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Chapter 9: Problem 119

A pitcher can pitch a "curve ball" by putting sufficient spin on the ball when it is thrown. A ball that has absolutely no spin will follow a "straight" path. A ball that is pitched with a very small amount of spin (on the order of one revolution during its flight between the pitcher's mound and home plate) is termed a knuckle ball. A ball pitched this way tends to "jump around" and "zig-zag", back and forth. Explain this phenomenon. Note: A baseball has seams.

Short Answer

Expert verified
A knuckleball 'jumps around' and 'zig-zags' due to the Magnus effect which causes a pressure difference when the ball spins, air resistance causing turbulence, and the ball's seams creating further irregularities in the air flow. This leads to unpredictability in the ball's trajectory.

Step by step solution

01

Understand the Concept of Spin

When a ball is thrown with spin, it creates a pressure difference across itself because of the Magnus effect. The Magnus effect states that a spinning object flying in a fluid creates a swirl of fluid around itself, and the induced pressures lead to a force on the object, causing its curved path. A ball with no spin will not experience this effect and thus follow a straight path.
02

Consider Air Resistance and Turbulence

The ball, while moving in air, experiences air resistance which interacts with the spinning motion. Due to this spin, the ball's trajectory tends to change direction, which is seen as 'zig-zag' movement. The unpredictability of this movement is due to the air turbulence around the spinning ball.
03

Role of Baseball's Seams

The seams on a baseball change the air flow around the ball and further increase this turbulence. This is especially pronounced on a knuckle ball due to its minimal spin, making the interactions with the seams more significant. This results in an even more unpredictable 'jumping around' motion.
04

Conclusion

A knuckle ball thrown with minimal spin tends to 'jump around' and 'zig-zag' due to the Magnus effect, the turbulence caused by air resistance, and the ball seams altering the air flow. Together these factors cause unpredictable changes in the ball's trajectory.

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