How to calculate the minimum friction
Publish: 2021-05-09 20:09:28
1. The calculation method of sliding friction is different from that of static friction. When solving the friction, we must distinguish static friction from sliding friction.
I. calculation of sliding friction
1= μ FN calculation.
(1) according to the force condition of the object, calculate the positive pressure FN.
(2) according to the formula F= μ 2. Two force balance method: the object is in the state of equilibrium (uniform velocity, static positive), according to the two force balance conditions to solve; 2、 Calculation of static friction force
1. Balance condition method is used to solve static friction force. As shown in Fig. 3-3-19, a static object is placed on the horizontal plane. When a person pushes the object with horizontal thrust, the object is still. According to the two force balance condition, the static friction force is equal to the thrust force,
2, The value range of static friction is O & lt; F ≤ Fmax is independent of the positive pressure between objects, so the formula F cannot be used= μ FN solution.
3. Maximum static friction Fmax The value is equal to the external force along the contact surface when the object is just sliding, which is the maximum value of static friction and greater than sliding friction. Generally speaking, they are equal, and the maximum static friction is Fmax.. The formula is Fmax= μ Static FN, μ Static is the static friction coefficient and specific dynamic friction coefficient between objects μ Slightly larger
I. calculation of sliding friction
1= μ FN calculation.
(1) according to the force condition of the object, calculate the positive pressure FN.
(2) according to the formula F= μ 2. Two force balance method: the object is in the state of equilibrium (uniform velocity, static positive), according to the two force balance conditions to solve; 2、 Calculation of static friction force
1. Balance condition method is used to solve static friction force. As shown in Fig. 3-3-19, a static object is placed on the horizontal plane. When a person pushes the object with horizontal thrust, the object is still. According to the two force balance condition, the static friction force is equal to the thrust force,
2, The value range of static friction is O & lt; F ≤ Fmax is independent of the positive pressure between objects, so the formula F cannot be used= μ FN solution.
3. Maximum static friction Fmax The value is equal to the external force along the contact surface when the object is just sliding, which is the maximum value of static friction and greater than sliding friction. Generally speaking, they are equal, and the maximum static friction is Fmax.. The formula is Fmax= μ Static FN, μ Static is the static friction coefficient and specific dynamic friction coefficient between objects μ Slightly larger
2. Determination of friction force:
when determining the size of friction force, special attention should be paid to whether the friction force between objects is static friction or sliding friction, because the change of the size of the two is different. The size of sliding friction force is proportional to the pressure FN, and has nothing to do with the size of the external force causing the sliding friction force; The magnitude of static friction is independent of the pressure FN, which is determined by the external force causing the friction, but the magnitude of the maximum static friction is related to the pressure FN. Therefore, when the magnitude of friction is determined, the magnitude of static friction should be determined by the external force causing the static friction instead of F= μ FN calculation.
the common formula of sliding friction is f= μ The value of static friction is usually determined by the equilibrium condition.
1. It is determined by the equilibrium condition.
[example 1] when a wood block with mass m slides on the board placed on the table, and the board is still, its mass m = 3M. The sliding friction coefficient between the board and the wood block or between the board and the table is known to be 0 μ. The friction force of the board on the table is ()
a μ mg B.2 μ mg C.3 μ mg D.4 μ Mg.
[answer] A.
2. Determine according to Newton's second law.
[example 2] put a block of wood m on the horizontal disc, and the block will rotate at an angular velocity with the disc ω When the object rotates at a constant speed, the distance between the object and the rotating shaft is R. what is the friction force on the object? What's the direction
[analysis] when an object rotates with a disk, it needs a centripetal force, while the vertical object is balanced by Mg and FN, so it is impossible to provide a centripetal force. When the object rotates around the axis, it tends to be centrifugal, so it is subject to the friction force pointing to the center of the circle. The centripetal force comes from the friction force between the disk and the object.
[solution] the friction force acts as a centripetal force. According to Newton's second law, f = f = M ω^ 2R
extension: if the dynamic friction coefficient between the object and the disc is known to be μ, How far is the object from the axis to make it stationary relative to the disk
analysis: the farther away the object is from the axis, the greater the centripetal force is required. When the centripetal force required is greater than the maximum static friction, the object will slide centrifugally relative to the disk.
analysis: when the centripetal force required is greater than the maximum static friction, the object will slide centrifugally relative to the disk μ mg=m ω^ 2R R= μ g／ ω^ two
when determining the size of friction force, special attention should be paid to whether the friction force between objects is static friction or sliding friction, because the change of the size of the two is different. The size of sliding friction force is proportional to the pressure FN, and has nothing to do with the size of the external force causing the sliding friction force; The magnitude of static friction is independent of the pressure FN, which is determined by the external force causing the friction, but the magnitude of the maximum static friction is related to the pressure FN. Therefore, when the magnitude of friction is determined, the magnitude of static friction should be determined by the external force causing the static friction instead of F= μ FN calculation.
the common formula of sliding friction is f= μ The value of static friction is usually determined by the equilibrium condition.
1. It is determined by the equilibrium condition.
[example 1] when a wood block with mass m slides on the board placed on the table, and the board is still, its mass m = 3M. The sliding friction coefficient between the board and the wood block or between the board and the table is known to be 0 μ. The friction force of the board on the table is ()
a μ mg B.2 μ mg C.3 μ mg D.4 μ Mg.
[answer] A.
2. Determine according to Newton's second law.
[example 2] put a block of wood m on the horizontal disc, and the block will rotate at an angular velocity with the disc ω When the object rotates at a constant speed, the distance between the object and the rotating shaft is R. what is the friction force on the object? What's the direction
[analysis] when an object rotates with a disk, it needs a centripetal force, while the vertical object is balanced by Mg and FN, so it is impossible to provide a centripetal force. When the object rotates around the axis, it tends to be centrifugal, so it is subject to the friction force pointing to the center of the circle. The centripetal force comes from the friction force between the disk and the object.
[solution] the friction force acts as a centripetal force. According to Newton's second law, f = f = M ω^ 2R
extension: if the dynamic friction coefficient between the object and the disc is known to be μ, How far is the object from the axis to make it stationary relative to the disk
analysis: the farther away the object is from the axis, the greater the centripetal force is required. When the centripetal force required is greater than the maximum static friction, the object will slide centrifugally relative to the disk.
analysis: when the centripetal force required is greater than the maximum static friction, the object will slide centrifugally relative to the disk μ mg=m ω^ 2R R= μ g／ ω^ two
3. Determination of friction force:
when determining the size of friction force, we should pay special attention to whether the friction force between objects is static friction or sliding friction, because the size changes of the two are different. The sliding friction is proportional to the pressure FN, but not to the external force; The value of static friction has nothing to do with the pressure FN, which is determined by the external force causing the friction, but the value of maximum static friction is related to the pressure FN. Therefore, when determining the magnitude of friction, the magnitude of static friction should be determined by the magnitude of the external force causing static friction, instead of F= μ FN calculation
the common formula of sliding friction f= μ FN, and the static friction force is usually determined according to the equilibrium condition
1. Determined by equilibrium conditions
[example 1] when a wood block with mass m slides on the board placed on the table, the board is still, and its mass m = 3M. It is known that the sliding friction coefficients between the boards and the boards, and between the boards and the table top are all 0 μ The friction force of the board on the table is ()
a μ mg B. 2 μ mg C. 3 μ mg D. 4 μ mg
[answer] a
2. According to Newton's second law
[example 2] a wooden block m is placed on the horizontal disc, and the wooden block moves with the disc at an angular velocity ω The distance between the object and the axis of rotation is r. What is the friction force on the object? What's the direction
[analysis] when the object rotates with the disk, it needs centripetal force, while when the vertical object is balanced by Mg and FN, it cannot provide centripetal force. When the object rotates around the axis, it tends to be centrifugal, so it is subject to the friction force pointing to the center of the circle. The centripetal force comes from the friction force of the disc on the object
[answer] friction acts as a centripetal force. According to Newton's second law, f = f direction = M ω^ 2R
extension: if the dynamic friction coefficient between the object and the disc is known to be μ， How far is the object from the axis to make it stationary relative to the disk
analysis: the farther away the object is from the axis, the greater the centripetal force required. When the required centripetal force is greater than the maximum static friction, the object slides centrifugally relative to the disk
answer: μ mg=m ω^ 2R R= μ g／ ω^ 2
I hope I can help you. If you have any questions, you can ask ~ ~ ~
I wish you progress in your study and make progress
when determining the size of friction force, we should pay special attention to whether the friction force between objects is static friction or sliding friction, because the size changes of the two are different. The sliding friction is proportional to the pressure FN, but not to the external force; The value of static friction has nothing to do with the pressure FN, which is determined by the external force causing the friction, but the value of maximum static friction is related to the pressure FN. Therefore, when determining the magnitude of friction, the magnitude of static friction should be determined by the magnitude of the external force causing static friction, instead of F= μ FN calculation
the common formula of sliding friction f= μ FN, and the static friction force is usually determined according to the equilibrium condition
1. Determined by equilibrium conditions
[example 1] when a wood block with mass m slides on the board placed on the table, the board is still, and its mass m = 3M. It is known that the sliding friction coefficients between the boards and the boards, and between the boards and the table top are all 0 μ The friction force of the board on the table is ()
a μ mg B. 2 μ mg C. 3 μ mg D. 4 μ mg
[answer] a
2. According to Newton's second law
[example 2] a wooden block m is placed on the horizontal disc, and the wooden block moves with the disc at an angular velocity ω The distance between the object and the axis of rotation is r. What is the friction force on the object? What's the direction
[analysis] when the object rotates with the disk, it needs centripetal force, while when the vertical object is balanced by Mg and FN, it cannot provide centripetal force. When the object rotates around the axis, it tends to be centrifugal, so it is subject to the friction force pointing to the center of the circle. The centripetal force comes from the friction force of the disc on the object
[answer] friction acts as a centripetal force. According to Newton's second law, f = f direction = M ω^ 2R
extension: if the dynamic friction coefficient between the object and the disc is known to be μ， How far is the object from the axis to make it stationary relative to the disk
analysis: the farther away the object is from the axis, the greater the centripetal force required. When the required centripetal force is greater than the maximum static friction, the object slides centrifugally relative to the disk
answer: μ mg=m ω^ 2R R= μ g／ ω^ 2
I hope I can help you. If you have any questions, you can ask ~ ~ ~
I wish you progress in your study and make progress
4. Train number, departure time, departure time and fare mileage
0575.a057 Shaoxing long distance passenger transport center Suzhou bus south station 16:40-19:40 Yutong 80 223
0575.a058 Shaoxing long distance passenger transport center Suzhou bus south station 07:05-10:00 Yutong 80 223
0575.a059 Shaoxing long distance passenger transport center Suzhou bus South Railway Station 09:50-12:50 Yutong 80 223
0575.a057 Shaoxing long distance passenger transport center Suzhou bus south station 16:40-19:40 Yutong 80 223
0575.a058 Shaoxing long distance passenger transport center Suzhou bus south station 07:05-10:00 Yutong 80 223
0575.a059 Shaoxing long distance passenger transport center Suzhou bus South Railway Station 09:50-12:50 Yutong 80 223
5. You can take the express bus from Shaoxing passenger transport center directly to Hangzhou south station, every 15 minutes, and the ticket price is 24 yuan
you can also take the minibus from Shaoxing west station directly to Hangzhou south station, and there are many buses, and the ticket price is 17 yuan.
you can also take the minibus from Shaoxing west station directly to Hangzhou south station, and there are many buses, and the ticket price is 17 yuan.
6. Shaoxing to Suzhou, take the bus to Suzhou north station, so it usually takes about two and a half hours to three hours. The most frequent flight I took was from Shaoxing passenger transport center at 2 p.m. to Suzhou north station at 5 p.m. the cost used to be 72 yuan, but now it may have risen to 82 yuan. For details, you can call Shaoxing passenger transport center at 88222222.
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