Write detailed process and formula 1. A car honks its horn in front of the mountain and hears the echo after 25s (the echo is 340m / s). If the car approaches the mountain at the speed of 72km, how far away is the horn honking from the mountain? 2. He stood between the cliffs and fired at a certain position. After 1 s, he heard the echo for the first time. After 0.5 seconds, he heard the echo again. Given that the sound speed is 34m / s, what is the distance between the two cliffs?

Write detailed process and formula 1. A car honks its horn in front of the mountain and hears the echo after 25s (the echo is 340m / s). If the car approaches the mountain at the speed of 72km, how far away is the horn honking from the mountain? 2. He stood between the cliffs and fired at a certain position. After 1 s, he heard the echo for the first time. After 0.5 seconds, he heard the echo again. Given that the sound speed is 34m / s, what is the distance between the two cliffs?

1. S = 25s (340m / S + 20m / s) △ 2
=25s × 360m / s △ 2
=9000m÷2
=4500m
2. S = 340m / S (0.5s + 1s) △ 2
=510m÷2
=255m

A problem about Coulomb's law
The distance between the centers of two equal charged metal spheres with radius R is 4R, which is related to the electrostatic force between them,
The force with the same positive charge is F1, the force with the same positive charge is F2, and the force with the different charge is F3
The relationship between the three forces is:?

With positive charge: positive charges repel each other, will make charge center away from each other, so F1 minimum
Negatively charged: negative charges repel each other, just as positively charged, F2 = F1
One positive and one negative: mutual attraction, charge centers close to each other, F3 maximum!

As shown in the figure, the identical metal balls a and B have the same amount of heterogeneous charges, and a light insulation spring is connected in the middle. When they are placed on a smooth insulation horizontal plane, the compression of the spring is x0. Now contact the uncharged metal ball C which is identical to a and B with ball a, and then take it away. After rebalancing, the compression of the spring is x, then ()
A. x＝12x0B. x＞12x0C. x＜12x0D. x=x0

At first, the ball a is in equilibrium, and the Coulomb force is equal to the spring force, which is kX0 = kq2r2. When the ball C is in contact with the ball a, the charged amount of the ball a becomes half of the original, assuming that the compression amount becomes half of the original, we know that the elastic force is f = kx02, and the Coulomb force F ′ = kq22r ′ 2

A problem in Coulomb's law
There is a problem... Two balls are hung on the same point with insulated ropes. We know that when they are not charged, they will touch each other. But if they are charged, they will be separated for a certain distance and then they will be in static equilibrium. That is to say, both balls are forced. At this time, I want to know, How to use Coulomb's law? Does f in it refer to any f or something? If you know the angle between mass and a ball, is there any way to find the distance between them

Is face f any f or what?
F is the Coulomb force on any ball
According to the force balance, mgtana = F
F=kQ1Q2/R²
Only when we know their charge can we find R
R=√(kQ1Q2/mgtana)

Each vertex of a square with a side length of a is placed with a point charge of Q. if their positions are kept unchanged, the resultant force of each charge by the electrostatic force of the other three charges is ()
A. 22kq2a2B. (22+1)kq22a2C. 3kq2a2D. 17kq22a2

As shown in the figure, assuming that the fourth charge q is placed at point D, the Coulomb repulsion force given to it by the charge at point B on the diagonal is F1 = KQ2 (2a) 2, and the Coulomb repulsion force given to it by the charge at point a and C is F2 = F3 = kq2a2. According to the law of force composition, the electric field force on the point charge q is f = KQ2 (2a) 2 + 2kq2a2 cos45 ° = (12 + 2) kq2a2 & nbsp; so select: B

Why is Coulomb's law applicable when two charges are stationary or only one charge moves, but not when two charges are moving?

Because Coulomb's law is to calculate the force between two point charges through the electric field
When the two point charges are stationary, there is only an electric field between them, and Coulomb's law is applicable;
When only one point charge moves and the other is stationary, the moving charge can produce a magnetic field, but the magnetic field has no effect on the stationary charge, so Coulomb's law is still applicable;
When two point charges are moving, because the magnetic field acts on the moving charge by magnetic force, the force between the two moving point charges is the combination of the force through the electric field and the force through the magnetic field. This resultant force can not be calculated by Coulomb's law, that is, Coulomb's law is not applicable to calculate the total force between them

Problems in Coulomb's theorem
For example, two metal spheres with different charges will attract each other and touch each other. Then, there will be no distance between the two centers? R = 0. Then, how can many problems give them a distance? Is there an external force? And how can the different charges without point charges be solved by Coulomb's theorem?

If we regard it as a point, we don't need to care about the size of the force after it is attracted. The closer it is, the greater the attraction is. There is a distance because you can calculate the size of their attraction at this moment. Generally, there is no external force. If we regard it as a ball instead of a point, then the distance between two dissimilar charged metal balls should be the distance between the center of the ball minus two radii, If it's two metal spheres with the same charge, then the distance is the distance between the centers of the spheres plus two radii

The point charge problem of Coulomb's law
Seeing an object as a point charge, in fact, he did not use the point charge to do experiments. How can he know that the point charge satisfies the conditions?
How to know that shape and size have little but not great influence on the research problem?
Even if the object is very small, how can we know that the distribution of charges (or the distance between charges) will not have a great influence on the force between charges?

The experimental accuracy can only reach certain conditions, not complete accuracy and perfection
Moreover, the conclusion derived from this law is consistent with the experiment
 
Shape and size have little influence on the research question & nbsp; rather than great influence
This is relative
Look at the research,
 
The influence is relative
It can not be calculated as a point charge
The sum is calculated as a point charge
You'll know how different it is
 
thanks
 

Equilibrium law of three free point charges only affected by Coulomb force
In vacuum, two freely moving point charges are located at a and B respectively. The point charge at a is positively charged with Q, and the low high charge at B is 9q. Another freely moving point charge Q3 is taken at C. if the distance between AB and ab is D, how far away from a should Q3 be placed in the equilibrium state? What is the charge quantity?

Three points are collinear (three points are on the same line)
Two identical charges and one different charge
Near small far large (the amount of charge close to the charge is small, the amount of charge far away is large.)

Electrostatic force formula
When did you learn it

In physics elective 3 - 1 book, when the specific study may be inconsistent