What is Friction: Definition and 999 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. Twigg

    Reducing friction at interface between a sphere and a plane?

    I have a flat planar part made of crystalline sapphire (about ~2k weight, and polished to a mirror finish) that rests on three ball bearings, and I want to minimize the static friction at these 3 interfaces. The ball bearings are fixed so they cannot roll, and the sapphire part can only slip...
  2. E

    How do I accurately calculate work done against friction?

    I'm unsure on where to begin with this question, i've tried many different formulas that aren't giving me the right answer. I believe to start I need to convert the kilo newtons to newtons. I tried w = fs + mgh w = 10500 x 8.9/sin(13.9)+(1845.69 x 9.8 x8.9) = 549986.46 J and then convert to...
  3. YehiaMedhat

    Pulley with friction, how to get the friction in the pulley?

    First, I assumed that the tension in the rope connected to the block A equals the static friction ##\sum{}^{} F_x =0 \rightarrow T=N_A*0.5=100N##, then the W weight or force equals to the tension in the pulley and the tension T ##W=100+\frac{2W}{\pi} \rightarrow W=275.2N## It's the first problem...
  4. M

    Damping and friction in syringe equation of motion

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  5. Volt582

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  6. BurpHa

    Static Friction Required to Keep the System from Moving (Two Boxes)

    Ok, logically, it must be that the static friction force of block A equal to the force of gravity on block B, so mass of block A is: m_A * 9.8 * 0.30 = m_B * 9.8 m_A * 2.94 = 2 * 9.8 m_A * 2.94 = 19.6 m_A \approx 6.7 kg. However, when I look at block A individually, there is one thing...
  7. Argonaut

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  8. BurpHa

    Static Friction Between a Box and the Floor

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  9. phos19

    Falling stick problem (no friction): What is the kinetic energy?

    Since there is no friction : $$ m \ddot{x} = 0 $$ (no x motion).For the kinetic energy , I've tried: $$ K = 1/2 I_{cm} \dot{\alpha}^2 + 1/2 m v^2_cm = 1/2 I_{cm} \dot{\alpha}^2 + 1/2 m \dot{z}^2$$ . Giving me a weird expression , shouldn't the kinetic energy just be half the the moment...
  10. haruspex

    Insights Subtleties Overlooked in Friction Questions: Object Slides Down Ramp

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  11. nav888

    Friction and an object stopping

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  12. A

    The Work of Friction: Explained in .32m

    The answer is .32m. I set the elastic potential energy as equal to the work, but at first I put the force in the work equation as (F elastic - F kinetic friction) times distance and rearranged. 1/2kx^2 = (kx-Ff) d (0.5) (22) (0.035)^2 = (22 x 0.035-0.042) d 0.013475= 0.728 d 0.013475/0.728 = d...
  13. K

    How do I find the coefficients of static friction and kinetic friction?

    I have found that μstatic= 5.6 (found on graph) / 18.20 = 0.3077 I don't know how to find Force of friction from the graph to calculate μKinetic
  14. Alaraity

    Static and Kinetic Friction -- Tilting a ramp until a block starts sliding

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  15. arhg

    A Normal force and force of friction inside a tube

    Dear all, Me and some colleagues (non-physicists) are discussing how force works when passing a cylinder (which we are holding) into a narrow tube. As we insert more of the cylinder into the tube, the force we are exerting is increasing. My theory is that the normal force is increasing and his...
  16. F

    I I would like a better understanding of friction and hysteresis

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  17. S

    Distance travelled by a car considering only air friction?

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  18. Clockclocle

    I What friction causes objects to decelerate?

    When I exert enough force that overcome the static friction. The object start moving and surface create kinetic friction on object if I exert harder overcome the maximum of friction it start accelerate. When I release the object will the kinetic friction disappear immediately proportional to...
  19. Clockclocle

    I Need help in understanding friction

    When we push an object on the surface, if I push hard enough such that it reach the maxium static friction of the object then it start moving with a constant speed and F_push = F_friction. But when I realease the object F_push immediately become zero remain only F_friction. Does the friction...
  20. M

    B Matter and Friction: Adhesion, Deformation & Hydraulic Press

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  21. F

    I Reaction force of and its relation to normal force and friction

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  22. J

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  23. Yarno

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  24. P

    Why are tension and friction in the same direction?

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  25. Y

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  26. tbn032

    B Why is static friction necessary for pure rolling?

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  27. O

    I F= - mg - kv ? (forces of gravity and friction on a mass)

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  28. sachin

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  29. T

    I What is the difference between the resistance force and the friction coefficient?

    Hi I have been wondering what is the difference between resistance force and friction coefficient. While doing research online I could not find a clear answer. Maybe someone here will now:bow:
  30. Differentiate it

    Friction causes a torque - then why do we sum it as a regular force?

  31. tbn032

    B Rolling of non-deforming sphere on a non-deforming rough surface?

    According to my current understanding rolling friction rolling friction is the static friction (parallel to the surface on which the object is moving) applied by the frictional surface (rough surface) on the contact point or contact area of the object whose v≠Rw(v is translational velocity and...
  32. tbn032

    B Trouble understanding what rolling friction is

    In pure rolling the of the sphere contact points of the sphere are at zero velocity,how is friction opposite to the motion of sphere being applied to these points?how the frictional force f is bieng applied to the sphere?
  33. tbn032

    B Friction on pure rolling non deforming sphere?

    How will the friction work on a sphere which is purely rolling on a horizontal surface such that both the sphere and surface does not deform. The sphere at any time t will only have one point of contact, which would continuously changing as the sphere rolls. Will The friction be applied to the...
  34. P

    B Why doesn't static friction move the other object?

    Consider a block sitting on top of a table. When we push the block rightwards, static friction opposes its motion by acting leftwards on the block. By Newton's 3rd Law, static friction should act rightwards on the table. There is now a net rightwards force on the table. Why doesn't the table...
  35. pixel

    I Friction in rolling without slipping

    Consider an object, say a ball, rolling at a constant speed without slipping to the right on a horizontal surface. Let's consider the ideal case, so no deformation of ball or surface. For rolling without slipping to occur, there has to be friction (static friction as the point on the ball that...
  36. P

    Static Friction Question — A wood block sliding on a tabletop

    I have no idea how to start this other than that I think that Fn is 17.64N and Fnet is 0. But I don't have the mew, so I am not sure how I am going to get the force of static friction.
  37. link223

    Friction Coefficients: Determining Force Direction?

    Do I determine this from the friction coefficients? Such that because mu_b > mu_a I know that it'll push on B because it has a bigger friction coefficient. such that this has nothing to do with the mass of the blocks? or does it? f.e. can I take it to be in an arbitrary direction and then...
  38. S

    Direction of friction on rolling object

    A force is given to the center of the object so the object rolls to the right without slipping. I understand that to provide clockwise rotation the static force should be directed to the left. But if the force F is located at the very top of the object, the static friction is directed to the...
  39. L

    Direction of friction for three bodies stacked one on top of the other

    I have drawn three free body diagrams, one for each box and then I applied Newton's Second Law after choosing a reference frame rotated clockwise by ##\alpha##, with ##x## pointing south-east and ##y## pointing north-east and I got: ##\begin{cases}m_{1x}: -T+m_1g\sin(\alpha)+F_{fr_{12}}=m_1...
  40. S

    Conservation of energy problem with friction included

    so I haven't looked at the solution yet, but I know that a 100% the velocity needs to be bigger, but analytically, I get a - sign instead of a + sign as you'll see at the final square root. So for the first 15meters of the motion all you should know is that ##v_1 = 10.458 m/s##. for the 2nd...
  41. C

    I Friction of a rolling cylinder on an incline

    Hello everyone! I'm watching this Walter Lewin lecture and am at 5:58 part of the video I'm wondering how there's a frictional torque applied to the cylinder, my reasoning is that the object has forward velocity, and on a perfect cylinder, the slope of the incline touches the cylinder at a...
  42. C

    I Understanding relative friction

    Hello everyone! I'm reading this book and trying to get a more concrete understanding of friction and it's relation to Newton's third law. So in the solution he writes$$D - f - F_R = m_ta_x$$Where D is the driving force of the truck, f is the frictional force of the box on the truck, and...
  43. ROOT0X57B

    Hertzian contact with friction

    Hi there! First of all, I do not have a clear homework statement or relevant equations to give but I still put this thread in here because it's related to school work and I've previously got a warning for not doing so. I'm a french student who is going to start his second year after high school...
  44. James Brown

    B How come friction can make a car turn?

    How is it possible that friction makes car turn? From what I know, frictional force is acting along to the direction of the wheel turns. When the car turns, the direction of the frictional force now act opposite to the direction of the wheel turns! I'm sure if you rotate the steering wheel to...
  45. S

    I Exploring the Direction of Sparks from Steel-on-Steel Friction

    when a railroad engine wheel slips on a rail as it tries to move forward i can see sparks mostly going in the rear direction. while thinking about what might happen in other situations, such as a moving engine (being pushed by its massive 100 car load, perhaps), i came to a simplistic case that...
  46. F

    Intuition on the direction of friction in a rotational dynamics problem

    The figure illustrates the situation. The radii of the larger and smaller discs are 2R and R, respectively. Their masses are M and 2M, respectively (the largst disc has the smallest mass). Also, m=5/4 M, where m is the mass of the suspended object. The pulley is "massless" (negligible moment...
  47. V

    How can we model a rolling truck on an incline using rotational motion analysis?

    This is a problem that was posted here in 2003 and is now closed for replies. This question can be found at https://www.physicsforums.com/threads/friction-problem.662/ The answer in that old post didn't seem clear to me probably because it was highly summarized. There was no mention of static...
  48. F

    Horizontal impulse on a ball at rest on a plane (with friction)

    Summary:: I'd like to check my understanding of standard problems where a billiard ball resting on a plane is hit horizontally at some height above its center So the situation is that a ball of mass ##m## and radius ##r## is at rest on a horizontal surface. There is friction between the ball...
  49. 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...
  50. rudransh verma

    B Why does friction decrease with repeated force application?

    Suppose you have a heavy body placed on a surface. We apply a horizontal force such that it is on the verge of moving (maximum static friction). We measure this force and note it down. Now we cut the body from the top and remove a thin layer and again do the same thing. We apply a force and...
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