15. The server horizontally shoots two ping-pong balls with different speeds forward from the same height (ignoring the influence of air). The ball with higher speed crosses the net, but the ball with lower speed doesn't cross the net; The reason is (c)
It takes more time for a slower ball to fall the same distance.
A ball with a smaller speed will have a larger speed in the vertical direction when it drops the same distance.
C it takes shorter time for a ball with higher speed to pass the same horizontal distance.
D in the same descending time interval, the ball with higher speed will descend a long distance.
18. Scanning alignment microscope (STM) can be used to detect the surface morphology of atomic samples. In order to effectively isolate the interference of external vibration on STM, several pairs of copper thin plates are symmetrically installed around the circular chassis along its radial direction, and a magnetic field is applied to quickly attenuate its tiny vibration. As shown in the figure, when there is no disturbance, apply a constant magnetic field to the copper sheet according to the following four schemes; After the disturbance, the most effective scheme to suppress the vibration of copper thin plate is (a)
2 1. as shown in the figure, fix ONe end o of the soft light rope on, tie a heavy object at a certain point m in the middle, and pull the other end n of the rope by hand. Initially, OM is vertical and MN is straight, and the included angle between OM and MN is α (α >: π/2). Now slowly pull the weight to the upper right, keeping the included angle α unchanged. When OM is pulled from vertical to horizontal (AC)
A. the tension on Mn increases gradually.
B. the tension on Mn first increases and then decreases.
The tension on com is gradually increasing.
The tension on D.OM first increases and then decreases.
The above are some questions and answers of a comprehensive science examination paper in the national college entrance examination for reference only.