Science Buddies: "Ask an Expert"

Hello, I am doing a project like the mythbusters did where they test if impact force is dependent on the surface tension of water. So i was planning to drop an object from around 20 ft, and then compare the impact forces after changing the surface tension of the water. So i need to know: 1.How to change the surface tension of the water? 2. How do i measure the force of impact of the object to campare results. (I know that Mythbusters and SportsScience use some equipment to measure force all the time.)
Thanks

I'm having a difficult time understanding how surface tension of a liquid applies. Please describe your experimental concept in more detail. What kind of object are you intending to drop (material, size, shape, mass)? What are you dropping it on?

Impact forces are some of the most difficult to measure accurately because they are definitly dynamic short lived forces.

This could be an interesting experiment. I agree with Craig, I don't understand how surface tension might affect impact forces, but I can think of one simple approach that might work to investigate this.

First, you can control the surface tension of a pool of water simply by adding liquid soap. That's how soap does its job. It reduces the surface tension of water so that dirt can dissolve in it more readily. Add enough so that the water feels good and slippery, but doesn't get foamy.

Next you need to be able to measure impact force. One way to do that is to measure the deformation of an object after an impact. You'll need some soft material - modeling clay perhaps. But that might be too stiff. Perhaps you could find a recipe in a craft magazine or book on how to make "Play Dough" at home. That way you could adjust the amount of water to make the material stiff enough to hold a shape, but not too stiff so that it will deform.

Then you could start by making a simple ball of dough. Drop it from various distances and measure how much it deforms when it hits the water. Remember, always make multiple experiment trials at every distance. You could try different shapes too, like a long cylinder or a cone or a bullet shaped object. This would allow you to measure the effect of shape.

After you have dropped a number of shapes from different distances, then add liquid soap to the water, reducing its surface tension, and repeat your experiments. Because your "Play Dough" might start drying out and changing its stiffness, it would be good to make a control drop every once in a while on a fixed, hard surface and measure its deformation. Actually, it would be a good idea to do that first to see how much variability you might get in the deformation of the object on a controlled surface.

I think you could have a lot of fun with this experiment. Enjoy!

Ed Neu
Buffalo, MN