What Makes Rubber Balls Bounce?

Rubber balls get there bouncing properties from their molecular structure.

Drop most items onto the ground and they will stay where they fall, sometimes broken into many pieces. Drop a rubber ball and it bounces right back up. The properties of rubber that make balls bounce are fairly straightforward, but the chemical and molecular characteristics of rubber that create these properties are more complex.

Strength

Rubber is a very strong material. It can withstand severe deformation of its shape without becoming physically damaged. This is very important to the bounce of a rubber ball because when you drop a ball, its shape is deformed as it hits the ground. If this deformation damaged the ball, it would not be able to bounce.

Elasticity

Elasticity is crucial to the ability of a rubber ball to bounce. Elasticity is the tendency for an object to retain its shape. When you drop a rubber ball, the part of the ball impacting the ground becomes flattened. Rubber's high elasticity causes the ball to spontaneously return to its round shape. This spontaneous return to its original spherical shape pushes the ball back up into the air.

Polymers

Rubber gets its bouncing properties from its molecular structure. Rubber is a polymer. Polymer molecules are very long and are arranged, more or less, in a straight line. These long, linear polymeric chains are the key to rubber's ability to bounce.

Flexibility

Rubber's long molecular chains are very flexible. These long chains of molecules can physically rotate around the chemical bonds that hold them together. Because the molecular chains can rotate this way, rubber is able to deform its shape, but only momentarily.

Crosslinking

Crosslinking is the other molecular characteristic that helps to give rubber its bounce. The long chains of molecules that make up rubber are crosslinked to other chains of molecules. The chains are attached to one another. If the chains were not linked together, rubber would remain deformed because individual chains of molecules would tend to straighten out. This crosslinking gives the chains of molecules stability, ensuring the ball returns to its spherical shape.

References

• Resonance: Why Does a Rubber Ball Bounce?; V. G. Geethama & Sabu Thomas; 1997