# Magnus Cup

## Experimental procedure and explanation:

• This experiment is about the lift acting on a body that is rotating while moving. The same force will act when the body is stationary, but rotating in a flow.
• When you launch an object with backspin, the flow becomes slower below the object because it is rotating in the direction opposite to the flow. Therefore, the pressure increases. However, above the object, the direction of rotation is the same as the flow, making the flow faster, thereby decreasing the pressure. The difference in pressure between the top and bottom creates lift and the object will rise.
• Alternatively, if topspin is applied, the flow below the object becomes faster, and the pressure decreases. However, above the object, the flow is reduced and the pressure increases. The pressure difference between the top and bottom causes negative lift (lift is defined as a force perpendicular to the flow, and “negative” lift can take place), and the object falls. Once it starts to go down, lift will act in the launch direction and the object will start to turn back.
• The phenomenon in which lift is generated on an object rotating in a flow is called the “Magnus effect.”
• In sports activities, backspin is applied when you want the ball to go farther (in baseball, a straight pitch or a batted ball with home-run potential, or a drive ball in golf), and topspin is applied when you want the ball to stay in a court (tennis, table tennis, or a drive serve in volley ball).

 [Keywords] Magnus effect, Bernoulli’s theorem [Reference] “The Wonders of Flow,” Japan Society of Mechanical Engineering, Koudansha Blue Backs, pp. 170-173 “Illustrated Fluid Dynamics Trivia,” by Ryozo Ishiwata, Natsume Publishing, pp. 196-197 and 90-95.
Last update: 9.7.2013