Gifted Issues Discussion homepage Forums Learning Environments Silly Putty Question

I need help with a question DS just asked me.

Why does Silly Putty bounce when you roll it into a ball, but not when it's flat?

I don't understand why any ball bounces, so that is where I need help.

(If you haven't already noticed, I'm not gifted. Duh.)

That's a silly Silly Putty question.

It's pretty perceptive to realize that understanding why any ball bounces is the first step to answering the question.

Stretch an elastic band. By stretching it, you are changing its shape. Because it's elastic, it bounces back when you let go.

When a ball hits the ground, the ball changes shape at the point of contact. Because the ball is bouncy (duh, that's what makes it a bouncy ball), the elastic energy in the ball bounces back, pushes back against the floor, and pops up.

Silly putty has aspects of both an elastic object (bounces back) and a plastic object (something that changes shape and stays that way). When it's a ball, the deformation is all right where it touches the floor -- check it, there'll be a flat spot where it hit -- but there's enough elasticity in the nature of Silly Putty for it to bounce back. When the SP is flattened, the contact with the ground changes the shape plastically more than elastically, not leaving enough "spring" for it to bounce.

(I actually teach elastic vs plastic behavior using silly putty in my university classes...)

Thinking about it some...Playdoh doesn't bounce when it's rolled into a ball. Perhaps Silly Putty does because of the chemical make-up? Some sort of rubber?

Wow! Thanks for that explanation.

Now to have DS read it and see if he understands.

He seems to understand. I think I'll get a ball of rubber bands to explore this more.

He read the explanation only needing help with the pronunciation of "elasticity". He said, "elas tick it e".

Oh, but it gets even better with Silly Putty... Silly putty is composed of a non-newtonian liquid which have awesome properties.

Alternate test:
Drop a metal ball onto flattened Silly Putty from various heights. At some height the force of impact will compel the Silly Putty into its elastic behavior and the ball will bounce. This shows that it is a question of the material properties reaction to force and not strictly a shape question.

He may also enjoy making a non-newtonian liquid using just corn starch and cold water. Stir a lot of cornstarch into the water until when the spoon moves too quickly it gets stuck. If he taps the surface with his finger at various speeds he'll get the transition effect. Fun but not as elastic as Silly Putty.

Silly putty trick!

Yay! Go MatSci!

Non-Newtonian fluids are a kid's best friend, I always say.

You can, however, bounce a ball off of the cornstarch-water version.

The Mythbusters have done some fun things with non-Newtonian fluids.

If you are REALLY keen to explore this-- make your own silly putty. If you vary the amount of catalyst, you can get a variety of cross-linking/doping in the resultant material, which produces varying macroscopic properties.

It's one of the clearest demonstrations of intuitive stat mech that I know of, in fact.

DD always found it just slightly off-putting that this kind of thing turns BOTH of her parents into giddy and slightly scary geeky people who are willing to try just about anything to 'see what happens' by the way.