How to write the "do it" process of estimating the exposure time in the required course in physics

Exposure time of the camera shutter

After learning the laws of free fall motion, we arranged such a question in the "Do it" column: "... from in front of a certain brick wall Make a stone fall freely from a high place, and take a photo of the stone in the air as shown... It is known that the average thickness of each brick is 6 cm, and the height of the starting and landing point of the stone from the ground is about 2.5 m. How to estimate the exposure time of this camera? ? ”

Photos of falling stones

The solution suggested by the editor is as follows.

The height of point A is estimated by the number of layers of bricks - 60 cm, and the length of AB is estimated by the number of layers of bricks - 12 cm; the height of the stone from the starting point to point A is 190 cm. From this, the speed of the stone passing point A is calculated to be 62 m/s. The time it takes for the stone to pass AB at such a speed is s=0.19 s, that is, seconds.

Some students think that it is inappropriate to divide the length of AB by the speed of the stone at point A, because the stone is still accelerating during the movement from A to B. Instead, h = gt2 should be used to calculate the speed of the stone. The difference between the time taken at two points A and B is the time taken from A to B.

The second solution is of course correct, but it is not necessary. On the one hand, judging from the original material, the data we obtain through the thickness of the bricks is quite rough, and the calculation does not need to be so precise; on the other hand, if you know something about photography technology, you will know that a small deviation in the exposure time will not affect the quality of the photo. What impact will it have? In fact, even if the camera is marked with a shutter speed, its nominal value is not very accurate.