High School Physics!! Urgently, please explain, high school physics !!

There are two situations in which an object is stationary: 1 It is not subjected to force, 2 It is balanced by force, for example, if you hold a rod in your hand and move it horizontally, the rod is in motion, but it is subjected to static friction that is vertically upward, so that it does not fall.

D The friction force is generated from the extrusion of two objects A and B after contact, so that the object has a tendency to move or move relatively, and the friction force is equivalent to A being prevented from moving by the force generated by another object B, so the direction of sliding friction must be opposite to the direction of motion of the object, and the direction of static friction is opposite to the relative direction of motion of the object.

A stationary object can be subject to sliding friction, e.g. a block slides down on an inclined plane, the inclined plane is stationary, but it is subject to sliding friction from the slider, so term c is wrong.

a 。Let's just say there's a wall. Then use a rope to attach a wooden block to the wall.

There is a large wooden block underneath the wooden block. Now pull that big block of wood underneath. Although the wooden block on it did not move.

However, it is still subject to sliding friction. can exclude c

b。。I haven't been on the elevator. Just thinking about putting an object on the elevator to move. If there is no friction. The object will slide on the elevator. That is, it proves that a moving object can be subjected to static friction.

d can be understood to be that sliding friction always hinders the relative motion of an object.

Let me give you a few examples;

A: Put a piece of wood on an iron block, put the iron block on the rough tabletop, and use a horizontal force to pull the block, if the static friction of the iron block on the table is greater than the sliding friction force of the wood block, then the iron block will not move with the wood block, so "a stationary object can be subjected to sliding friction".

b: Two wooden blocks of different sizes, the small one is placed on top of the large one, and the large wooden block is pulled by a horizontal force f, and the small wooden block will move together, and the small wooden block is subjected to the static friction of the large wooden block, so it is said: "The moving object can be subjected to static friction".

d: There are no examples, or they are all examples, if there is sliding, and there is sliding friction, then the direction of sliding friction must be opposite to the direction of motion of the object.

The answer is D....

A: According to the deterministic formula of electric potential (I don't know if your teacher talked about it, I remember that our teacher just mentioned it at that time, saying that the textbook does not require mastery. =kq r, q is the amount of charge of the charge that produces the electric field, you need to substitute the symbol, r represents the distance from the point to the charge of the field source, when you encounter a situation like this problem where there are two electric fields superimposed to find the electric potential of a certain point, since it is a scalar, just find the algebraic sum.

As you can see from the diagram, ra b a is wrong.

B: The strength of the electric field is judged by the density of the electric field lines, and it is clear that EA>EB B is wrong.

c: wab = quab (ab is the corner mark), uab is a- b, in the a option we already know that uab should be a positive value, but because it is a negative charge, q is less than 0, so wab is less than 0, that is, the electric field force does negative work c error.

d: From the formula e=q, since q is positive, we have already said a> b, so ea>eb d is correct.

I don't know if I made it clear.

d a: The two points of potential are equal when it is -2q. b：

The density of electric field lines is different, so the field strength is different. c: The direction of the electric field force on the negative charge is a certain angle of the fourth quadrant, because the electric field strength is an angle pointing towards the second quadrant, so negative work is done.

d: The electric potential of point A is high, so the potential energy of the positive charge at point A is large.

Answer: D Explanation: According to the knowledge that the equipotential surface is perpendicular to the electric field line, you first make two equipotential surfaces through the two points A and B, because the electric field line is directed from the higher potential surface to the low equipotential surface, the density of the electric field line indicates the strength of the electric field, the electric field force does positive (negative) work, and the electric potential energy decreases (increases), then you can choose the correct option.

The straight line where d ab is located is greatly affected by the positive charge, and the negative charge on the left side is removed at the limit, and everything is clear.

As shown in the figure, in the uniform magnetic field along the horizontal direction, the charged ball A and B are in the same vertical line, in which the ball B is positively charged and fixed, and the ball A is in contact with a horizontally placed smooth insulating plate C and is in a stationary state If the insulating plate C is removed in the horizontal direction, the following statement is correct.

A Ball A may still be at rest.

b Ball A will likely move along trajectory 1.

c Ball A will likely move along trajectory 2.

d Ball A will likely move along trajectory 3.

Removing the board, the ball A accelerates upwards to produce velocity because it is only subjected to an electric field force greater than the gravitational force, so the answer A is incorrect.

Judging from the direction of the Lorentz force, it can be seen that if a is a positive charge is forced to the left, otherwise it is to the right. Combined with the upward velocity condition, ball A can only move along trajectory 1. i.e. b is the correct answer. Only the velocity down c and d can be correct.

First, it is judged that A is positively charged, so that A can be stationary.

The force analysis of a is carried out, and the downward weight is heavy, and the upward Coulomb force is affected. There may or may not be support from C to A.

When gravity = Coulomb force, take out c and a is still at rest.

When the gravitational force is Coulomb force, there is a downward support force, and when C, a has an upward acceleration, it can be judged that there is a leftward Lorentz force.

Remember, in physics problems, when there is force, the problem is solved from the force analysis.

Develop good problem-solving habits. I wish you success in your studies.

The small ball A can be kept motionless under the plate, which shows that A and B are positively charged and can be repulsed, but the repulsion force is greater than or equal to the gravity of A, if the repulsion force is equal to the gravity of A, after taking the plate, A will naturally remain still, choose A, if the repulsion force is greater than the gravity, A moves upwards and is subjected to the left Lorentz force, so choose B, the repulsion force cannot be less than the gravity, so it cannot move downward, so CD is wrong!

Answer] C Analysis] The moon and the earth move in a uniform circular motion around o, and the gravitational force between them provides their respective centripetal forces, then the centripetal forces of the earth and the moon are equal. And the Moon and the Earth are always collinear, indicating that the Moon and the Earth have the same angular velocity and period. So there is v v=r r=m m , so , the linear velocity is inversely proportional to the mass, the correct answer c.

c。This o-point is the center of mass of the Earth and the Moon binary star system, and the angular velocity of their rotation is the same. The linear velocity ratio is the reciprocal of the distance to the center of mass, which is the reciprocal of the ratio of the masses of the two.

Looking at the figure, using the kinetic energy theorem, the maximum speed can be solved by the two formulas.