Junior high school about friction, the difference between junior high friction and high school frict

Please note that this is junior high school physics and does not involve friction coefficients.

1.Wrong. Front wheel obstruction, rear wheel power The rear wheel gives the ground a backward force, and the ground gives the rear wheel a forward friction force to push the bicycle forward; The force exerted by the front wheels to the ground forward, and the friction force exerted by the ground to the front wheels backwards.

2.Yes, see 1

3.Not necessarily, when you accelerate, the friction becomes greater (the rear wheels are forward, the front wheels are backward, and the rear is greater than the front), and the minimum at constant speed) the rear wheels are forward, the front wheels are backward, and the size is equal), and when the front brake is braked, the friction becomes greater (the rear wheels are backward, the front wheels are backward, and the front is greater than the rear). When braking the brake, the friction becomes larger, all backwards, and the rear is greater than the front.

4.It depends on the friction coefficient and contact area of the road surface and tires, the weight and load of the bicycle, and even the air resistance.

5.Note that "squeezing" means that the block is at rest.

20 N, the vertical value of friction is upward, opposite to the direction of gravity, equal in magnitude, is a pair of balanced forces; The pressure on the block (50 N) and the support force of the wall on the block (50 N) are a pair of balanced forces.

20 Ox, the reason is the same as above.

The friction of the front wheel is backward, hindering the progress of the bicycle, and the friction of the rear wheel is forward, which is the driving force of the bicycle to move forward. The person pedals, the chain transmits the pedal force to the rear wheel, and the rear wheel has a force to the ground backwards, so that the ground gives the rear wheel a forward friction force to push the bicycle forward, in this process, the front wheel because the bicycle moves forward, the ground gives it a backward friction force to block it from moving forward. These two sets of forces are the relationship between the action force and the reaction force, when the friction of the front wheel and the friction of the rear wheel are balanced, the bicycle moves forward at a uniform speed; When it is less than it, it is acceleration, and when it is greater than it, it is braking.

See (but only when pedaling).

Suppose that the person is on the center line of the gravity of the bicycle, the center of gravity of the person and the center of gravity of the bicycle are on a plumb line, and always maintain, that is, the positive pressure of the front and rear wheels on the ground remains unchanged, and the friction of the front wheel remains unchanged under the condition that the friction coefficient remains unchanged; The friction of the rear wheel is determined by the force of the person pedaling when moving forward, and the friction of the rear wheel is closely related to this: the bicycle chain disc is a fixed pulley, so the friction force on the rear wheel is equal to the pedaling force of the foot, if you do not pedal the pedal, this friction force is reversed backwards, and the front wheel is the same as f = n.

See , specific problem analysis.

If the wooden block is stationary, it means that it is balanced by force, and the friction force f=20n, vertically upward; If the block slides, then the friction force f= n=50;

Ibid.

1 False, the bicycle is powered by friction, so the total friction is in the direction of forward.

2 Correct, the power of the bike is provided by the rear wheel, and the direction is forward.

3. False, in the process of driving, the friction force is the sliding friction force, and there is a formula to know that when the gravity is unchanged and the dynamic friction coefficient is unchanged, the friction force is unchanged.

4 can be seen from f=umg. m is the sum of the gravity of man and the gravity of the car.

5. At this time, the object is not moving, and the frictional force is not equal to the sliding friction force. At this time, gravity and friction are opposite, so the friction force is 20n.

In the same way, it is also 20n.

1.Because friction is the force that hinders the relative motion of an object, that's right.

2.The friction between the rear wheels and the ground is also backwards.

3.The friction doesn't change, because the pressure and the roughness of the contact surface don't change.

4.That's an outrageous question, isn't it?

20n~

1, The front wheel is backward, which is the force that hinders the bicycle from moving forward, and the rear wheel is forward, which is the force that helps the bicycle to move forward. Because assuming that the front wheel is not subject to backward force, then it must not move, so why is it moving now and turning backwards, because it is subject to backward friction, and the rear wheel, studying the contact point with the ground, the rear wheel wants to move backwards, but the ground does not give it, so the reverse point moves forward by itself, that is, the power.

2. At the beginning of the start, there may be a problem of maximum static friction, this is a little bigger, but junior high school or even high school does not talk about this, so it can basically be considered unchanged.

3. How big? It depends on the coefficient of friction and the positive pressure on the ground, multiplying the two, paying attention to the SI units.

4. If the wooden block is not moved, it is 20 Ox. Because assuming that it is not 20, but 30, then the force analysis, the wooden block has a downward gravity of 20 N, but the upward friction force is 30 N, so isn't it going to move upward? So it can't be.

As long as it does not move, it means that it is balanced by force, then it has gravity, that is, it only needs one more equal amount of upward friction, which has no direct relationship with the force used by people, and the product of manpower and friction coefficient (that is, the maximum friction) has a certain relationship, that is, this value must be greater than or equal to gravity, when it is greater than that, the actual friction force is less than this value, just equal to gravity, when it is less than, it slides.

It's a bit winding, normal, understand slowly, look carefully, I'm sure you can understand my explanation.

Difference Between Middle School Friction and High School Friction: Different Requirements, Different Learning Content.

1. Different requirements: junior high school physics.

The requirements of the outline for friction are "knowing" and "remembering"; High school requires "understanding" and "memorization", and the actual situation is that from stationary to sliding, the friction force has a momentary jump and decreases a little. The junior high school is approximately considered to be equal, which is convenient for us to remember.

Second, the learning content is different: sliding friction judgment Huaiqing friction force.

It is equal to the kinetic friction factor.

Multiply by the positive pressure, the maximum static friction.

It is equal to multiplying the static friction factor by the positive pressure, but the static friction factor is more complex and difficult to understand, so it is not explained in middle school. However, in high school, when the maximum static friction is used, you will often be told that "the maximum static friction is equal to the sliding friction".

Characteristic properties. Friction is related to objects rubbing against each other, so the description of friction in physics is not generalized and not as precise as other forces. If there is no friction, the laces cannot be tightened, and the screws are screwed.

and nails that cannot hold objects.

The biggest distinction within friction is between static friction and other frictional forces. Some people believe that static friction should not actually be counted as friction. All other frictional forces are related to dissipation: it reduces the relative velocity of objects rubbing against each other and reduces the mechanical energy.

Converted into heat energy.

Hello, here is the answer for you: "Mo Mo and Tan Que Rubbing Force" is studied in junior high school and high school, but the learning and difficulty and focus are different. The friction learned in junior high school textbooks focuses more on the formation of a preliminary perception of friction.

High school physical friction focuses on the combination of qualitative analysis and quantitative calculation of slippery denier cremation dynamic friction, static friction and maximum static friction.

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.

Sliding friction: A force that occurs between two objects that are in contact with each other and slide relative to each other, hindering the relative sliding between them.

The direction of friction is opposite to the direction of the relative motion of the object or the direction of the relative trend of motion. and not in the opposite direction of motion of the object. Friction can be used as a driving force as well as a drag force.

You can use the formula to find the f=UMG or f=uf positive pressure.

To solve the problem of friction, you have to understand the concept of friction, i.e., the conditions under which friction is generated.

The so-called friction force is the force that hinders the relative motion of an object, and the direction of friction is opposite to the direction of motion or relative motion of the object.

The conditions for friction to arise are: (1) two objects must be in contact; (2) there is extrusion between the two objects; (3) There is a tendency of motion or relative motion between two objects.

1. An object is stationary on a horizontal plane, and if it is not affected by push or pull, it is not affected by friction. If a push or pull force is received, it belongs to the category of static friction, and the magnitude of the frictional force is equal to the magnitude of the pull or thrust force, and has nothing to do with the gravity or pressure of the object. Only the maximum static friction is related to the pressure or gravity of the object.

2. If the object is pressed against the wall, the magnitude of the frictional force is equal to the gravitational force experienced by the object.

3. As long as the object moves in a uniform linear motion on the horizontal plane, the object is affected by the equilibrium force in the horizontal direction, that is, the push or pull force is equal to the friction force.

4. If the object is not pushed by a force of 10n, the object is subjected to static friction, and its magnitude is equal to the magnitude of the thrust. If the force of 15N is just propelled, the friction force experienced by the object is the maximum static friction force, and if the thrust is increased in the future, the friction force will not change. Theoretically, the sliding friction of an object is less than the maximum static friction, but they are generally considered to be equal.

5. If the object with a weight of 10N is pulled with a force of 15N and moves vertically upwards at a uniform speed, the friction force is f=15-10=5N

Friction is divided into sliding friction, static friction and rolling friction. The sliding friction force (the relative motion between objects) is f=ufn, and the static friction force only has a relative tendency to move, and the magnitude is judged by Newton's third law, which is equal to the magnitude of the tensile forceThe key to hope for adoption is to summarize.

As long as the object is at rest or moving in a straight line at a uniform speed.

Then the frictional force is always equal to the magnitude of the resultant external force exerted on that object, in the opposite direction, as long as the object is in motion.

Then the frictional force is equal to the kinetic friction factor of positive pressure (the pressure of the object in the perpendicular direction of the contact surface) in the opposite direction to the direction of motion.

The formula for friction is f=un u is the coefficient of friction and n is the pressure on the plane. Direction: The direction of the object is the opposite of the direction of the force on the object. That is, it is the opposite direction of what the object wants to move (or has already moved).

An object rests on a flat surface (not subject to friction, the object is only subject to gravity and support), and if it is pressed against a wall (subjected to static friction, the magnitude and gravity are equal).

If an object pulls it hard and moves it at a constant speed to the right, with a force of 10n (the friction force is also 10n), the object is pushed with 10n without movement (the friction force is static friction force of 10n) and it is pushed with a force of 15n (the friction force is 15n).

Use 15N to pull an object pressed against the wall (the object weighs 10N) (the friction force and gravity are equal to 10N).

The above problems are actually not difficult, the key is to draw the diagram, according to the constant speed or rest, you can get the balance of the force of the object, the horizontal and vertical force balance, which is the key to solving the problem.

On the plane, this is whether there is friction or not to see the pulling force, when the pull is not moving, the friction force = the pulling force, and the friction force = the friction coefficient when pulling positive pressure.

force, pressed on the wall according to gravity, do not dare to calculate according to pressure,. If an object pulls him hard to the right at a uniform speed of 10n (horizontal plane), and then the friction will not change, pushing the object with 10n has no movement, the friction is 10n, and the force is pushed with 15n, then the friction is 15n, which is the friction will not become larger, with 15n pulling the object pressed on the wall (10n in the object) upwards to do a uniform motion, the friction is 10n, the friction force is not moving when the object is not moving and the force is equal to the inclined plane, when moving with the formula friction = friction coefficient positive pressure, to calculate, if the vertical speed or deceleration is calculated by gravity, the acceleration should be discussed,

At this time, the trolley is subjected to two pairs of balancing forces: a pair of gravity and support, and a pair of friction and thrust.

According to the two-force equilibrium condition, the supporting force is equal to the gravitational force, i.e., t=g=mg=50kg 10n kg=500n;

The frictional force is equal to the thrust force, i.e., f=f=100n

equal, because a uniform pull indicates that the 2 forces are balanced.

It is to increase the elastic force between the object and the contact surface.

No, the object is subjected to a pulling force, a frictional force consistent with the direction of gravity, and a gravitational force The resultant force of friction and gravity is equal to the pulling force.

Yes, friction is a vector, there is a size, there is a direction, in junior high school generally do not study its direction thanks you question

Factors influencing the magnitude of sliding friction - junior high school physics easy learning to share a set of interesting teaching courses, which is more fun, including all junior high school physics knowledge points, synchronous course explanation, and after-class exercises. Like friends like, favorite,**. Follow me and continue to share quality teaching every day**.

1. Use force to pull the wooden block, and the wooden block moves in a straight line at a uniform speed.

It shows that the force of the wooden block is balanced at this time, and it can be seen that the friction force on the object is.

The sliding friction experienced by the object is independent of the state of motion, so no matter how much external force it is subjected to, or how much other external force it is, as long as the wooden block is moving, the sliding friction force is.

Sliding friction is related to two factors, one is the roughness of the contact surface and the other is the amount of pressure. In this problem, the roughness of the contact surface has not changed, and the pressure has not changed, so the sliding friction is always It's just that this is the linear motion of the wooden block at variable speed, because the tensile force is greater than the friction force, so it is not the equilibrium force, so the linear motion of variable speed is done. The known condition is that there is a wooden block of mass on the table, and the wooden block is pulled by force, and the wooden block moves in a uniform linear motion tells us that the friction force is, because the friction force and the tensile force cancel out (balance force), and become a balance force, and the wood block will continue to rely on inertia to advance, and it is a balance force, so it moves forward at a uniform speed.