Curve Ball

Thrown for a curve

Introduction

Build a simple device to make a spinning ball will curve when it moves through the air.

Material

• A Styrofoam ball between 2 and 4 inches in diameter (5 to 10 cm)
• a cardboard tube slightly larger than the Styrofoam ball in internal diameter at least 2 feet long (60 cm)
• a saw

Assembly

Use the saw to cut the tube in half lengthwise. (A bandsaw might be easier.)

To Do and Notice

Hold the tube in one hand with the ball placed in the groove near your hand.

Whip the tube around and throw the ball.

Notice that the ball curves as it moves through the air.

What's Going On?

When a spinning ball or cylinder moves through the air it experiences a sideways force known as the Magnus force.

The faster the ball spins, and the faster the ball moves through the air the larger the magnus force.

For a rough ball such as this Styrofoam ball, a baseball with seams or a golf ball, the direction of the magnus force is the direction of the spin of the ball viewed from in front of the ball as it moves through the air.

For a ball viewed from above moving to the north and rotating counterclockwise the magnus force will be to the west.

Wind tunnel photographs show that when air flows around a spinning ball the airflow is deflected to one side. This ball exerts a force on the air to deflect it while the air exerts an action-reaction force on the ball pushing it to one side.

So What?

Baseball pitchers put spin on a ball to make it curve.

Baseball hitters put backspin on the ball to give it lift.

Golfers put backspin on the ball to give it lift.

Golfers put sidespin on the ball to make it hook or slice.

Volleyball, soccer, and table tennis players also spin the ball to use the Magnus force and make the ball curve.

 Scientific Explorations with Paul Doherty © 2007 22 February 2007