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Blurry Spokes by Archan Baldev Luhar

Honorable - Contrived Category

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School: Medfield High School

Teacher: Mr. Richard Shapiro

If one looks at the wheels of a moving bicycle or a car on the road, a strange phenomenon can be observed. As I rode across the lawn, a camera, with a set timer, captured the wheel of my bicycle. The question is though, why are the top spokes of the bicycle wheel blurred while the bottom ones are sharp?

The spokes near the top of the wheel are more blurred than those near the bottom of the wheel because they are moving faster.

The wheel pushes on the ground at a certain speed to move the bike in an opposite direction but with an equal speed. The bike and thus the bottom of the wheel translate at a speed equal to but in an opposite direction to the rotational velocity of the bottom of the wheel. The pure rotation and the pure translation cancel each other yielding a net velocity of zero at the bottom of the wheel. Because the very bottom of the wheel has a zero net velocity, the camera captures it sharply.

At the top of the wheel, the pure rotation and the pure translation yield a net velocity in the same direction as and twice the translational velocity of the bike. This higher net velocity results in the spokes near the top being blurred because the camera captures them at multiple positions in the 1/160 of a second its shutter is open.

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