The Myth of Top Spin???

[Patrick Johnson]

...would it not be true that while the bottom tick mark is rotating in a westerly direction, it is in fact, moving east relative to its original position on the earth?

Try visualizing a wheel with a much larger flange: say the train's built for a wider track so its 1-inch radius axle rolls on top of the track but its 12-inch radius wheels hang off the sides like huge flanges.

Before the train moves paint a red dot on the bottom edge of a wheel, 12 inches away from its center (and also 12 inches away from the center of the axle, which is in the center of the wheel). Now visualize the axle rotating forward just 90 degrees - the center of the axle (and the train) will have moved forward about 1.5 inches (1/4 the circumference of the axle), and the red dot is still 12 inches away - but now it's directly behind the axle. The train has moved forward 1.5 inches and the red dot has moved backwards 10.5 inches.

The same thing happens with a smaller flange on a bigger wheel, it's just harder to visualize because of the smaller differences.

pj
chgo

 

[av84fun]

Try visualizing a wheel with a much larger flange: say the train's built for a wider track so its 1-inch radius axle rolls on top of the track but its 12-inch radius wheels hang off the sides like huge flanges.

Before the train moves paint a red dot on the bottom edge of a wheel, 12 inches away from its center (and also 12 inches away from the center of the axle, which is in the center of the wheel). Now visualize the axle rotating forward just 90 degrees - the center of the axle (and the train) will have moved forward about 1.5 inches (1/4 the circumference of the axle), and the red dot is still 12 inches away - but now it's directly behind the axle. The train has moved forward 1.5 inches and the red dot has moved backwards 10.5 inches.

The same thing happens with a smaller flange on a bigger wheel, it's just harder to visualize because of the smaller differences.

pj
chgo

Thanks for that visualization. I get it now.

Regards,
Jim

 

[8JIM9]

The myth of top spin

According to "The Owner's Manual for the Complete Pool Player", the difference between Force-follow and a normal rolling cueball is it's final path after hitting an object ball. With a normal rolling cueball the cueball leaves the object ball on an angle between 0 and 30-something degrees. With a Force-follow shot, the cueball ends-up on a path that is close to parallel to its original path before impact. That's as simple as I can put it. The illusion that the cueball is spinning faster than its rolling, occurs when the cueball impacts an object ball. On impact the cueball loses some or all of it's forward momentum but none of its angular momentum or spin; which is determined by how hard it was rolling down the table before impact.
8JIM9

 

[thoffen]

It has to do with friction. And there are two types of friction: static and kinetic.

Static friction would be a rolling ball. Think of it like a car driving on pavement. The friction of the tires on the pavement drives the car on its path.

Kinetic friction would be a sliding ball. Think of it like a car on ice. No matter how fast you spin the wheels, you're going to slide in whatever direction you were moving until you collide with something or your wheels slow down their spinning enough for static friction to take over.

Without kinetic friction, draw could never exist. How else could you get the ball to go backward?

Follow can be achieved with a rolling ball if the OB doesn't absorb all of the impact energy. It can also be achieved with a sliding ball where the collision with the OB causes static friction to take over and cause the ball to roll forward.

 

[Bob Jewett]

According to "The Owner's Manual for the Complete Pool Player", the difference between Force-follow and a normal rolling cueball is it's final path after hitting an object ball. With a normal rolling cueball the cueball leaves the object ball on an angle between 0 and 30-something degrees. With a Force-follow shot, the cueball ends-up on a path that is close to parallel to its original path before impact. ...

I think this doesn't make any sense physically. A forceful follow shot will slide out to the side more, but it will usually take a path parallel to the path that a less forceful follow shot will take. The only way I can think of to get a straighter-through follow angle is to make the cue ball jump a little. If the author is referring to the possibility of a very slight overspin, then the follow angle will change only very slightly -- maybe a couple of degrees.

 

[Patrick Johnson]

According to "The Owner's Manual for the Complete Pool Player", the difference between Force-follow and a normal rolling cueball is it's final path after hitting an object ball.

There is no difference between force follow and a normal rolling cueball. Force follow just means "hit harder than normal with follow". When you hit harder than normal with follow the cueball rolls normally, but faster.

A cueball that's rolling faster (force follow) will take a different path after hitting an object ball at an angle than a cueball that's rolling slower, but it's not because of the spin; it's because the faster rolling cueball will carom wider before curving forward.

pj
chgo

 

[LAMas]

Bottom of CB seems to contact the felt without sliding/going backward befor contacting the OB. So is a forced follow shot actually a higher velocity follow shot?

http://www.billiards.colostate.edu/high_speed_videos/new/HSVA-144.htm

 

[randyg]

CB would never travel past OB if it didn't. Must be easy to get the BCA instructor cards!

It's so easy, everybody gets one.

What do you have against quality instruction??????....SPF=randyg

 

[Bob Jewett]

Bottom of CB seems to contact the felt without sliding/going backward befor contacting the OB. So is a forced follow shot actually a higher velocity follow shot? ...

I think that's the way that a lot of people use the term "force follow". I've also heard it used for follow shots where the cue ball starts close to the object ball, but that seems to be a less common usage.

 

[mbippus]

Isn't physics fun. (I was a comm. major)

Your mileage may vary, but I usually find that when I don't understand what Mr. Jewett is saying, it's because he's a step or two ahead of me.

He will agree there's no part of a moving car going backwards, and therefore as you said, a car can't kick a stone backwards (so long as it's not peeling out, that is).

But a train differs from a car in a significant way. There's the flat bottom of the train wheel that sits on the rail, and then there's a part of the wheel called the flange that sticks down lower on the inside of the rail (to keep the wheel from sliding out) In other words if the business part of the wheel is a circle of 40 inch diameter, then the wheel with the flange is a circle of, say, 46 inch diameter. The portion of the flange below the rail is moving west.

 

[av84fun]

Relative to the earth, the bottom point of the flange would be moving west.

I found a graph that illustrates. Scroll down, and you'll see the exact problem we're talking about.

http://www.physclips.unsw.edu.au/jw/rolling.htm

And Mike's answer to your question is the same as mine. Yes, it is not true.

Fred

I got it from Patrick's description but THANKS for the link of the ANIMATED diagram.

Cool beans!

(-:
Jim