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As shown in the figure, the parallel metal plates a and B placed along the horizontal direction are respectively connected with the positive and negative poles of the power supply. There is a small hole in the center of the two plates along the vertical direction. Now there is a positively charged droplet. The point P directly above the small hole falls freely from rest, and the speed after successively passing through the two small holes is V1. If the plate a does not move, if the key K is kept open or closed, B When the plate is translated upward or downward for a short distance, the same droplet still falls freely from rest at point P, and the velocity after passing through two small holes successively is v2. The following statement is correct without air resistance () A. If key K remains closed and plate B moves downward, V2 > v1 B. If key K is closed for a period of time and then disconnected, and plate B is moved downward, V2 > v1 C. If key K remains closed, V2 = V1 regardless of moving board B up or down D. If key K is closed for a period of time and then disconnected, no matter moving board B upward or downward, V2 < v1
A. If the key K remains closed and the work done by gravity and electric field force remains unchanged, it can be seen from the kinetic energy theorem that the velocity of charged droplets remains unchanged, then V2 = V1, so a is wrong;
B. If the key K is closed for a period of time and then disconnected, move the B plate downward, the negative work done by the electric field force decreases, and the droplet velocity becomes larger according to the kinetic energy theorem, that is, V2 > V1, so B is correct;
C. If the key K remains closed, no matter moving the B plate upward or downward, the work done by gravity and electric field force remains unchanged. According to the kinetic energy theorem, the droplet velocity remains unchanged, that is, V2 = V1, so C is correct;
D. If the key K is closed for a period of time and then disconnected, move the B plate upward, the work done by gravity remains unchanged, and the work done by the electric field force increases. According to the kinetic energy theorem, the droplet velocity decreases, that is, V2 < V1; If the B plate is moved downward, the work done by gravity remains unchanged and the work done by the electric field force becomes smaller. According to the kinetic energy theorem, the velocity of the small ball becomes larger, that is, V2 > V1, so D is wrong;
Therefore, BC is selected
This is a physics problem. I hope to solve it as soon as possible! Along the 1000m long and 250m high slope, one vehicle weighs 6 × 10m ³ N mine car is pulled from the slope bottom to the slope top at the speed of 2m / s, if the tension is 2 × 10m ³ N. Find the friction force on the harvester?
The tension F of the tramcar is balanced with the friction F and the sliding component of gravity mgsina
F=f+mgsina
sina=250/1000=0.25
f=F-mgsina=2 × ten ³ N-1.5 × ten ³ N=500N
The small ball with air volume of M is tied at one end of the light rope and makes a circular motion with radius of R in the vertical plane. During the motion, it is subject to the action of air resistance. Suppose that at a certain time, when the small ball passes through the lowest point of the track, the tension of the rope is 6.5mg. After the breakpoint continues to pass, the small ball continues to make a circular motion and just passes through the highest point through half a circle, then in this process, How much work does the ball do to overcome air resistance?
Because the tension of the rope is 6.5mg when the ball passes through the lowest point of the track at a certain time, the ball continues to move in a circle after the breakpoint pass
F1 (centripetal force) = t-mg = (6.5-1) mg = 5.5mg
Therefore, the velocity at this point satisfies equation (V1): F1 = 5.5mg = m * V1 ^ 2 / R
v1^2=5.5*g*R
After half a circle, you can just pass the highest point
F2 (centripetal force) = mg
The velocity at this point satisfies the equation (V2): F2 = mg = m * V2 ^ 2 / R
v2^2=g*R
According to the kinetic energy theorem, the sum of work done by external forces = the amount of change in the kinetic energy of the object
Take vertical upward as positive
Wf+WG=Ek2-Ek1
Wf=-1/4*g*R
W G f = 1 / 4 * g * r
A physics problem, (question 9)
9. Set wet sand moisture content x% m wet sand = 100kgm water = xkgm dry sand = 100-x KGC water = 4180j / (kg ° C) C dry sand = 0.9 * 10 ³ J / (kg ° C) C wet sand = 1.2 * 10 ³ J / (kg ° C) C dry sand * (100-x) + C water * x = C wet sand * 1000.9 * 1000 (100-x) + 4180 * x = 1200 * 10090000-900x + 4180x = 12000
The physics problem of grade three hopes to have a detailed answer Gao Ming reads the newspaper through a convex lens with a focal length of 6cm and a diameter of 3cm. The distance between the lens and the newspaper is 4cm, so that the newspaper 16 cm away from the eye can be imaged 24 cm away from the eye. Set the eye on the main optical axis of the lens, and the newspaper is perpendicular to the main optical axis. If the newspaper is densely lined with words with width and height of 0.3cm, he can see at most several complete words on the same line through the lens
1/f=1/u+1/v
1/6=1/4+1/v
V = - 12cm (negative sign is virtual image)
Thus, it can be seen that the distance from the eye to the newspaper is 16cm; to the lens is 12cm; to the image is 24cm
The geometric relationship determines the image range of 6cm
k=v/u=3
Image size of each word = 0.9CM
N = 6 / 0.9 = 6 complete words
7. After learning the knowledge of thermometer, Xiaoming and Xiaohua took different methods to understand the temperature The method of directly feeling with his hands (as shown in Figure A and b) is adopted. The specific method is as follows: Xiao Ming first puts his hands into hot water and cold water respectively (as shown in figure a). When he first puts them in, the feeling of his hands is ____, After a while, the feeling of both hands _____, About a minute later, Xiao Ming puts both hands into the warm water cup at the same time (as shown in Figure b), then he feels _____ Xiaohua made a liquid thermometer by using a vial, a rubber stopper and a glass tube. She put the vial into different water cups and observed the height of the liquid column in the glass tube to know the temperature of different objects (as shown in Figure C). (fill in "same" and "different" in the above three spaces) (1) Through the judgment process of the two students, do you think____ The student's judgment is more accurate, and another student can't make an accurate judgment, which means ____________ (2) If Xiaohua finds that the liquid column in the glass tube of the thermometer does not change significantly during the experiment, please put forward an improvement suggestion__________________________
Different
identical
Different
floret
There is a difference between objectivity and subjectivity. Science should be objective
Put the thermometer into hot water first, and then cold water. The greater the temperature difference between hot water and cold water, the more obvious the change of liquid column
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