Ask a Physicist Answers

Why does a steel ball bounced on a steel surface bounce higher than any other ball on the same surface? - JL, California

balls-img

The chart shows the relative bounciness of various types of balls. Reproduced with permission. (c) 1997 Exploratorium

Bouncing is a matter of energy storage and recovery. When two objects collide, they dent and store energy. When they rebound, they undent and return some of that stored energy into the energy of motion. The more efficient the two objects are at storing and returning energy, the better the bounce. As you have noticed, steel bounces very efficiently from steel.

To understand this efficiency, let's start by examining steel. Steel is composed principally of iron atoms, but with significant amounts of carbon and/or other atoms distributed in strategic places throughout the steel. If the steel is stainless, it most likely contains about 18% chromium atoms and 8% nickel atoms. Like most metals, steel and iron are crystalline, meaning that their atoms are arranged in orderly patterns. Crystalline metals are susceptible to various types of permanent deformations. When rows of atoms slide across one another, a process called "slip," the crystals deform permanently. If a metal ball's crystals undergo slip during a bounce, some of the ball's energy goes into permanently deforming the ball and the bounce isn't very good. A lead ball deforms so easily that it barely bounces at all.

To minimize slip and other internal problems, many steels are hardened. Carbon and other added impurities disrupt the crystalline patterns and make it difficult for steel to deform. When hard steel collides with hard steel, both materials deform only temporarily and then return almost perfectly to their original shapes. Nearly all of the energy stored during the impact is returned during the rebound. That's why your steel ball bounces so nicely from a steel surface.

But when almost any other type of ball bounces from a steel surface, the steel has no opportunity to contribute to the bounce. While both surfaces dent during any bounce, the softer surface dents further than the harder surface and the softer surface ends up receiving and hopefully storing most of the energy. Steel is so hard that it barely dents at all and therefore doesn't store any of the collision energy. All the storing and bouncing is up to the ball. Most rubbers are pretty efficient at this job, but not as efficient as steel itself. Such balls just don't rebound as high as a steel ball when you drop them on a steel surface.

Answered by Louis A. Bloomfield of the University of Virginia