Regarding the question of the scientific ability of the first year of junior high school, the questi

According to Newton's third law, the action of forces is reciprocal.

If the object is at rest or in a state of uniform linear motion, the effects of these forces cancel each other, and we say that these forces are balanced.

Judging the equilibrium force: 1. Two forces act on the same object.

2. Each pair of forces or each pair of resultant forces is in the same straight line, opposite in direction, and equal in magnitude. (i.e. the net force is zero).

3. The object is at rest or in a state of uniform linear motion. *Note: Distinguish from interaction forces!

Equilibrium forces are also divided into multi-force balance and two-force balance, and the conditions for two-force balance are: the two forces are equal in magnitude, opposite in direction, acting on a straight line, and acting on the same object!

The interaction force is the mutual force between two objects.

Definition of Equipoise:

Several forces acting on the same point, the vector sum of which is 0, is a set of equilibrium forces.

To put it simply, for two forces, if they are equal in magnitude and opposite directions, acting on the same object, then these two forces are a pair of balanced forces. In junior high school, only one pair of equilibrium is examined.

A pair of equilibrium forces always act on the same object, on the same line!

If the object is only subjected to the action of the equilibrium force, then it will remain in a state of motion at rest or in a straight line with a uniform velocity.

Deepening a little, if the net force of the three forces acting on the same object is 0, then the three forces are a set of balanced forces ......By analogy, four or more forces can be obtained.

Action and reaction forces (i.e., interaction forces): When an object A exerts a force on another object B, object A itself must also be subjected to a force exerted by B. If the force exerted by the A object is called the applied force, then the force experienced by A is called the reaction force.

The action and reaction forces are equal in magnitude and opposite in direction, acting in the same straight line, and always occurring, increasing, decreasing, or disappearing at the same time.

The action and reaction forces always act on different objects!

Because the action of force is reciprocal, so the horse is both an object of force and an object of force, so it is right for the car to exert force on the horse as well, and it is wrong for man to have no attraction to the earth.

c；Because the wooden box must have experienced a force, D is excluded; The same direction of force as stated by A does not meet the conditions, and although the gravity of the wooden box is equal to the pressure of the wooden box on the ground, it is more accurate to say that it is "the pressure of the wooden box on the ground".

As for waxing the belt on the belt, it is equivalent to the sand on the runway when the athlete is running, because the belt is slippery.

1 The action of force is mutual.

2. Increase the friction and increase the pressure. 3b

Because the action of force is mutual.

It is to reduce friction. b

Solution, the force analysis of the wooden block has:

Vertically, the wooden block is subjected to gravity, m=mg=ox.

The tensile force t = 32 N, because the uniform speed of the movement, indicating that the force is balanced.

So the friction force f=t-m=bull, in the direction downward.

From the horizontal force balance, it can be seen that the pressure of plank B on block A is also 20 N.

In the vertical direction, the board is subjected to two frictional forces f A and F B, gravity g and tensile force ff A = f B.

g=mg=2*10=20

f = g + f A + f B = 32

Therefore, the friction between the boards fA = f B = 6n

If it's the total friction, add the two together).

Solution: The wooden block is subjected to four forces in the vertical direction, namely: downward gravity g, upward force f, and downward sliding friction between the wooden block and the two plates, both are f, then 2f+g=f, g=mg

Solution. f=6n

(1) 1, auditory imbalance. 2. Tail steam taste. 3. Bumps, sharp turns, or frequent.

4. Read a book in the car, play with your mobile phone, look at roadside buildings, trees, etc. That's all. Workaround:

1. Eat easily digestible foods. 2. Swing more. 3. Take motion sickness medicine.

2) A column: A glass of water and half a glass of water are different when you hit it, because the vibration frequency of the cup is different due to the difference in water, so there will be different sounds. In the same way, when you fill a kettle with water, the vibration caused by the collision between the water and the inner tank of the kettle will change as the water rises.

If you get a lot of water, you will become familiar with the sound of the water when it is fast and slow.

3) The problem of transmission mode, the recording is actually equivalent to you talking yourself, and then listening through the air, and usually the sound you hear when you speak is from the bone, the medium is different, so the sound will be different.