# Is a tennis ball technically a composite or a polymer?

1. May 21, 2013

### skyturnred

1. The problem statement, all variables and given/known data

Under which class of materials would the following materials be categorized?
What are/is the primary bond type(s) in each material/object?

a. Tennis Ball

2. Relevant equations

3. The attempt at a solution

We'll consider a very simple and standard tennis ball composed of a rubber material covered with felt.

Here is where I'm confused:

A composite material is any material that is made of two or more materials with significantly different properties.

Felt and rubber obviously have significantly different properties

However, rubber and felt are both polymers on their own

So I am having a hard time deciding whether a tennis ball can be classified as a composite or a polymer. Any ideas?

PS: Even the statement of "felt is a polymer" is confusing to me. Felt is a cloth made of woolen fibers. Woolen fibers are themselves protein fibres composed of more than 20 different amino acids.

Although the amino acids are polymers and protein fibers are polymers, is it correct to classify woolen fibers as polymers since they are composed of many different types?

Thank-you

2. May 22, 2013

### Staff: Mentor

Question is ambiguous, no wonder you have problems answering it. I would answer stating both rubber and felt parts are a separate polymers.

3. May 27, 2013

### Staff: Mentor

Even though the felt and rubber are both polymers, in terms of structural mechanics, they are considered separate materials, and a tennis ball is considered a composite. The cord/rubber plies of a radial tire are typically composites of rubber and steel cords. If the steel tire cords are replaced by Kevlar (polymeric) cords, the plies are still considered composites.

4. May 28, 2013

### Staff: Mentor

Isn't the felt put just outside? My understanding is that in composites properties of the final material are different from the properties of the components, here we have a rubber ball (that behaves like a rubber ball) covered with felt (that behaves like felt). It is no more composite than a painted steel is.

5. May 28, 2013

### Staff: Mentor

Maybe not exactly. It all depends on whether or not the felt has a non-negligible effect on the overall deformational response of the ball. I'm not sure whether the load-deformation behavior of a bald tennis ball is essentially the same as the load deformation behavior of a tennis ball with felt covering (and thus whether a bald tennis ball performs in play the same as a tennis ball covered with felt). My guess is that it doesn't. In the case of painted steel, the paint does have a negligible effect on the deformational response of the steel.

6. May 28, 2013

### Staff: Mentor

Interesting point. I always thought the felt is there just to add friction, making it easier to spin the ball.

7. May 29, 2013

### chemisttree

Just a note about your PS... wool is made up of a class of protein fibers composed of 20 different amino acids.

The protein is a polymer of amino acids. Amino acids are the monomers that make up the protein. AND, if you are interested, the wool itself can be described as a composite since the hair shaft is itself composed of several different keratins that make up the cuticle, fibers and matrix.