Path traced by a point on the periphery of the ball? Helix?

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Discussion Overview

The discussion revolves around the path traced by a point on the periphery of a ball that is launched vertically with an initial translational velocity and given angular velocity. Participants explore the nature of this path, hypothesizing that it resembles a helix and discussing the characteristics of the motion, including the effects of gravity and the relationship between the ball's rotation and its translational motion.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that the path will resemble a compressed helix and questions the ratio of distances between loops, proposing potential relationships such as arithmetic or geometric progressions.
  • Another participant proposes that the position of the point can be described as a combination of parabolic motion (due to the ball's center of mass) and uniform circular motion (due to the ball's rotation).
  • Several participants agree that the point will form a helix with the same radius as the ball, noting that the pitch of the helix will change as the ball's vertical velocity varies during its ascent and descent.
  • There is a discussion about how to calculate the ratio of the pitches of the helix, with references to the coordinates of the point and the need to integrate over time to find the height changes after each rotation.
  • Participants provide equations for the x, y, and z coordinates of the point, emphasizing the relationship between the ball's rotation and its vertical motion.

Areas of Agreement / Disagreement

While there is general agreement that the path resembles a helix and that the pitch changes with vertical velocity, there is no consensus on the specific ratios of the pitches or the integration methods required to derive them. Multiple viewpoints on the calculations and assumptions remain present.

Contextual Notes

Participants express uncertainty regarding the integration needed to determine the ratios of the pitches and the assumptions underlying their calculations. The discussion includes potential corrections to earlier claims about the period of rotation.

AbeerJoshi
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TL;DR
Path traced by a point on the horizontal periphery of the ball (right or left), when spinned and launched.
So, If we imagine a ball, give it some initial angular velocity ω, and launch it straight up, with a translational initial velocity u.

What will be the path traced by a point on the periphery of the ball (the point on the either of the two extremes horizontally, left or right)?
(air resistance is neglected, weight is not).

I believe, It's going to end up sort of like a compressed helix (somewhat like in the image below...)
Here L(i) is the distance between 2 consecutive loops...

My question here is, what will be the ratio of L1:L2:L3 here? anything unique? an AP? a GP? Do we have to integrate something? (sorry for bad representation, but it was the best I could draw)
Also will the loops formed, be of equal radii? (Because, I believe them to be..?)
I know that ΣL(i)=u^2/2g (since, that is the maximum height achieved by the ball.)
Sorry for any rookie mistakes, I am a high schooler...

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Simply write the position of the point on the perifery as the center of mass motion of the ball (parabolic motion) and the rotation of the point relative to the center of mass (uniform circular motion).
 
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Welcome to PF.

The point will form a helix with the same radius as the ball.

As the ball slows and then falls back to Earth, the pitch of the helix will change, being proportional to the vertical velocity of the ball.
 
Baluncore said:
Welcome to PF.

The point will form a helix with the same radius as the ball.

As the ball slows and then falls back to Earth, the pitch of the helix will change, being proportional to the vertical velocity of the ball.
Thank you!
But in what way can I find out the ratio of the pitches? What function do I assume to integrate and with regard to what?
 
AbeerJoshi said:
But in what way can I find out the ratio of the pitches?
The x and y coordinates are simply due to the ball rotating against time, t.
x = r⋅Cos(w⋅t)
y = r⋅Sin(w⋅t)

The z coordinate is the height of the ball in its parabolic flight against time.
z = u⋅t+ ½⋅a⋅t2
where a = -9.8 m/s/s; the acceleration downwards due to gravity.

The pitch of the helix is the change in the z coordinate, after each full rotation of the ball.
If you work out the period of w; T = 2π/w, you can compute z for each full rotation.
 
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Baluncore said:
If you work out the period of w, T = 1/w, you can compute z for each rotation.
Minor correction: The period is ##T = 2\pi/\omega##.
 
Baluncore said:
The x and y coordinates are simply due to the ball rotating against time, t.
x = r⋅Cos(w⋅t)
y = r⋅Sin(w⋅t)

The z coordinate is the height of the ball in its parabolic flight against time.
z = u⋅t+ ½⋅a⋅t2
where a = -9.8 m/s/s; the acceleration downwards due to gravity.

The pitch of the helix is the change in the z coordinate, after each full rotation of the ball.
If you work out the period of w; T = 2π/w, you can compute z for each full rotation.
It was indeed very helpful! Thanks a-lot!
 

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