What is Friction force: Definition and 235 Discussions

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:

Dry friction is a force that opposes the relative lateral motion of two solid surfaces in contact. Dry friction is subdivided into static friction ("stiction") between non-moving surfaces, and kinetic friction between moving surfaces. With the exception of atomic or molecular friction, dry friction generally arises from the interaction of surface features, known as asperities (see Figure 1).
Fluid friction describes the friction between layers of a viscous fluid that are moving relative to each other.Lubricated friction is a case of fluid friction where a lubricant fluid separates two solid surfaces.Skin friction is a component of drag, the force resisting the motion of a fluid across the surface of a body.
Internal friction is the force resisting motion between the elements making up a solid material while it undergoes deformation.When surfaces in contact move relative to each other, the friction between the two surfaces converts kinetic energy into thermal energy (that is, it converts work to heat). This property can have dramatic consequences, as illustrated by the use of friction created by rubbing pieces of wood together to start a fire. Kinetic energy is converted to thermal energy whenever motion with friction occurs, for example when a viscous fluid is stirred. Another important consequence of many types of friction can be wear, which may lead to performance degradation or damage to components. Friction is a component of the science of tribology.
Friction is desirable and important in supplying traction to facilitate motion on land. Most land vehicles rely on friction for acceleration, deceleration and changing direction. Sudden reductions in traction can cause loss of control and accidents.
Friction is not itself a fundamental force. Dry friction arises from a combination of inter-surface adhesion, surface roughness, surface deformation, and surface contamination. The complexity of these interactions makes the calculation of friction from first principles impractical and necessitates the use of empirical methods for analysis and the development of theory.
Friction is a non-conservative force – work done against friction is path dependent. In the presence of friction, some kinetic energy is always transformed to thermal energy, so mechanical energy is not conserved.

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  1. Adam Hanns F A Cong

    Find the man's tension pulling the rope

    The elastic force of the board,N=G_1+G_2=1000N. The kinetic friction f=μN=200N. The tension T=f=200N.
  2. I

    For the friction formula when to use Fn(normal force)×mu and when F×mu

    Is there some kind of rule when we are supposed to use Fn×mu compared to F×mu or mg×mu to calculate friction? Because I don't get the same answer when I use Fn×mu and F×mu. In magnitude the forces are the same but they are not same in directions. Why is it so? Here is the photo. Problem goes...
  3. ElectroMaster88

    Application of Newton's second law to a horizontal pulley

    I don't understand how to solve these, and I don't understand how an horizontal force can affect the friction force if it's defined by μ*N, and the additional force affect neither of those. I also don't understand how there is a range of possible forces that F can be to make the system static...
  4. S

    My Mistake? Understanding Friction Force & Work Done on Snowy & Icy Surfaces

    The answer is (D), but I don't understand why. Option (A) is wrong because the work done = 0. Then, I divide the motion into 3 parts: 1) motion on snowy surface Since the sledge is being pulled horizontally (let assume to the right), there will be tension force T to the right and friction...
  5. V

    Heating up of tyres on the road

    I have searched online for this and found that it's due to friction. But for a rolling tyre there is only static friction acting and static friction produces no heat. For heat to be produced the two surfaces in contact must slide over each other i.e. there needs to be kinetic friction. Perhaps...
  6. ols500

    Linking moments with friction force for a sliding block

    Hi so with this question, I am really confused. Even from the start. Like it doesn't tell us where the force is acting on. From top of block, middle of block etc? And I know how to determine the direction of moment for something on a pivot, not in this scenario though. The answer is A. They did...
  7. H

    Sliding resistance of eccentrically loaded block

    Summary:: What is the sliding resistance of an eccentrically loaded block on a flat surface? If a 1000kg block is placed on a flat surface (say coeff. of friction = 0.4), what force does it take to slide if it loaded eccentrically, i.e. at the far edge of the block? My thinking is there is...
  8. SpectraPhy09

    Plotting the graph of Friction Force vs. time (Laws of motion)

    F.B.D Of first block (I have shown only the horizontal Forces) f1(max) = μ (1kg)(g) = 0.5 * 10 = 5N F.B.D Of the second Block f2(max) = μ (3kg)(g) = 15N Now the string will become taut and the tension will start acting when f = t = 5N But for 0<f<5N there will be no motion between the 1 kg...
  9. D

    Friction force on each foot and on each hand of a person

    I get how to solve (a); my method involves finding the net torque about the man's hands and setting it to zero, which can be used to solve for the normal force acting on his feet and the normal force on his hands can be solved using Newton's first law. Then divide by 2 for each to get the normal...
  10. kara123

    Finding Friction Force & Work with No Applied Force

    -i had begun by finding the normal force =147 N -then found the force of friction=99.96 N -found the work of friction=-799.68 J after that I am unsure of where to go since I don't have a force applied
  11. vibha_ganji

    B What Happens When The Applied Force Equals the Static Friction Force?

    In my teacher’s notes, it said that when the applied force on an object equals the maximum static frictional force, the object begins to move at constant velocity. My question is if both the applied force and the frictional force cancel as they are equal in magnitude, what force accelerates the...
  12. FEAnalyst

    Friction - problem with one of the equilibrium equations

    Hi, actually it's not my homework, I'm just practicing some academic problems after a long break but it seems that I should post this here anyway. Here's a scheme of the problem that I want to solve: The task is to calculate minimum force ##P## for the system to stay in equilibrium. And...
  13. E

    Friction force of 2 connected block on steel surface

    hello there, I am wondering if my attempt at a solution for this particular problem correct?? I mean I know that the blocks won't move and the total friction force is 38.95N (at least lesser than that, since that is the maximum static friction). I saw someone attempted the solution but it is...
  14. Caio Graco

    Static friction force: Consider a block on a sheet of paper....

    Consider a block on a sheet of paper. If the sheet is pulled without sliding the block, who has done work on the block (since it has undergone a kinetic energy variation)? I can not see another force doing work other than the static frictional force on the block.
  15. S

    Determining position of an object after inelastic collision

    Homework Statement A 39,000 lb truck A and a 3968 lb sports car B collide at an intersection. At the moment of the collision, the truck and the sports car are traveling with speeds vA = 70 mph and vB = 30 mph. Assume that the entire intersection forms a horizontal surface. Letting the line of...
  16. J

    Determine the friction force in a stacked block scenario

    Homework Statement Assume that block A which has a mass of 30 kg is being pushed to the left with a force of 75 N along a frictionless surface. What is the friction force of block A on block B if the block B has a mass of 24 kg and is accelerating at 0.50 m/s2 to the right relative to the block...
  17. Digital_lassitude

    Is it possible to express friction force as a tensor?

    Homework Statement Consider the equation for the friction force Ff = m FN. is it possible to express the friction force as a tensor? If so, what rank tensor is it, and what are the ranks of the tensor m and the normal force FN? Homework Equations Ff = mFNThe Attempt at a Solution [/B] So I...
  18. E

    Help with a car under braking and friction force

    Got a question for you mechanics enthusiasts out there. Here https://i.imgur.com/Px0Hakg.jpg from what I gather is a basic force diagram depicting a car accelerating along a straight road, Dr represents the driving force of the engine, Fr the friction acting on the tyres against the direction of...
  19. Navin

    Two blocks on a plane connected with a Rod

    Homework Statement Two blocks m1 and m2 are connected by a weightless rod are on a inclined plane of ∅=37° and μ =0.25 .What is the common acceleration and Tension of rod if m1 is 4 kg and m2 is 2 kg such that the 4kg block is the upper block. OPTIONS A =4m/2 T=0 B =2m/2 T=5N C =10m/2 T=10N D...
  20. H

    Force of a Punch to exceed the Max Static Friction Force

    I heard that the average professional boxer's force of punch is between 500-750lb. If the coefficient of static friction of rubber (from shoes) to say concrete is at most 0.85 then why can boxers get hit (especially in the chest) without losing footing? Does it have something to do with the...
  21. babaliaris

    Friction Force: Why do we walk?

    I created this example to see if i can explain how humans walk but I can't. If I'm correct, when we push the ground to move forward we apply a force with the opposite direction (backward). Let's name this force Fp (for push). Because of that force, a friction force is being applied to us with...
  22. PhysicsIsKillingMe

    Friction force on an inclined plane

    Homework Statement A speed of a body of mass 8 kg is 6 m/s in position A. By the time it gets to B, the speed is measured to be 12 m/s. What is the frictional force opposing the motion? The incline is 30 degrees, the height of the ramp is 12 meters. Position A is at the top of the ramp...
  23. A

    Calculating Applied Force for a Moving Block on a Table

    Homework Statement You gradually apply a force to a 10. kg wood block on a wood table. What will the applied force on the block be once it is moving? The coefficients of static and kinetic friction are 0.5 and 0.3, respectively. The acceleration due to gravity is 9.8 m/s2. Homework Equations...
  24. Sergio Rodriguez

    Satellite fall and friction force problem

    Homework Statement A satellite of mass m_s orbits the Earth in a circular orbit of radius r_0. If the satellite orbits at the upper part of the atmosphere and the friction force f is constant, it would trace an spiral and fall to the earth. but if we suppouse that the friction force is small...
  25. Jeviah

    Can the only reaction force (to gravity) be friction?

    Homework Statement If you have an object say in a vertical pipe, which only has a gravitational force acting on it would the reaction forces only be friction forces? If this is the case how would you calculate the forces? would you calculate the friction force using the friction coefficient or...
  26. Blueberrymuffim

    The tension in the cord in this system

    Homework Statement A force F is applied horizontally to a block A of mass mA = 0.600 kg so that it slides on the floor with acceleration g/3. Block B of mass mB = 0.200 kg rests on top of A, but is kept in place by a cord which is tied to the wall at 45.0°. The coefficient of kinetic friction...
  27. E

    Displacement of an object with certain Ek stopped by Frictional Force

    Please do excuse if certain terms are not the most technical ones as I am translating the problem from another language. Do feel free to kindly inform me of the more technical term that substitues the sloppy one I've used. (And do excuse the generally bad English overall :-)) 1. Homework...
  28. Alexanddros81

    Derive the expression for the work done by the friction force

    Homework Statement 14.6 The coefficient of kinetic friction between the slider and the rod is μ, and the free length of the spring is ##L_0 = b##. Derive the expression for the work done by the friction force on the slider as it moves from A to B. Neglect the weight of the slider. Homework...
  29. M

    Speed after gliding with friction force present using work/E

    Homework Statement The question is as follows: A 50kg ice skater is gliding along the ice, heading due north at 4.0m/s. The ice has a small coefficient of static friction, to prevent the skater from slipping sideways, but kinetic friction =0. Suddenly, a wind from the northeast exerts a force...
  30. lichenguy

    Conceptual question about frictional force and equilibrium

    A uniform beam of length L and mass m is inclined at an angle θ to the horizontal. Its upper end is connected to a wall by a rope, and its lower end rests on a rough, horizontal surface. The coefficient of static friction between the beam and surface is μs. Assume that the angle θ is such that...
  31. H

    How Is Friction Force Calculated in Physics Problems?

    Homework Statement Box Mass = 64.7 kg T1 = 22.5° T2 = 54.1° F = 2327.9 N μdynamic = 0.19 μstatic = 0.37 What is the force of friction acting on the box? Homework Equations FF= mu x FN The Attempt at a Solution 64.7 x 9.81 = 634.707 0.19 x 634.707 = 120.69 Computer is telling...
  32. J

    Applied force compared friction force at constant velocity

    Homework Statement How does an applied force and a frictional force compare when moving at constant velocity? Homework EquationsThe Attempt at a Solution
  33. A

    Calculating Distance for a Train on a 1% Grade

    Can anyone help with the following question, please? 1. A 60,000kg train is being pulled up a 1 percent grade (it rises 1m for each horizontal 100m). The friction force opposing the motion of the train is 4,000N. Initial speed of the train is 12 m/s. Through what horizontal distance will the...
  34. A

    Friction Force with Incline and Pulley

    Homework Statement There is a mass m1 on an incline (angle α), connected to a pulley with a string, and on the other side of the pulley, after another string, there is a mass m2. See picture if it's unclear, I'm not sure how to express the problem. Anyway the plane has a friction coefficient...
  35. EEristavi

    Automotive Friction force, Coefficient of friction &Traction

    I have few (more or less) general questions: 1. I know that: "The coefficients of friction are nearly independent of the area of contact between the surfaces". I understand this, but I know from everyday life, when car tire tread is worn off, braking distance increases and maximum acceleration...
  36. L

    Finding Friction Force on a Break Pad

    A 30-cm-diameter, 1.6 kg solid turntable rotates on a 1.4-cm-diameter, 450 g shaft at a constant 33 rpm. When you hit the stop switch, a brake pad presses against the shaft and brings the turntable to a halt in 15 seconds. How much friction force does the brake pad apply to the shaft? I am...
  37. F

    Second derivative of friction force question

    I'm studying boundary layers. I am confused by what I am reading in this book. The book says the friction force (F) per unit volume = $$\frac{dF}{dy}=\mu\frac{d^2U}{dy^2}$$ They say $$\frac{dU}{dy}=\frac{U_\infty}{\delta}$$ This makes sense to me, delta is the thickness in the y direction...
  38. generalguiness

    Static friction force homework

    Homework Statement A 50kg object begins at rest on a surface with a static coef of friction at 0.6. If you push horizontally on the object with a force of 27N, how quickly will it move across the surface? Homework Equations The Attempt at a Solution[/B] This is from Trivium Praxis test prep...
  39. davidge

    Solving Boat's Friction Force and Speed

    Homework Statement A boat of mass 1000 kg is moving at 25 m/s. The friction force ##f## is proportional to the speed ##v## of the boat, ##f = 70v##. How many time will take for the boat to reduce its speed to 12.5 m/s? Homework Equations ##\vec{F_r} = m \vec{a_r}## The Attempt at a Solution...
  40. M

    Why is the tension force equal to friction force

    Homework Statement The 1.0 kg block in the figure is tied to the wall with a rope. It sits on top of the 2.0 kg block. The lower block is pulled to the right with a tension force of 20 N. The coefficient of kinetic friction at both the lower and upper surfaces of the 2.0 kg block is μk = 0.300...
  41. terryds

    Friction Force: When Does it Provide Motion?

    Is there any case example where the friction force DOESNOT oppose the motion, but rather PROVOKES the motion of some object? I wondered about this all day long :D
  42. Arman777

    Friction Force on 2 blocks on a frictionless surface....

    Homework Statement The two blocks ##(m=16 kg)## and ##(M=88 kg )## , are not attached to each other.The coefficient of static friction between the blocks is ##μ=0.33##, but the surface beneath the larger block is frictionless.What is the minimum magnitude of the horizontal...
  43. Arman777

    Calculating Acceleration on an Inclined Plane with Friction | Two Block System

    Homework Statement Two blocks,in contact,slide down an inclined plane ##AC## of inclination ##30^°##,The coefficient of kinetic friction between the ##2.0 kg## block and the incline is ##μ_1=0.20## and that between the ##4.0kg## block and the incline is ##μ_2=0.30##. Find the...
  44. Arman777

    Friction Force and Decelerating Boat

    Homework Statement A 1000 kg boat is traveling at 100km/h when its engine is shut off.The magnitude of the frictional force ## \vec f_k## between boat and the water is proportional to the speed v of the boat:##f_k=70v##,where v is in meters per second and ##f_k## is in Newtons.Find...
  45. U

    What is the static friction force acting on the object?

    Homework Statement A block sitting on a flat surface has a us of 0.5. The normal force acting on the block is 10N. If someone pushes the block with a horizontal force of 2N. What is the static friction force acting on the object? Homework Equations F=ma fs=usR (less than or equal to)The...
  46. M

    How can energy be lost to friction?

    hi everyone! I'm new here. I was wondering how energy can be lost to friction? How would one even calculate that? Thanks!
  47. ChrisBrandsborg

    Finding Acceleration and Stopping Time in a Multi-Block Lifting System

    Homework Statement A lifting system can be modeled as two blocks of mass m1 and m2 hanging from two pulleys, connected by ropes to a third block of mass M = 100 kg, lying on top of a horizontal surface. There is gravity g = 9.80 m/s2. a) Ignoring at first friction, find an expression for the...