Homework Help: How do I get the damping coefficient experimentally

1. Oct 7, 2007

williamshipman

1. The problem statement, all variables and given/known data

This problem is part of my project for Controls 3B. The project revolves around modeling a bouncing ball that is dropped from a certain height and bounces of the ground. I have been told to use the mass-spring-damper model to simulate the ball while it is in contact with the ground. The model for this system therefore consists of two equations: one for when it is falling through the air, with only gravity affecting it, and the second for when the ball is in contact with the ground.

My problem is that none of the physical properties of the ball are known before hand. Finding out the spring constant is easy enough as you simply need to squash the ball and measure how much force is being applied and the new size of the ball.

The damping coefficient is the one that is giving me problems.

2. Relevant equations

The second order differential equation describing the ball during the collision is:
$$\ddot{x}+\frac{b}{m}\dot{x}-\frac{k}{m}x=g-\frac{k}{m}r$$​

k is the spring constant, b is the damping coefficient, m is the mass of the ball, g is 9.81 - gravity, x is the height above the ground. I defined down to be the positive direction for my forces and velocity.

I have been thinking for this for a while and have put all of my thoughts into "bounce model idea.pdf". The file "prac1.pdf" is the project description and requirements given by the lecturer.

3. The attempt at a solution

From the pdf, I have some possible leads to follow. The only problem is that at each turn, I am limited by what I can measure. The easiest value to measure is the maximum height of each bounce. The time spent in contact with the ground is so small that I can't measure it without expensive equipment (which I don't have). The same problem exists for measuring the amount of deformation in the ball during the collision. My Internet research has turned up various terms such as damping ratios or damping factors, but only gives these in terms of the damping coefficient and the spring constant. No physical link is given, making it difficult for me to derive an experiment to determine b.

Any help would be greatly appreciated. Thanks.

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