2006 AAPT Summer Meeting
Bend it Like Bernoulli
Gabe Alba and Hsu-Chang Lu
Serin Physics Laboratories
136 Frelinghuysen Rd.
Piscataway, NJ 08864
alba ‘at’ physics ‘dot’ rutgers ‘dot’ edu
Bernoulli's Principle states that in a flowing fluid, a region of higher velocity has lower pressure while a region of lower velocity has higher pressure. In a translating and rotating soccer ball (or ping-pong ball), both regions are present – outside the half of the ball moving against the incoming air, and the half moving with the incoming air. The pressure differential results in a net force (Magnus Force) that deflects the ball in a direction perpendicular to its original motion. This enables David Beckham and others to curve the trajectory of a ball around a wall of defenders into a goal. This apparatus measures the Magnus Force.
Construction of Apparatus:
A thin rod is inserted through the center of a ping-pong ball and attached to a spindle of an inexpensive electric motor. This is then mounted on a U-shaped plexiglass frame through which two holes have been drilled. For stability and smoothness, the end of the rod opposite the motor is anchored to the frame via a wheel bearing. Rotation speed is controlled by a variable power supply and can be monitored with a strobe and a mark on the ball.
To measure the Magnus Force, the whole assembly is glued to the top of an electronic scale to prevent drifting due to the vibration. A high-velocity electric fan is used to simulate the incoming wind that a moving ball experiences, and is oriented perpendicular to the ball's axis of rotation.
Use of Apparatus:
Apparatus is stationary, compact and can be used to demonstrate both Bernoulli's Principle and Magnus Force without throwing a ball. In addition, it affords a semi-quantitative measurement of the Magnus force, which is limited by scale fluctuations due to apparatus vibration and non-uniformity of wind flow.