dr_dave
01-14-2008, 11:06 PM
Below is an e-mail question I received recently, along with my reply. I wanted to share it with you guys for comment and discussion. I've heard about this experiment from several people. If you haven't tried it yet, give it a try. The visual results are very interesting.
<blockquote><font class="small">Quote e-mail question:</font><hr>What you may find interesting would be to place a striped ball with the stripe tilted as the ball is at 0:18 in your NV B.7 (http://billiards.colostate.edu/normal_videos/new/NVB-7.htm) video. Instead of hitting it with sidespin only, also hit it with some draw. Hit it low, off center and on the stripe. It would be like you are back spinning the ball so it keeps the stripe in line spinning in reverse on a tilted axis as it goes down the table. ( the stripe aligned with the spin direction)
The ball should come up to a vertical spin (perpendicular to the table) and then keep rolling. It does... but not how you would think. The ball comes up to a vertical spin and the axis then actually flips over 90 degrees from the original axis that it started on. You can't see that with a measels ball.
If you put arrows on the starting spin direction and stop the ball in towels or use something to catch the ball on the opposite end of the table... the arrows will be facing in the complete opposite direction. That is why I mentioned the red/white ball. If the red side starts out facing one side of the table it will flip over and the red side will face the opposite direction. A complete ball flip or turn over.<hr /></blockquote>The reason why you can't see it with the measles ball is the spin axis really isn't flipping. It just looks that way because of how the 2-color or striped ball rotates sideways and forward as the drag converts to roll.
I have tried this experiment several times with a striped ball, a 2-color Elephant Practice Ball, and an Aramith "measles" ball. The experiment doesn't change my understanding. When you hit the ball with bottom-left English, the spin axis starts off pointing down and to the right. If you use your right hand and curl your fingers in the spin direction, your thumb will point in the spin axis direction. With just left spin, your thumb would point straight down, with your fingers curling in the clockwise direction (looking from above). With just forward roll and no sidespin, your thumb would point to the left, with your fingers curling in the forward roll direction. Now, with the shot in question, this is how the spin axis changes:
1.) The thumb (spin axis) first points down and to the left (at 4:30 o'clock), with the bottom-left spin.
2.) After the drag removes the bottom spin, only left idespin remains for a moment, and the thumb points straight down (6:00 o'clock).
3.) As forward roll starts develops, the spin axis drifts towards the left (towards 7:30 o'clock), as the ball now has forward roll and the left sidespin.
The spin axis does not flip. It gradually drifts through steps 1, 2, and 3. Again, the 2-color Elephant Practice Ball or a striped ball kind of creates a visual illusion (leading to your "flip" interpretation). It looks cool, but I don't think it is showing what you think it is showing. Try it with an Aramith measles ball, and the spin-axis changes described above are fairly easy to see. The measles don't lie.
Regards,
Dave
<blockquote><font class="small">Quote e-mail question:</font><hr>What you may find interesting would be to place a striped ball with the stripe tilted as the ball is at 0:18 in your NV B.7 (http://billiards.colostate.edu/normal_videos/new/NVB-7.htm) video. Instead of hitting it with sidespin only, also hit it with some draw. Hit it low, off center and on the stripe. It would be like you are back spinning the ball so it keeps the stripe in line spinning in reverse on a tilted axis as it goes down the table. ( the stripe aligned with the spin direction)
The ball should come up to a vertical spin (perpendicular to the table) and then keep rolling. It does... but not how you would think. The ball comes up to a vertical spin and the axis then actually flips over 90 degrees from the original axis that it started on. You can't see that with a measels ball.
If you put arrows on the starting spin direction and stop the ball in towels or use something to catch the ball on the opposite end of the table... the arrows will be facing in the complete opposite direction. That is why I mentioned the red/white ball. If the red side starts out facing one side of the table it will flip over and the red side will face the opposite direction. A complete ball flip or turn over.<hr /></blockquote>The reason why you can't see it with the measles ball is the spin axis really isn't flipping. It just looks that way because of how the 2-color or striped ball rotates sideways and forward as the drag converts to roll.
I have tried this experiment several times with a striped ball, a 2-color Elephant Practice Ball, and an Aramith "measles" ball. The experiment doesn't change my understanding. When you hit the ball with bottom-left English, the spin axis starts off pointing down and to the right. If you use your right hand and curl your fingers in the spin direction, your thumb will point in the spin axis direction. With just left spin, your thumb would point straight down, with your fingers curling in the clockwise direction (looking from above). With just forward roll and no sidespin, your thumb would point to the left, with your fingers curling in the forward roll direction. Now, with the shot in question, this is how the spin axis changes:
1.) The thumb (spin axis) first points down and to the left (at 4:30 o'clock), with the bottom-left spin.
2.) After the drag removes the bottom spin, only left idespin remains for a moment, and the thumb points straight down (6:00 o'clock).
3.) As forward roll starts develops, the spin axis drifts towards the left (towards 7:30 o'clock), as the ball now has forward roll and the left sidespin.
The spin axis does not flip. It gradually drifts through steps 1, 2, and 3. Again, the 2-color Elephant Practice Ball or a striped ball kind of creates a visual illusion (leading to your "flip" interpretation). It looks cool, but I don't think it is showing what you think it is showing. Try it with an Aramith measles ball, and the spin-axis changes described above are fairly easy to see. The measles don't lie.
Regards,
Dave