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This week's LabZone activity
Jan. 21, 2004
The Magnus Effect
What makes a thrown baseball curve?
Materials:
- 30-ft length of string
- wide, strong rubber band
- baseball
- table or drawer
For years, experts claimed that no pitcher throws a curveball and that curveballs were optical illusions. Scientists have since taken measurements that show that the ball does indeed curve. A baseball can be made to curve as much as 19 inches away from a straight line between the pitcher's mound and home plate (60.5 ft to the front edge of the plate).
Why do curveballs curve? A baseball thrown without spin flies off with practically no curve except for the descent caused by gravity. In order to make a ball curve, the pitcher must throw it so that it spins in flight. When a spinning ball moves through air, a force develops that is greater on one side than the other. The greater force pushes the ball to the side. The force is called the Magnus force after Gustav Magnus, who published a scientific article on it in 1853.
You can observe the Magnus effect by spinning a baseball on a pendulum.
Obtain several feet of soft string (such as the white string use to wrap bakery boxes). Slip one end of the string under a heavy rubber band that fits tightly around a baseball.
Hang the other end of the string from a support (open drawer handle, or tabletop) so that the ball hangs freely. It is important that the support does not move at all during the experiment.
Draw the ball back and let it go to check that it swings back and forth in a straight line. Stop the motion.
Twist the string for 50 or more turns. This is most easily done by slapping the ball to keep it going around. When the string is twisted, catch the ball with your hands to keep it from untwisting.
Pull the ball straight back as before and release it. Observe the path.
To which side is the ball pushed outward?
When the baseball swung back and forth without spin, its path did not curve. The spinning baseball was different. Its path began to curve after a few swings. As it moved forward, the path of the spinning baseball curved as shown in the figure below.
 | A pendulum with a spinning ball can illustrate the Magnus effect. The curve is exaggerated. | |
The narrow oval of the swing was not large, but it was distinct. As can be seen from the figure, the Magnus force pushes outward on the side of the ball where the spin goes in the opposite direction to the airflow. This results in the ball being pushed outward when the pendulum swings in one direction, and outward again when the pendulum reverses. Hence, a loop forms. What causes the Magnus force?
Magnus measured air pressures on both sides of a spinning object while air was flowing past it and found that there was a difference in pressure on the two sides. The greater pressure acted as the force that pushed the ball.
How does the Magnus force develop on a baseball? When a baseball moves through the air, a layer of air (the boundary layer) is held between the stitches and spins with the ball as it rotates. As the spinning ball moves through the air, the air layer on one side of the ball is moving in the same direction as the air through which it passes; the air on that side of the ball speeds up. On the other side of the ball, the boundary layer is moving opposite to the motion of the air that the ball is traveling through; the two motions oppose each other, so the flow of air on that side slows.
It is the difference in speed of the air on opposite sides of the spinning ball that causes the pressure difference. The effect of air pressure difference on two sides of an object was first discovered by Daniel Bernoulli, a Swiss scientist (1700–1782). He found that the faster air exerts less pressure on an object. So the slower air pushes the object to one side.
The spinning boundary layer held by the stitches on the baseball interacts with the passing air to cause the path of the ball to curve.
Reprinted with permission from Sports Science Projects: The Physics of Balls in Motion by Madeline Goodstein. © 1999 by Enslow Publishers (www.enslow.com).
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