Re: How do I find the hardness of play-dough?

How do I find the hardness of play-dough?

I am working on a science fair project, and I need some advice. My experiment is on the effect of evaporation on play-dough. I plan to leave the play dough out for certain periods of time and then test the hardness (elasticity maybe?). So far the only idea that I have came up with is to drop a ball from a certain height and look at the spread (measured before the play dough is left out and after). I could also make a scale of 1 to 10 to show the hardness; but this would be done by my touch and therefore would not be that accurate. Please email me any suggestions. Your help is greatly appreciated!!!
Sincerly,
CEP

Hardness is a rather technical and arbitrary concept and usually is defined in terms of the ability of one substance to scratch another. However, Play-Do^TM is not a uniform substance but a heterogeneous mixture of different materials; as such, the Moh's Scale hardness quickly loses any real meaning.

Actually, your method sounds like a very good one and simply means that you are defining hardness in your own (but still a perfectly valid) way. My only suggestion would be to refine your method by using identically-sized balls of different composition (for example, soft Play-Do^TM; rubber; glass; wood; steel) and controlling for mass differences by allowing the balls to strike with constant momentum.

The way to ensure the different balls strike with constant momentum is as follows:

1. Remember that momentum is mass ´ velocity.

2. Remember that velocity of a dropped ball is related to the distance it falls by the relationship v = (2gd)^½, that is the square root of the quantity 2´g´d.
   1. g is the acceleration due to gravity, and has the value 9.8 m/s² or 32 ft/s².
   2. d is the distance the object falls.

3. Weigh your different balls.

4. Design your experiment so that the momentum of each ball is the same; that is, so that the quantity m´(2gd)^½ is a constant.
   1. This means that balls of different mass will need to fall a different distance.

The other test that comes to mind is to measure the force required to break a Play-Do^TM sample by bending or twisting; however, this requires sophisticated and expensive machinery to do reproducibly. The "dropped ball" method is much cheaper!