Physics in the third year of junior high school, please point out your fingers urgently

The teacher said that it was a uniform linear motion or stationary meaning.

When all external forces are gone, the ball can be in an ascending or descending state.

It can also be a very coincidental point in time, that is, the moment when the ball is just 0, that is, the ball moves upwards and decelerates evenly, and when the speed is just 0, all external forces disappear, and the ball is stationary.

There are three states in the process of vertical throwing, rising, stationary to the top, and descending. In the process of rising and descending, all external forces disappear and move in a uniform straight line, and when they reach the top, all external forces disappear and they will be stationary.

If there is an object moving parallel to the table tennis ball and at the same speed, then relative to this object, the table tennis ball is stationary, I hope it can help you, hehe.

When the table tennis ball still has speed, all external forces disappear, and the table tennis ball moves in a straight line at a uniform speed.

At the moment when the speed of the table tennis ball is 0 (the highest point), all external forces disappear, and the table tennis ball is stationary.

According to the great Newton's first law, an object remains at rest or moves in a straight line at its original speed without being acted upon by external forces. That's how it works, in fact, you just need to focus on the key points when doing this kind of problem! I wonder if I can?

All external forces disappear and must be motioning in a straight line at rest or at a uniform speed, following Newton's first law.

The teacher was right. This is Newton's law. When an object is not subjected to an external force, it is moving at a uniform speed or at rest.

Depending on the movement state of the table tennis ball at the moment when all external forces disappear, the teacher should estimate the fact that the external force disappears when the table tennis ball is at its highest point.

After throwing, the key is when the "after" will come, whether it is rising or just the highest point;

For junior and senior high school physics problems, more attention should be paid to special positions, such as motion turning points, image inflection points, etc.; Especially in the option to fill in the blanks.

Flip through the book to find the answer by yourself and deepen your impression What others are saying, you may not necessarily understand it from the root You find the answer by yourself is the best Physics is a test of logical thinking From your questions, you can see that your physics foundation is not old.

There will be differences depending on the reference, and the force is the reason for the change in the physical state.

This is the velocity-time image;

A represents uniform motion (equilibrium state), while B is accelerated motion (non-equilibrium state);

So A is subjected to a balanced force, while B is subjected to an unbalanced force;

So choose C I hope it will help you

cA velocity does not change, so it is a uniform linear motion subject to equilibrium force.

B velocity increases with time and is therefore a uniform acceleration linear motion with acceleration and unbalanced forces.

c, because A accelerates at a uniform speed and B accelerates at a uniform speed.

If it is changed then, both will regain the balance force, because A is at rest and B has a constant velocity.

Do you know the definition of maximum static friction? When the maximum static friction is exceeded, the object of Hesong ab is relatively slippery, remember? Suppose that the maximum static friction between AB is 30N, and that object B is placed on a smooth plane.

When pulling a B object with a force less than 30N, ab moves together, you want to ask why? Reason: Because a force of less than 30N is used to act on B, the friction between AB provides power for A, so that AB can move together, but the friction between AB can only provide a maximum force of 30N (maximum static friction), that is, the acceleration (assumed value of 5) produced by the force of 30N on A is the maximum acceleration of A, when the force of pulling B object is large, When the resultant force of these two forces (tensile force and maximum static friction) produces acceleration (assumed value of 10) for b when it is greater than the maximum acceleration of a (value of 5), that is, 10 5, ab will slide relatively, and a will fall behind relative to b, so it will fall off object b (according to the length of the body of b object, some problems will ask how long you need to fall, what is the displacement of ab when falling, etc.), note:

This kind of problem needs to be analyzed for each object in order to be properly explained.

Pull the object with 100n.

What is the frictional force experienced by an object if it does not move?

A: 100n.

Pull the above object with 200n.

Constant motion. What is the friction involved? A: 200n. Solution: Force analysis.

If you still have questions, you can ask them directly.

There are a lot of physics teachers here, I don't dare to say who is the best, but your question is not difficult teacher, I can help you comment, but I will not help you cheat in the exam

Third question:

Do not use a rheostat and a voltmeter, directly use 5, 10, 20 ohm resistance measurement, although the same power supply is used, but due to the different current in the circuit, the current inside the power supply is also different, so that the voltage drop inside the power supply is different, the voltage output of the power supply will also change: in fact, the voltage added to the resistor at both ends of the 5, 10, 20 ohm resistor is different, so it can not be predetermined to conclude.

Since the sliding rheostat is 20 ohm 1 ampere. When the fixed value resistance is 20 ohms, the rheostat slide moves to the maximum value of 20 ohms, at this time, the voltage representation is only half of the power supply voltage, because there is voltage inside the power supply, so half of it will be less than 3 2=, so the u value is better to choose greater than. The answer is b.

When u=, connect the 5 ohm fixed value resistance, the current in the circuit exceeds, the internal resistance of the power supply is generally about 1 ohm, so the voltage in the power supply will be greater, the output voltage will be less than the total voltage, so that only make u less than or equal to appropriate.

The first question is that the current of a conductor is directly proportional to the voltage.

When connected to 1, the current is the same, the power is i square * r, so r2 is twice as much as r1, let r2 = 2, r1 = 1, the voltage is unchanged, so u (1 + 2) = 2 3 * u (1 + r3), so r 3 = 1, the next is a simple power calculation of the series resistance, calculate it yourself.

Because the ratio of r2:r1 power is 2, r2=2r1 is obtained from p=i*2r, and because the ratio of the current representation is 2:3, r1+r3=2r1 is obtained according to i=u r

r1=r3, so u2=utotal 2, u3=2utotal 3 according to the formula p=u*2 r to get p3:p2=4 9u*2 2r:1 4u*2 r from this the ratio is 9 8