Science Buddies: "Ask an Expert"

Hi Science Buddies!

I'm doing a science experiment on 'Does the angle of a kick effect contact force?' . (The kick part is about a Tae Kown do-- or martial arts-- kick.) Can you please help me with some ideas on how to measure the contact/ impact force? I'm going to be measuring the angle by videotaping the kick and later freezing the shot to show the impact. I will then measure the angle.

Thanks so much!

I can think of two approaches. One of the force equations is Force = mass * acceleration. If you weigh the object you are kicking ahead of time, you will know its mass. If you use the video tape to measure where the object goes on a frame by frame basis, you can estimate the acceleration. The problem with this approach is that the object motion maybe complex and different parts of the object moving in different directions at different speeds.

If you use a spring to attach the object being kicked to some structure that won't move, another force equation Force = -k * d where k is the spring constant that you have to determine by using a calibration step and d is the displacement or how far the spring is compressed.

I agree with Craig. The motion of the objects in question here will be quite complex. Human motion is by its very nature, complex, and trying to make measurements on videotape can be tricky, particularly if you don't have a high speed camera to slow the motion down enough to eliminate the blurring effect. This is not to discourage, just things to consider as you set up your experiment.

As for the kick...

Because of the complexity of the kinematics of the human body, it is near impossible to replecate a move such as a tae Kown do kick exactly. Tiny differences, such as foot placement, balance, experience level, even forcefulness of the "kiup" can have great affect on the power produced by a technique. So, each time the technique is done, the results could be quite different each time. Two ways I can suggest to get around this:

1) A large sample size. Perform the kick at each desired angle numerous times (I would suggest at least 10 of each) The more you do, the better, but can lead to an overload of data. By averaging the force imparted to your target across the kicks in each angle category, you get an average force at that impact angle. To take it a step further, you could perform the same experiment with students of different skill levels, say a white belt, a blue belt and a black belt to see if the angle of impact affects the imparted force as much for a highly skilled vs novice student. I created a notional data graph, but am having trouble getting it to post. If I figure it out, i'll post it later.

2) Build a kicking machine. An analog to a real leg could be constructed from 2x4 planks cut to mimic the proportions of real human legs and attached via a heavy duty hinge to mimick a knee. The power of the kick can be provided by streaching cargo bungee cords from the "thigh" to the "shin" across the "knee" area (opposit the hinge). A padded "foot" could even be attached on the end for realism. This way, the power can be calibrated, by adding or subtracting bungee cord, to that of a novice, an expert, or even superhuman. The benifit is that you eliminate all those other variables and isolate angle. Additionally, the desired angle of impact can be determined prior to the trial just by setting your kicking robot in the right position with respect to the target. Additionally, the "kick" will always land the same way with the same force.

Ok, this is alot, so i'll end here. post back with any additional questions.

In addition to Craig's ideas for a device to measure the force of the impact I would add the "ballistic pendulum", a device commonly used to measure the impact of bullets. You can read a detailed description of this device in Wikipedia, but the simpler explication here,

http://hyperphysics.phy-astr.gsu.edu/hbase/balpen.html

and, with more details,

http://hyperphysics.phy-astr.gsu.edu/hb ... alpen.html

may be a better starting place.

You may find it more practical to use a "bifilar suspension" design and measure the horizontal displacement with a simple pusher rod as described in detail here:

http://www.fas.harvard.edu/~scphys/cour ... /E1a_4.pdf

You can find these links and many more with a Google search om "ballistic pedulum".

Good luck, hope this helped.

Thanks Science Buddies! I appreciate it.