Bead on Hoop

Bead on a Rotating Hoop

1. Introduction

Consider a bead of mass m sliding freely on a hoop of radius R rotating with angular velocity ω₀ in a constant gravitational field with acceleration g as shown in Figure 1.

Since the bead is on rotating hoop, it moves on the surface of the sphere of

radius R. We use the spherical coordinates to describe the position of the

bead. The position of the bead is given as follows

[1.1]

Figure 1: Bead on a rotating hoop

The velocity components of the bead are as follows:

[1.2]

2. Lagrange’s Equations of motion for bead

The kinetic and potential energies of the bead are given as follows

[2.1]

[2.2]

Where θ = 0 is the zero potential energy point. Therefore the Lagrangian L = T-V is

[2.3]

The Lagrange’s equation for θ is

[2.4]

Thus one gets the equations of motion for bead on rotating hoop:

[2.5]

In the equation (2.5) frictional forces are not considered. We can easily include the velocity dependent frictional forces in the equation. Let v be the frictional coefficient of the bead on the hoop, on adding the frictional forces the equation of motion becomes

[2.6]

3. Numerical Integration of Equation

The equation of motion (2.5) can be time integrated to know the trajectory/ position of the bead at every time step using methods like Euler method, Runge-Kutta method etc,. Runge-Kutta method is better and more accurate. To apply these techniques the equation of motion is transferred to two first order differential equations as follows:

[3.1]

Equation (3.1) defines the state of the bead. Its first order time derivative is given by

[3.2]

To solve the above first order differential equations initial values are required i.e. θ and θdot are required.

4. Code for Bead on a Rotating Hoop

A code is written in MATLAB for the bead on rotating hoop. The equation of motion is solved using ODE45 of the MATLAB. Animation of the bead motion is plotted. The following are the values used in the code and can be changed accordingly.

Acceleration due to gravity, g = 9.81 m/s²

Mass of the bob, M = 10 N

Radius of the rotating hoop, R = 2 m

Frictional coefficient of the bead, V = 0.5

Angular Velocity of the rotating hoop, ω₀ = 20 rad/s

The following are the initial values used in the code

Initial values can be changed accordingly to the requirement. The following figure shows the plots obtained for the bead with the above values run for 20 seconds.

The values of the bead in the code can be changed and the motion of bead can be studied. One can study the interesting behavior in the motion of the bead by playing around with the code changing the values.

A Code is written in MATLAB for the Bead on Rotating Hoop. The position of the bead as the hoop rotates is animated. Below video shows the animation done with the CODE written in MATLAB. The boundary conditions in the CODE can be changed and the interesting motion of the bead on the hoop can be studied.

CODE can be obtained from the following link:

http://www.mathworks.in/matlabcentral/fileexchange/33425-bead-on-a-rotating-hoop

Your suggestions and comments are appreciated if you use the CODE.

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