### Re: what is the best way to measure shock absorption of shin guards?

Date: Wed Feb 15 16:35:57 2006
Posted By: David Ellis, Researcher, NASA Glenn Research Center
Area of science: Physics
ID: 1137463317.Ph
Message:

The shin guard you describe has a hard outer cover to prevent penetration of the backing (and your shin) and padding to absorb energy. The hard outer shell is also important because it will spread the impact over a greater area and therefore improve shock absorption.

There are many ways to measure the amount of energy a material will absorb. Materials scientists can use impact testers to measure the amount of energy a specimen will absorb prior to breaking. While important for sports equipment, it is not really suitable for your project.

A more sophisticated test would be to send a pulse of energy such as a sound wave through the material and measure either the attenuation of the wave as it travels through the material or the time it takes for the material to stop vibrating once the sound source is turned off.

To do properly, both require some specialized equipment. A sound pressure level meter can be used to measure attenuation accurately. You would have to measure the sound level with and without the shin guard in place to determine how much energy is absorbed. You would also have to convert the sound pressure level into an energy value to determine how much energy is absorbed.

To measure the vibration of a material it is possible to use a contact microphone and oscilloscope (analog, digital or PC) to determine how fast the vibration decays. This can be related to the energy absorption of the material. This method is specific to the frequency of the sound since different structures and different materials have different resonance frequencies. That is why you will need to use a sound source with a specific frequency. Placing the base of a tuning fork on the shin guard to vibrate it is one option.

Using a metal ball or other small impactor as you suggested is also an excellent method for determining the total energy absorbed by the entire shin guard. The potential energy of the ball can be determined by its height above the impact point using the formula PE = m * g * h where m is the mass in kilograms, g is the gravitational constant (9.8 meters/second2) and h is the height in meters. The unit for energy in the metric system is joules (J). If you measure the height before the steel ball is dropped and the height that the ball rebounds, the difference is essentially the energy absorbed by the shin guard.

Some practical points both for the experiment and to get extra points from the judges:

• You need to drop the ball vertically onto a flat, horizontal portion of the shin guard to get it to bounce back up vertically. Using something like a vice to hold the shin guard and a tube that is set to vertical using a level to guide the ball the first couple of inches will help accuracy and running the experiment immensely.
• Scientists would use a high speed digital or film camera to observe the balls motion. A good digital video camera and some computer video editing software can substitute for it. Use a ruler or other measuring device behind the ball to measure the rebound of the ball. If you do not have a video camera, watch the ball rebound by eye and estimate the height
• One data point is not enough. Especially for a test like this you are going to see a lot of scatter in the data. That is normal and expected in science. Do a lot of repeats (dropping the ball) and measure the energy absorbed for each test/repeat. Ten is a good number of repeats, and three is the absolute minimum. Get help if needed from your parents or use a spreadsheet's AVERAGE function to average the data and report that as the energy absorbed. Do not throw out the data, though! That needs to be recorded in your log book.

Best of luck on your project!

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