Dynamics
Spin and Ball Sports
As anyone who's ever played tennis, soccer, golf, baseball, cricket, polo (normal variety), billiards, dodgeball, polo (elephant variety), ping pong, or polo (Segway variety) can attest, rotation plays a vital roll in almost any sport with a ball. Tennis balls, baseballs, golf balls, or any other kind of roughly spherical object that's thrown, kicked, hit, booted, walloped, or otherwise generally moved through the air from one point to another don't just slide along—they rotate.
This rotation is called the ball's spin, and it can be in any direction. However, we generally categorize spin into two buckets: topspin and backspin.
Topspin occurs when the top of the ball rotates forward in the same direction as its travel:
Backspin, on the other hand, is when the top of the ball rotates backwards, opposite the direction the ball is traveling in:
The two kinds of spin have very different effects on the path a ball follows. Imagine a soccer player kicking a ball (or a tennis player hitting one, a baseball player throwing one, whatever). With no spin, the trajectory might look something like this:
Topspin will tend to push the ball down as it travels:
While backspin will tend to lift the ball slightly:
The change in trajectory caused by spin is due to something called the Magnus effect.5 A spinning ball that flies through the air will deflect the air behind it as it travels. Topspin tends to lift air up; backspin tends to push air down. Newton's laws tell us that if the air moves, the ball must move in the opposite direction—and sure enough, topspin lifts air up and pushes the ball down, while backspin pushes air down and lifts the ball up.
This effect can be seen using a wind tunnel, a fog machine, and hopefully lasers because any time you use a fog machine without lasers is a wasted use of a fog machine. In the wind tunnel, a strong wind can be blown over a stationary spinning ball. This both simulates the ball traveling through the air and allows the air deflection to be seen due to the fog:
In this picture, the wind comes in from the right, simulating a ball traveling to the right. The ball is spinning counterclockwise (backspin), and you can see the air pushed down in its wake. This creates an upward force on the ball, lifting it from its normal trajectory.
Of course, by the time Gustav Magnus figured this out, athletes had been using the idea to play golf or tennis for (literally) centuries. Whether Magnus ever escaped the lab and tried his theory out for himself on the tennis court is up for debate.