achieved. Decompression should be like time reversal.
We can represent the motion of the ball as a sum of a linear motion and a rotation in any number of ways. If we centre on the part that contacts the cushion, the rotation about that point is immaterial to the forces in the impact (except as second-order effects over a longer time). It's like a rolling contact.TSny said:I can see how this argument would apply to a particle striking the cushion. But I don't see (yet) why the argument is valid for the sphere striking the cushion. If you consider a small volume element of the ball at the ball's surface where contact is made with the cushion, then that small element is going to experience not only the force from the cushion but also complicated forces from neighboring volume elements of the ball. It's the sum of all of these forces which brings the volume element momentarily to rest.
The relationship between rolling ball and impulse can be described by Newton's second law of motion, which states that the force applied to an object is equal to its mass multiplied by its acceleration. In the case of a rolling ball, the impulse (change in momentum) of the ball is directly proportional to the force applied to it, as well as the time duration of the force.
Yes, the surface on which the ball rolls can affect its impulse. A rougher surface will provide more resistance, resulting in a shorter time duration for the force to act on the ball and therefore a smaller impulse. On the other hand, a smoother surface will provide less resistance, resulting in a longer time duration for the force to act and a larger impulse.
The mass of the ball does not directly impact its impulse, as the impulse is determined by the force and time duration of the force, not the mass. However, a heavier ball may require a greater force to achieve the same impulse as a lighter ball.
Yes, the velocity of the rolling ball can affect its impulse. The impulse of an object is equal to its change in momentum, which is directly proportional to its velocity. Therefore, a higher velocity will result in a larger impulse, while a lower velocity will result in a smaller impulse.
No, the impulse of a rolling ball is a fundamental physical concept and cannot be "too easy to be true." However, the calculation of the impulse may be affected by external factors such as air resistance or friction, which may make the results seem unrealistic. In these cases, it is important to consider all factors and conduct further experiments to ensure accurate results.