In the first book, "Entertaining physics" described loupe time", based on the use of the camera. Here will tell you about another way to achieve the same effect, based on the phenomenon that was discussed in the previous article.

We already knowthat when the disc blackened sectors, making 25 revolutions per second, is illuminated every second 100 lamp flashes, it seems to be an eye fixed.

The drive to determine the speed of rotation of the engine.
Imagine, however, that the number of outbreaks has become equal to 101 per second. During the period between such two consecutive increase of the flash drive will not be able to turn, as before, for a full quarter turn, and, hence, the corresponding sector will not reach the original position.

The eye will see him behind at a hundredth part of the circle. During the next outbreak it will appear behind another on a hundredth part of a circle, and so on, We find that the disc rotates back, making one revolution per second. Movement slowed by 25 times.

It's simple to understand, as you can see the same slow rotation, but not in the opposite direction, and in the normal direction. To do this, the number of flashes of light does not increase, but decrease. For example, if 99 flashes per second disc will seem rotating forward, making one revolution per second.

We have here a "microscope time" with a 25-fold slowdown. But it is possible to obtain an even greater slowdown. If, for example, the number of outbreaks reported to 999 10 seconds (i.e., 99,9 per second), the drive will appear to be doing 1 turn in 10 seconds; he has, therefore, 250-fold slowdown.

Any rapid periodic motion can be described by taking slow for our eyes to the desired degree. This provides a convenient opportunity to study the characteristics of the movements are quite fast mechanisms, slowing their movement is our "microscope time in 100, 1000, etc. time *.

Describe in conclusion, the method of measuring the speed of a bullet based on the ability to accurately determine the number of revolutions of a rotating disk. On a rapidly rotating shaft wear a cardboard disk with blackened sectors and curved edges, so that the disk has the form of an open cylindrical box (see Fig. below). Arrow puts a bullet along the diameter of this box, punching the wall in two places. If the box was fixed, both holes lying at the ends of the same diameter. But the box was rotated, and while the bullet flew from edge to edge, the box had a little turn, substituting the pool instead of point *b* point *with*. Knowing the number of revolutions of the box and its diameter, the value of the arc *bc* to calculate the velocity of the bullet. This is a simple geometrical problem, which can easily cope readers a little familiar with mathematics.

Measuring the speed of a bullet.
* Reviewed the underlying principle used in practice instruments - strobe lights that are used to measure the frequency of quick-variable processes. Strobes give an extremely high accuracy of measurement (for example, the measurement accuracy of the electronic strobe reaches of 0.001 %). (Approx. as amended).