Sicifang photography

This used to be a college entrance examination question.

The picture shows a bullet passing through an apple taken by a high-speed camera. After enlarging the photo, the distance between the bullet impact and the back is about 1%~2% of the bullet length during the exposure time. It is known that the flying speed of the bullet is about 500m/s, so it can be estimated that the exposure time of this photo is the closest.

a、 10^-3s B、 10^-6s

c、 10^-9s D、 10^- 12s

Answer: The length of the bullet is about 5cm, and t = s/v =1%* 0.05m/(500m/s) =10-6s.

Suppose the length of the bullet is 10 cm, and s = 0.1* 0.01=10-3. Divide by speed to get the time of 2* 10-6.

In 2007, Beijing College Entrance Examination Physics 18, the basic skill of establishing the estimation model is to grasp the "order of magnitude" structure. The exposure time required by the topic (order of magnitude) is equal to the distance between the front and back of the bullet image (the key point of model establishment) divided by the bullet speed, so it is necessary to know the order of magnitude of the distance between the front and back of the bullet image and the bullet speed (step1); According to the meaning of the question, the distance between the front and back of the bullet image is about 1-2% of the bullet length, but what is the bullet length generally? Students can actually measure photos with the measuring tools in their hands, estimate them in proportion, or estimate them according to their own life experience (the longest anti-aircraft machine gun warhead is only a few centimeters, and the shortest general pistol warhead is about 1 cm) (step 2), so that the length of the warhead is in the order of 10 minus 2 meters (centimeter level); Then, according to the meaning of the question, "the front-back distance of the bullet image is about 1-2% of the length of the bullet" (step 3), then the order of magnitude of "the front-back distance of the bullet image" should be minus 4m of 10, and the order of magnitude of the bullet speed of 500m per second is10m per second (step 4); So the closest exposure time is 10 minus 6 seconds.