[quote author=wan1201 link=topic=2288.msg8523#msg8523 date=1315214855]
[b]Three kinds of techniques help you master physical kinematics[/b]
Mechanical motion is the most basic forms of exercise in physics, study mechanical movement rules of branch called kinematics. High school physics, the application of kinematics rules is not only reflected in the mechanical parts, also penetrates in heat and electromagnetism of, so kinematics knowledge is not only an basic knowledge, is also the important knowledge, the application of the kinematics rules solve problems nature is a focus of the university entrance exam proposition, in this paper, several of kinematics skills in mathematics, a simple summary for the classmates reference.
Qiao choose references
Motion is absolute, but we description of the sport is always relative, that is, to study a the movement of objects, must choose a reference. In theory, the selection of reference is arbitrary, but the selection of reference to solve some extent will have obvious effect, so we in the solution, the kinematics analysis in cet4, and on the basis of frame of reference, to select the suitable the question of the study and simplified, solve the problem rapidly.
Example 1 as shown, in a column of the elevator hanging a small balls, ball distance elevator floor for h. If the elevator to accelerate to move up, a acceleration some point to the line is cut and ask: how long ball after fall to the floor?
Method 1: with the ground, epigastric trails line balls do vertical movement, the elevator is still throw up do acceleration in the movement. The process of the elevator and ball displacement h1 and hq, respectively their relationship as shown in figure, so have: hq + h = h1
Use opportunely image
In high school the application of physics image is very wide, this is because the image can image to express laws of physics, intuitively describe the physical process. By image problem solving, is to use the slope of the image, intercept, area, peak, and has the intersection of physical meaning, qualitative analysis and quantitative calculation. The information provided by using graph line problem solving, is an important problem solving method.
Example 2 two vehicles the same car of A and B flat highway in tandem with the same speed driving at A constant speed. Now the sudden braking, taxi distance S stop after. Car drivers stop in after the moment immediately, and brake process of brake acceleration and the same. To avoid two trains collided, two car in uniform road, should at least how far apart? Put two car brake process do slow movement and.
Method 1: two car movement process as shown in figure, the call is in s. The figure may have the relationship between displacement:
S + s = v0t + s and s = v0t / 2
Solution: s = 2 s
Method 2: according to two car at the same rate and brake at the same speed to add process conditions, it is not hard to push two car brake process of gliding distance and time are the same. From A car start braking, at the same time draw two car coordinate system of v-t lines, as shown in figure. As can be seen from the graph, from A car started to brake to B last stop car of time, A car is to make the displacement S slash the area of A triangle, the car is right B displacement of the area by the graph, the trapezoid it's easy to see the SB = 3 S, so to avoid two cars collided in uniform down the car at least two apart: ? S = 3 S-S = 2 S.
Use opportunely inverse extrapolation
Downstream thinking is physical processes occurring in accordance with the order of the working party, because of the fruit of the way of thinking, reverse thinking is will the physical process for inversion, holding fruit cable for way of thinking, popular version is "in turn think about it". Using reverse thinking problem solving method is inverse extrapolation. To solve in the problem sets, if the actual kinematic process is slow movement, so its well inverse process is acceleration in the movement, and zero velocity of the acceleration in the movement has a group of scale relation, make use of these proportional relations can be easily solve the problem.
3 a sample particle do well slow movement, through 36 m stopped. If this period of displacement into three parts, and through each time equal particle, sought to the length of the first paragraph.
Method 1: if the thinking way toward solving, according to a size for a, acceleration every time for T, it is:
From the above three breathing: S1 = 20 m
Method 2: if according to reverse thinking mode, will the movement to solve inverse into the opposite direction started from the early do zero speed acceleration in the movement, is the third length is for. Because of the zero velocity acceleration in the movement for equal time in the ratio of displacement for:
S1: S2: S3 = 1:3:5
Can see the second solution than the first one simple solution. Shippo middle school teachers LiuShuTian "
I loved the way you explained. Kinematics is a totally a concept based chapter.