# How can a bowling ball curve in both directions?

There is a popular video on facebook that I just don't understand. A man spins a bowling ball very fast then slightly taps it down the lane. Somehow it does half of a figure eight, curving around two pins. Can anyone explain the physics behind it? Intuitively, if it is spinning one way, it can only curve one way.

Update: The answers have left me more confused b/c they disagree, so I assume the correct answer lies between both of them. B/c more work was given to draw up a diagram, I have given the bounty to him.

• @СимонТыран Hey Simon Tyran; Gyroscope is clear stuff. But it doesn't explain this; "if it is spinning one way, it can only curve one way." Or rather please just draw a picture with force vectors filling the Newtons laws. It shouldn't be too much work asked. – Jokela Mar 6 '16 at 10:26

The phenomenon you see here is precession.

The bowling ball is acting like a gyroscope (spinning top). Its angular momentum vector is almost perfectly vertical, but not quite - this is giving rise to the "drift". However, there is a small amount of torque on the ball because it's not quite symmetrical. As Carl pointed you, this may be something that was deliberately introduced by the manufacturers; perhaps it was due to the friction between the ball and the lane; or maybe the presence of the holes on top (but not quite centered) may have been sufficient. We can't tell from the clip; but we can deduce that there is angular momentum, and precession - we conclude there is some source of torque.

A gyroscope with torque will precess - the axis of rotation will slowly move around. As the axis of rotation shifts, the point where the ball is touching the lane will have velocity in different directions, and thus produce a small net force first one way, then the other.

• Some interesting points on all the ways in which the mechanics of bowling balls is tuned can be found here – Floris Feb 28 '16 at 13:05
• This Precession is the reason why the Spintops are travelling a circular paths. Othervice they would only travel direct paths. The Newtons laws you know.... But show my a spintop which Travels S-path without external cause. It's pretty easy to test this at home and to found out that your answer doesnt explain the S-path. – Jokela Mar 4 '16 at 7:58
• @JokelaTurbine remember that the first part of the lane is oiled, and has low friction. This allows the ball to move "against the direction of spin" unlike a spintop, which does not slip. The video that David Hammen referenced clearly shows this without the impact with a bowling pin. – Floris Mar 6 '16 at 3:39
• And What exactly then makes the ball to go circular path in the Video of David Hammen, if it's not the friction? "Wind?" No, it's elastic collisions when the ball bounces from the track at the beginning, before it starts to travel with full and stabile track-contact. Friction changes not the direction, it changes only the amount. If there would be really a layer of fluid, there might be such an effects, as there would be various front wave patterns. But these amount would require a frequent oiling after every throw, or the paths of previous shots would influence to the prevailing ones – Jokela Mar 6 '16 at 10:32

Modern high-end bowling balls are extremely non-homogeneous in construction. If you look around at vendor sites, you'll see that there's an off-center "dumbell" of different density from the body of the sphere. All sorts of eccentric motion can occur as a result.

There is one important point missing here on the given answers. The main point of the question actually. It's important that the ball hits to the second pin. This contact is the clue to change the direction of the bowling ball.

As it's stated in the question, there is no way to do an half-eight or S; You can have J-curve or C-curve, but S needs an external force, or it would be a violation against newtons laws.

At the picture is shown how this goes.

1. Is the starting position of the bowl.

2. Shows how the contact in Pin (RED) causes a force described with green arrows, and makes the rotating axle to flip. RED arrows shows where the flip momentum comes; the floor friction and the collision point are at different heights-> Rotation follows

3. is the initial position of bowl which causes it to curve in another direction.

• This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review – John Rennie Mar 3 '16 at 6:58
• @JohnRennie I think the judgement about this should be left to the Author. I have already -many time- produced the accepted answer with such a Review results. – Jokela Mar 3 '16 at 7:06
• I'm usually not one to refer to youtube, but watch 35 seconds of this youtube video, starting at 56 seconds. You may want to withdraw this answer. – David Hammen Mar 5 '16 at 22:30
• @DavidHammen "English" and "Sunday driver"? They seem to be only "J's" There is some "S" just plainly by the fact that the ball is jumping at the beginning, but after the full track contact has developed, it's "J"-shape all the way. – Jokela Mar 6 '16 at 10:15
• David's video clearly shows, at 0:59ff, termed "double hook", a similar shot to the one in the question, so the contact with the pin cannot be necessary to explain the answer. – Emilio Pisanty Mar 13 '16 at 19:04