Rolling Ball inside a Cylinder

ratio: golf ball radius to cylinder radius

entry angle from horizontal, α

initial speed,

initial pitch rate,

ρ0

gravity, g cm/

sec



980

number of revolutions

0.5

1.5

reset to golf ball parameters

decouple initial pitch rate from initial speed

time

A ball rolling along the inside of a cylinder without slipping performs a surprising return to the starting height. This is related to the phenomenon of a golf ball rising out of the cup after it starts to fall in. This Demonstration uses the equations of motion to show the rolling ball together with the axis of rotation and an energy plot. A key point is the proper coordinate system for studying the axis of rotation. The rotation axis is given by

(

)

, where

refers to the vertical axis through the center of the ball,

refers to the horizontal axis through the point of contact and normal to the cylinder, and

refers to the horizontal axis perpendicular to that of

. The third component of the angular velocity,

, turns out to remain constant. The graph on the right shows the breakdown of two types of energy. Total energy is constant, but the energy can be divided into a conventional part (kinetic energy plus angular energy due to rotation of

and

plus potential energy) and a surprising part, the spinning around the

axis,

. The

-spinning is due to a coriolis torque and it is this that causes the sign of

to reverse; because

′

, the reversal of

-spin causes the reversal of the

-motion.

The controls:

The ratio control gives

r/R

where

is the ball radius and

the cylinder radius.

is always set to the golf ball cup radius of 5.4 cm. The golf ball setting for

is 2.1 cm.

is the acceleration due to gravity in

cm/

sec

is the entry angle, in radians measured from the horizontal.

is the initial speed of the ball.

ρ0

is the initial rate of spin, in radians per second, about the horizontal axis normal to the cylinder—the

axis. For example, a golf ball will have such spin as it enters the cup. The default ties this to

via

ρ0

, but there is a checkbox to decouple these.

The number of revolutions sets the number of horizontal revolutions of the ball around the cylinder.

Time is given in seconds.

Details

The physics of a ball rolling inside a cylinder is worked out in the paper by Gualtieri et al; they analyze the various forces involved, notably a Coriolis torque that acts around the

-axis. A physical demonstration of the process is described in the paper by Matsuura. We enhanced the work of Gualtieri et al by allowing an initial spinning

ρ0

around the

-axis. This matches the behavior of a golf ball, which would be spinning at a rate proportional to its speed as it falls into the cup (see second snapshot). Solving the differential equations yields a formula of the form

sin(λt)+

cos(λt)

. But in order to learn where points are at time

requires the numerical solution of a differential equation so that the angular velocity at each instant is as it should be. The angular velocity is computed from

z(t)

by the following equation, where

is the radius of the ball,

is the radius of the cylinder, and

is a constant that represents the (negative of) the angular speed of the ball around the central axis of the cylinder:

(

)=-

ρ0

′

ΩR

. Note that

is a linear function of

. The program for the soccer ball was taken from a Demonstration by Greg Wilhelm.

References

M. Gualtieri, T. Tokieda, L. Advis-Gaete, B. Carry, E. Reffet, and C. Guthman, "Golfer's Dilemma," American Journal of Physics, 74(6), 2006 pp. 497–501.

A. Matsuura, "Strange Physical Motion of Balls in a Cylinder," Proc. of Bridges Conference, Banff, 2005.

External Links

Metamorphic Soccer Ball

Permanent Citation

Dan Curtis, Alexi Radovinsky, Stan Wagon

"Rolling Ball inside a Cylinder"
http://demonstrations.wolfram.com/RollingBallInsideACylinder/
Wolfram Demonstrations Project
Published: September 1, 2009