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Magnus Force-1.doc

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Physics 2065A/B
Bob Sica

Magnus Force: Pitching a Baseball and Shooting a Puck The Magnus Force -When we discussed the affect of air drag we ignored the fact an object may also be rotating -Rotation causes the air to affect the object differently on either side -This difference in force on the object due to air resistance is called Magnus Force -By rotating the air on the side of rotation will be moving more quickly, causing the ball to feel an extra force making it move horizontally -Consider the motion of air around the ball -The air’s speed near the bottom of the ball is increased by the ball’s rotation and, vice versa, the air’s speed is lowed down near the top fo the ball due to its rotation ->Hence, the air on the bottom of the ball moves faster than the air on top of the ball -Therefore, the pressure on the top of the ball is greater compared to the bottom -Recall the Bernoulli Effect: as the air’s velocity increases along a streamline the pressure decreases -The difference in speed( and hence, pressure) causes the indicated Magnus Force - The Magnus force acts at right angles to the air resistance and the ball’s velocity Equation(copy from slides) - 1/2ClpAv2= ClA(1/2pv2 -Dynamic pressure= 1/2pv2 Fmagnus is propotional to w*v where w is Rotation of a Pitched Baseball -The pitch is thrown from a raised mound which is 60ft 6 inches (18.4m) from home plate and is 10” (25cm) above the infield -A typical ball is thrown at a speed btwn 85-100mph (135 to 160km/h) -The batter has about 0.4s to react once the pitch is thrown Strike Zone The Ball -Stitching that holds the ball together is very pronounced and helps create resistance and provide grip -Pitchers put different kings of spin on the ball to use the roughness of the surface and the seams to alter the Magnus and drag forces on the ball -The drag coefficient for a smooth ball is much greater than a rough ball at the speed of a pitch (see chart) -The broken line shows where Fdrag=Fgravity - Measurements of the Drag vs. Magnus Force for a Pitched Ball -Magnus force is shown here for a ball rotation of 1800 revolutions per minute (RPM; that is 30 times/second) a typical curveball rotates 20 to 30 times in a second -Magnus is roughly constant with speed Types of Pitches -Rotation of the ball seen by the batter for an overhand pitch throw by a right-handed pitcher -The direction of the rotation is also the direction of the Magnus Force *The pitcher sses the ball cruve to the left while the batter sees it curve to the right -The curve ball is thrown by the pitcher spinning the ball as it leaves his hands -Though the curve ball travels significantly slower than the fast ball, it can be extremely difficult to hit because of its deflection -The figure shows the path of the ball thrown by a right-handed pitcher (from overhead) *The ball is rotating counterclockwise Trajectory of the Curve Ball - 1959: former National Institute of Standards and Technology Director Lymen Briggs used a wind tunner and professional pitchers to prove a curve ball was not an “illusion” -He found spin was more important than speed in determining the break of the ball Air Motion Behind a Curve Ball -This ball in a wind tunnel spins 1000 rpm counterclockwise (CCW) at right angles to the wind -Which way would the Magnus force point in the picture?: pointing up the screen Brigg’s Measurements -The ball leaves the pitchers hand at 70 mph (113km/h) spinning at 1600 RPM (16 revolutions to the plate) counterclockwise -The ball passes the plate 0.6s later traveling 61 mph (98km/h) Flight of the Curve Ball -The curve ball follows a nearly constant radius path, but the deflection increases as the square of the distance -In other words, roughly ¼ of the deflection (roughly ¾”=8.6cm) happens by the time the ball is ½ way to the plate -By the time the pitch makes it to the plate it has deflected 14.4” (37cm) Hitting a Curve Ball -A curve ball is hard to hit because during the last 25% of the flight half the deflection occurs in about 1/6s (167ms) -It takes a batter 1/5s (200ms) to swing the bat, so they have to start their swing before the ball curves much (and drops as well) Sagitta Sagitta: the largers deviations from a straight line drawn from the beginning to the end of a ball’s flight, here 3.4” -the ball deflects 37cm relative to where it was aimed but only deviates 8.6 cm from its flight path Interactive Simulation -We can also study the air flow around the ball using a model developed by NASA scientists for flow around an airfoil -Note: the simulation shows 2 views of the motion, “overhead stadium” and “overhead wind tunnel” -Baseline case is using a right-handed pitcher, ball rotates CCW -This i
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