Science about force is a science question in junior high school, which is mechanics

1 False are in contact with each other and have a tendency to move relatively.

Definition of frictional force: If two objects in contact with each other have a tendency to move relative to each other or move relatively, a force that hinders the relative motion will be generated on the contact surface of the two objects, and this force is called friction. If there is a relative tendency to slide between two objects in contact with each other but have not yet sliding, the force that hinders the movement on the contact surface is called static friction.

If two objects in contact have a relative sliding, the force that hinders the movement on the contact surface is called sliding friction. The magnitude of friction is related to the material and smoothness of the contact surfaces of the objects in contact with each other, and also related to the positive pressure between the objects.

When calculating the magnitude of friction, it is necessary to determine whether the friction is sliding friction or static friction. Then use the corresponding method to find out.

The formula for calculating the magnitude of sliding friction is f = n, and the dynamic friction factor in the formula is also called the sliding friction coefficient, which is only related to the roughness of the material and the contact surface, and has nothing to do with the contact area; n is positive pressure. 2 false.

1.False, if an object is subjected to friction, there must be another force acting on it, and two objects that are in contact with each other have no friction if they are not subject to other forces, such as a wooden block placed on a table.

2.Right, like hovercraft and touching the lube on objects.

1.False Contact does not have to have friction, there must be friction and elasticity.

2 False can reduce beneficial friction.

1 False: Both objects are stationary and have no tendency to move relatively, there will be no friction. 2 pairs, so that the dynamic friction factor is small.

First mistake

I'm standing on the floor! But there was no friction

2 Wrong. Maglev trains.

Speaking of your picture, I can't even see it if you work so hard to draw it

I don't need to explain to you in the diagram, the initial state of the ball contains kinetic energy and gravitational potential energy;

Because the initial height, velocity, and mass are the same, the initial kinetic energy and gravitational potential energy of the 3 balls are exactly the same;

According to the law of conservation of energy, then when the 3 balls land, the gravitational potential energy is 0, so the kinetic energy at this time is also the same, and according to the kinetic energy formula, the velocity can be known to be the same (different directions).

If you don't have a picture, you can't say it in terms of speed, you can only tell you the answer from the energy point of view first, and if you need it, send a picture, and then explain to you in detail why this is happening.

A and C are exactly the same at the moment of landing, A first moves upward, then falls, and then when it falls back to the initial position, its velocity and muzzle velocity are the same, and the direction is downward, which is exactly the same as that of C.

Their minute velocity in the vertical direction is not the same, as it should be.

It should be viewed from the point of view of conservation of energy. Their height mass is also the same, then they have the same potential energy. When you land, all the potential energy is converted into kinetic energy, and the initial kinetic energy is the same, so the final kinetic energy is the same, so the velocity is the same.

You don't consider the potential energy factor.

The initial kinetic energy of A, B and C is the same, and the work done by the gravitational potential energy of the landing is also the same {the initial height is the same}, so the final kinetic energy = initial kinetic energy + potential energy. It's the same with speed.

m A = m B = m C, v0 A = v0 B = v0 C, h A = h B = h C, so the total mechanical energy at the beginning of the three balls is equal, from your answer you can know that ball A is thrown upward, ball B is thrown horizontally, and ball C is thrown downward. The ball thrown upwards should completely convert the initial kinetic energy into potential energy, so the height of the fall is the maximum, and the ball thrown horizontally is the same as the height of the ball thrown downward. At the moment when the three balls land, the potential energy is completely converted into kinetic energy, and the mass and mechanical energy of the three balls are equal, according to the law of conservation of mechanical energy, it can be known that when the three balls land, the kinetic energy of the three balls = mechanical energy, and the mechanical energy when the three balls land is equal.

Whereas mechanical energy = mgh + 1 2mv 2H=0 when landing, so E A = E B = E C = 1 2mv 2, and the mass of the three balls is equal, so the velocity when landing is equal.

Two forces balance three elements: the acting closed beam is on the same object, equal in size and opposite in direction.

The train moves at a constant speed, and although the coin is thrown out in a state of thought, it only displaces in the vertical direction, and still advances at the original speed in the horizontal direction, and the bird is relatively stationary with B, so it still falls in B's hand.

According to Hooke's law, f=kl (k is the modulus of elasticity, l is the deficit length of the spring pull).

When hanging 10g weights, the spring is elongated by 2cm, and when hanging 50g, the spring is elongated by 2*50 potato empty loss of 10=10cm, so the original length of the spring is 25-10=15cm

I can explain 3 questions, and the rest is not good to search for the meaning of not learning, should choose B

Because the coin did not move in the rolling position of the carriage, and neither did Ant Li B, the coin fell vertically down into B's hand.

On a horizontal tabletop, the gravitational force on the object and the supporting force of the table on the object are a balancing force.

The equilibrium force is characterized by the fact that the object is at rest or at a constant velocity and moves in a balanced and closed straight line. There are 3 requirements for a balanced force: 1 There are two forces acting on an object and in the same straight line2 The magnitude of the force is equal. 3. The direction of the force is cracked and reversed.

We learned this knowledge in the first chapter of the force part of physics, and you learned it very well in the first year of junior high school

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 balance force: 1. Two rising force 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).

This 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.

To go deeper, 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!

The wooden block and the trolley do a uniform linear motion to the right, when encountering an obstacle, the trolley stops, and the wooden block slides from point A on the car to point B (as shown in Figure A), because the wooden block has inertia The wooden block can stop at point B, indicating that the wooden block is subjected to the frictional force to the left (fill in "left" or "right") in the sliding process In order to measure the magnitude of the friction force, Xiao Ming fixed a spring dynamometer on the obstacle to pull the wooden block, and pulled the trolley to the left with his hand (as shown in Figure B), then the friction direction of the wooden block is to the left (fill in "left" or "right"), The size is n (you have to see the picture yourself).

1. Inertia. 2, right.

3, left. 4. How much is on the spring. I can't see clearly.

Answer BCB12 Both questions are propositions with a clear cover rolling off the balance force, this kind of question should grasp a key point: when we talk about the balance force, there is only one (instead of two) force objects, and the object is subjected to a pair of forces in the opposite direction of equal magnitude, which is called the balance force. The object is subjected to a balancing force.

Easy to work with. Interaction of objects.

Confusion, when two objects interact, there are also two interacting forces of equal magnitude and opposite directions, but these two forces act on the two objects respectively.

The third question can actually be said to be a common sense question, and the key is to drive at a constant speed. Think of a coin as an object, and a train and a person as an object. Start the coin in B's hand, at the same speed as the train; After being tossed, the coin is only subject to gravity and is not subject to the net force in the horizontal direction, so it still moves at a uniform speed in the horizontal direction, and the rate is the same as at the beginning, so it still falls in B's hand.

What happens if the train accelerates or slows down? Think for yourself.