17. As shown in the figure, the semicircular smooth track is fixed on the horizontal ground, and the diameter of the semicircle is vertical to the ground. A small object slides into the orbit from the lower end of the orbit at a speed v and flies horizontally from the upper end of the orbit. The distance from the rapid landing point of small objects to the lower end of the trajectory is related to the trajectory radius. When this distance is maximum, the corresponding trajectory radius is (g).
Audio and video? / 16g B.v？ /8g C.v？ /4g D.v？ /2g
The answer to this question is B.
19. As shown in the figure, Neptune orbits the sun in an elliptical orbit, with P as perihelion, Q as apohelion, M and N as the two endpoints of the short axis of the orbit, and the running period is T0. If only the interaction between Neptune and the sun is considered, Neptune is in the process of moving from P to M and Q to N.
The time from p to m is equal to T0/4.
B. from q to n stage, mechanical energy gradually increases.
C. from p to q, the speed gradually increases.
D from m to n, gravity does negative work on it first, and then positive work.
The answer to this question is CD.
20. There is a uniform magnetic field between two parallel dotted lines, and the direction of magnetic induction intensity is perpendicular to the paper. The square lead frame abcd with a side length of 0. 1 m and a total resistance of 0.005 Ω is located in the paper, and the cd side is parallel to the magnetic field boundary, as shown in Figure (a). It is known that the lead frame has been moving to the right in a straight line at a constant speed, and the cd edge enters the magnetic field at t=0. The graph of induced electromotive force changing with time in the wireframe is shown in Figure (b) (when the direction of induced current is clockwise, the induced electromotive force is positive). The following statement is correct.
A. the magnitude of magnetic induction intensity is 0.5t.
B the moving speed of the lead frame is 0.5m/s.
The direction of magnetic induction intensity is perpendicular to the paper.
D during the period from t=0.4 s to t=0.4 s, the ampere force on the lead frame is 0.1n.
The answer to this question is BC.
The above are some of the questions and answers in the two-volume science comprehensive examination paper of the national college entrance examination for reference only.