Physics Motion and force, what is the relationship between various forces in physics in junior high

From the question stem, we can know that the two cars of B and C are either moving in the direction of return, or stationary, or the speed is less than A but the same as the speed of A.

1. According to the description in the question, car C is stationary relative to the ground: if car A is driving to the left, the student will see car C driving to the right; If vehicle A is driving to the right, the student will see vehicle C driving to the left.

2. According to the description in the question, train A and B are driving in the opposite direction: if car A is driving to the left, the student will see car B driving to the right; If car A is driving to the right, the student will see vehicle B driving to the left.

Based on the analysis results of 1 and 2, no matter whether car A is going left or right, the students in car A must see that car B and C are driving in the same direction.

However, the question said that the student saw two cars B and C driving in the opposite direction, which of course could not happen.

The meaning of the title is that B, C and 2 trains are reversed.

Or is B and C the opposite for A?

Car C is stationary and moves southward with respect to A; Car B also moves southward relative to car A, so the relative direction of movement of car B and car C must be the same.

If C is stationary, A is exercising, and it is in the positive direction, and A is used as a reference, then C's exercise direction is in the negative direction, and B and A are reversed, then B is also in the negative direction, so that B and C are in the same direction, which is inconsistent with the opposite motion of B and C in the question.

Because then you will see B and C in the same direction.

1.There is no direct relationship between the various forces [in other words, it cannot be directly said that one force is necessarily equal to another, only the action and reaction forces.

Equilibrium has a relationship between equal magnitude and opposite direction].

2.For gravity and pressure, an example can be given:

An object is placed on the flat ground at a high level and is analyzed by force.

It can be known that the gravitational force and the ground support force on the object are equal in magnitude and opposite in direction, and are a pair of equilibrium forces, while the supporting force and the pressure of the object on the ground are a pair of action and reaction forces, equal in magnitude and opposite in the opposite direction. So we can know this fact: gravity and pressure are equal.

Of course, if the object is placed on an inclined plane, gravity is not equal to pressure, which is easy to know because of friction.

Put sand in the machine. 3.Different surfaces have different coefficients of friction.

For example, the coefficient of friction of wooden planks is the same as that of ice surfaces.

Of course, [for the same object, the same kind of object, the friction coefficient is a fixed value], but if you add water, lubricating oil or something to this surface, the friction coefficient will change.

Vertical pressure is not necessarily equal to gravity, it is established without the action of external force, for example, if you give a downward pressure is equal to the force of gravity plus the force you give, the pressure you give an upward force is gravity minus the force you give to the chain, pressure refers to the positive pressure on the surface, if a surface is tilted, then the pressure on the surface is equal to the component of the gravitational force perpendicular to the surface.

Gravity is not equal to pressure, to name two examples. When an object is stationary in the horizontal plane, you press down on the object or lift the object upwards with your hand (keeping the object in its original position), and the support force and gravity of the object on the horizontal plane are not equal. In addition, if you take the elevator upstairs, the process that has just started is an acceleration process, and at this time you are supported by the elevator more than gravity.

There are many examples of other situations as well.

The study of the relationship between forces is generally the study of the relationship between forces acting on the same object. For example, some topics may refer to the motion of multiple objects in the moving carriage, and in the study of the force on the carriage itself, the motion of the objects in the carriage can be regarded as internal forces, that is, the carriage and the objects in it are analyzed as a whole.

When analyzing the relationship between forces, it is necessary to analyze the motion of the object on which the force acts. For example, the net force of an object moving at a uniform speed is 0, while the net force of an object moving at a variable speed is 0.

A person is in train A, and sees train B with train A as a reference, and train C moves in the opposite direction, so with the ground as a reference, the following statements about the movement of three trains are impossible (

a.Trains A and B travel in the same direction, and train C travels in the opposite direction.

b.Trains A, B, C, all travel in the same directionTrain A and B travel in the opposite direction, and train C is stationary

Trains A and B travel in the same direction, and train C is stationary, please explain the reason. If a is true, then the speed of A" the speed of B, the speed of C does not require b If it is true, then the speed of A "the speed of B", and the speed of A" the speed of C C If it is true, there is no requirement for the speed of the train of A, B C If it is true, then the speed of A "The speed of B" and the speed of C are not required so it should be abcd is possible, as long as the speed of A is "the speed of B", and the speed of A is the speed of C, B is established, regardless of the speed of A, B and C, C must be established as long as the speed of A and the speed of B, D is established.

To solve this type of problem, first draw a diagram to represent several objects.

A as a reference. A has three situations relative to the ground.

1 When A is at rest, both B and C are moving in opposite directions.

2. A is moving forward, B and C are moving in opposite directions, a or B and C are moving in the same direction (slower than A), C or B and C are stationary.

or B is at rest with C or B is at rest with C or B is at rest with C or C is at rest with B.

3.A moves backwards, and B C and A move faster in the same direction than A.

c. So select all for comprehensive analysis.

Force is the cause that makes an object move is wrong, and force is what changes the state of motion of an object. (Newton's First Law).

If an object is originally stationary, then in order for it to move, it must be forcefully involved. That's the right idea.

It is a question of the relationship between force and motion. This question has always bothered many small pot friends.

First of all, the motion of an object does not require force to sustain it. Because Newton's first law is known that an object always maintains its original state of motion when it is not subject to an external force, it does not require force to maintain the motion.

Secondly, the external force is the reason for changing the state of motion. Newton's first law knows that once an object is subjected to an external force, the original state of motion changes, which also produces acceleration, so the external force is the cause of acceleration.

So the force is not what makes the object move, but what changes the motion – like acceleration or deceleration.

The answer is [c].

Feeling that the back of the seat has a force on oneself indicates that the person's body is moving backwards relative to the car due to inertia, and the car is in a state of acceleration.

a.Hasten. b.Starting, the instructions are also accelerating.

c.Brake, the car does a deceleration motion, the person should lean forward due to inertia, contrary to the title dClimbing up the hill at a constant speed, because on the hillside road, the car has a certain angle with the horizontal direction, and due to the gravity of the human body, it will lean on the back of the chair (which can be understood by drawing).

c Obviously, according to inertia, the chair will give us a thrust when accelerating or starting, so that we can continue to follow the car, otherwise we will keep our back in its original state of motion due to inertia (i.e., stationary in the same place or moving at the previous speed of the car, unable to keep up with the car).

To be honest, the meaning of the title is not clearly explained, whether the person is facing the direction of the car's movement or facing away from the direction of the car's movement, so it is possible. Perhaps the so-called standard answer is c

The resultant force of the two supporting forces is straight upwards and forms a pair of equilibrium forces with gravity, at which point the ball can be seen as only receiving two forces straight up and straight down, and the object is at rest and these two forces must be balanced.

c c It is also possible to slow down when going up a hill, but the acceleration is smaller, and the back of the chair also has a force on itself.

But relative to the other three is the least likely.

c, the inertia of the brakes will lean forward.