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J. Perelman
"Entertaining physics". Book 1.
Chapter 1. Speed. The addition of movements

THE MYSTERY CART WHEELS

Attach the side to the rim of wagon wheel (or tire bike) colored paper and watch it while driving carts (or bike). You will notice a strange phenomenon: while the paper is in the lower part of the rolling wheels, it can be seen quite clearly; in the upper part of it flashes so fast that you don't have time to see.

Goes like that the top part of the wheel moves faster than the bottom. The same observation can be done if you compare the upper and lower spokes rolling wheel of some of the crew. Will be seen that the upper spokes merge into one continuous whole, the lower visible separately. Again, it is as if the upper part of the wheel moves faster than the bottom.

What is the explanation of this strange phenomenon? Yes, just that the upper part of the rolling wheels onactual moves faster than the bottom. The fact seems at first sight incredible, yet simple reasoning will convince us of this. Because each point on a rolling wheel immediately makes two movements: turns around the axis and at the same time pushed forward with that axis. Happens - as in the case of the globe - the addition of two movements, and the result for the upper and lower parts of the wheels is different. At the top of the rotational motion of the wheels is added to the progressive, as both movements are directed in the same direction. At the bottom of the same rotary motion in opposite direction and, therefore, subtracted from progressive. That's why the top of the wheel moves relative to a stationary observer faster than the bottom.

What it's really so easy to understand at a simple experience that should be done when convenient. Plug in the land of the stick near the wheel standing carts so that the stick had against the axis. On the rim of the wheel at the top and at the bottom of its parts, make a mark with chalk or charcoal; notes will, therefore, against the stick. Now, roll back the cart slightly to the right (see figure), so that the axis away from the stick centimeters by 20 - 30, and notice how moved your notes. It will appear that the top mark A has moved significantly more than the bottom one In which only barely stepped away from the stick.


How to make sure that the top part of the wheel moves faster than the bottom
Compare the distances of the points a and b rolled wheels (right drawing) from the stationary poles.

Entertaining physics J. Perelman

 




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