What is Friction: Definition and 998 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. mazia

    Question(s) Regarding Magnitude of Force (Window Washer Problem)

    So, ignore the -0.72, I was just trying to see if I had a sign error (I then remembered magnitude is absolute value) but basically: Since the weight is 12.0N, theta is 53.1, and the coefficient of kinetic friction is 0.100, I just plugged those values into the equations above...
  2. Lotto

    How many discs will slide off the board as it decelerates?

    I don't undertand the equation. It is Newtons's second law of motion, so it decribes a force that acts on a single disc relative to the ground. So when the force is proportional to velocity, shouldn't it be ##-bv##? Because the dics's velocity is ##v## relative to the ground. Relative to the...
  3. Lotto

    What force is needed to push out this slab on the bottom of the stack?

    The picture is here: I understand that friction I need to overcome is ##5mg\mu##, but why do I need to accelerate the slab on acceleration ##g\mu##?
  4. M

    Box on treadmill connected to a spring

    First i show the sketch of the setup: My first attempt was just to balance out the forces on the box. On the sketch below i have shown the situation where the spring is stretched distance L. In this situation we get the equations: Which when solved leads to All good. I then looked at the...
  5. C

    Finding the time for an object to start rolling without slipping

    For this, I don't understand why they don't have a negative sign as the torque to the friction should be negative. To my understanding, I think the equation 5.27 should be ##I\frac{d \omega}{dt} = -F_{friction}R## from the right hand rule assuming out of the page is positive. Noting that ##f_k...
  6. Juanda

    I Solving Wheel Coming to a Stop: FBD, Friction & Hysteresis

    Simple question. Let's say a solid cylinder has an initial speed ##v_o## and it's rotating on infinitely hard ground without air resistance. The cylinder will come to a stop eventually. There are two sources of friction. Since the wheel/cylinder is deformed at the contact patch, there is some...
  7. cutielollipop

    Coefficient of Friction question for a cart going down a wooden ramp

    (mentor note: moved from Classical Physics forum hence no template) Hello, I am having trouble with this question: Imagine in real life there was a coefficient of kinetic friction of 0.4 between the plastic wheels of the cart and the wooden ramp. If there is only friction on the flat part of...
  8. wrobel

    I A pendulum with viscous friction

    Consider the standard pendulum with a weightless rod of length b and a mass point m and mg is applied. In the hinge there is a torque of viscous friction which is proportional ##\omega^2##. Now release the pendulum from the horizontal position. What biggest height does the point m attain after...
  9. Lotto

    B How will object's velocity change when a frictional force increases?

    Let's say that the mass of the objest is suddenly bigger, so when I want to maintain the constant movement, my force must increase as well. But will the velocity have the same magnitude? I think that the velocity will be smaller, so if I want to have the same initial velocity, I must apply an...
  10. Ben2

    Solving Rotary Friction Problem at Ye Olde Swinging Door

    Homework Statement: A door opened at 75 degrees from the latch remains at rest. When moved to a position 60 degrees from the latch, it slowly closes on its own. Is there another possible cause besides a sloping floor? Relevant Equations: \tau = r x F; v = \omega x r; a_T = \alpha x r, where...
  11. al4n

    B Need help understanding some concepts about friction

    We have 2 objects, m1 and m[SUPlB]2[/SUB] Friction is present between the two objects but not between m1 and the floor. A force is exerted on the bottom object which causes it to accelerate parallel to the floor. The thing I'm wondering for while now is, how do I prove that the acceleration of...
  12. jaketodd

    B Things giving rise to static and dynamic coefficients of friction?

    Topography of both the object and the surface. Mass/inertia. Moisture, but that can probably fall under topography. I suppose atmospheric pressure, maybe. Or wind. Magnetism. Any others?
  13. H

    Calculation of Box's Acceleration on a Trailer with Friction

    Question picture: My solution: Where: S is the lineforce Ff is the force as a result of friction a is the resulting acceleration F is the acting force The answear is supposed to be a=(F-2mg(mu))/(m+M) Any idea what i could have missed? Thanks for your help on beforehand!
  14. A

    Why is the work done double its expected value? (conveyer belt)

    The question was this: My calculations show that the answer should be equal to work done on crate to make it reach the same velocity which is equal to 216 J but the answer given is 432 J It is believed that extra energy is needed to overcome friction but friction is an internal force and...
  15. Mohmmad Maaitah

    Shouldn't this be kinetic friction instead of static friction!?

  16. Mohmmad Maaitah

    Minimum stopping distance so a box on a truck doesn't slip (friction)

  17. Argonaut

    Work-Energy Theorem and Friction

    My final answer is different from the official one in the back of the book, and I can't figure out what I did wrong. This is my attempt: Let block 1 be the vertically moving block and let block 2 be the horizontally moving one. Also, let ##m_1 = 6.00 ~\rm{kg}##, ##m_2 = 8.00 ~\rm{kg}##, ##v_0...
  18. 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...
  19. C

    How to find coefficient of friction of a body sliding down the slope?

    So basically I need to find the coefficient of friction given the listed information. What bothers me is that I am getting two different accelerations for two different approaches. When I calculate acceleration using Fg=mgsin60 I do it this way: Fg=mgsin60 -> ma=mgsin60 ->a=gsin60 -> a=8.66. But...
  20. per persson

    B How does the front wheel have no friction?

    So I have difficulty understanding this https://physics.stackexchange.com/questions/331173/why-is-friction-only-on-the-back-wheel With constant speed cycling uphill the front wheel has no friction. But if that would be the case how does the wheel even spin?
  21. Daniel Guh

    Mechanical Physics: Coefficient of Friction

    First of all, the pulling force is 300N cos(30) = 260 N At this point, I try to find the friction force Fn = mg = 20kg * 9.81 m/s^2 = 196.2 N Then, Ff = μ * Fn = 0.5 * 196.2 N = 98.1 N So after canceling the horizontal forces, 260N - 98.1N = 161.9N And the acceleration will be 161.9N / 20kg =...
  22. G

    A Radiation Friction: Solving Abraham-Lorentz Eq for Non-Physical Solutions

    There is a well-known Abraham-Lorentz equation describing radiative friction. Suppose a particle moves in an electromagnetic field. ma(t)=q(E+vxB) + m(tau)a’(t) By solving this equation numerically, I get non-physical solutions(runaway solutions) Although, it would seem that an electron in an...
  23. H

    Finding the work done by a block

    We want to figure out how much work friction does on a block as it slides down an inclined plane with a rough surface. we find the force due to gravity that pulls the block down the ramp, that's found by M * g * sin(θ), The normal force on the block is given by M * g * cos(θ). The force of...
  24. A

    Direction of static friction between a vehicle and circular dome

    So the only problem I am having is determining the direction of static friction. I did the same problem but while they were going in a vertical circular motion instead, where the static friction force was in the direction of centripetal force (pointing to the center of the circle). Would it be...
  25. A

    Friction, Mass and Acceleration: Analyzing Block Motion

    The block starts to slide if friction can no longer hold the block. F=u*n and F=(m1+m2)a so: (m1+m2)a=uN=>am1+am2=uN=>am2=(uN)/(am1) So:am2=(uN)/(am1) is the force. The answer is F=(u*m1g(m1+m2))/m2 I do not see how the acceleration terms are canceled. Is my answer equivalent to this?
  26. M

    I Can free body diagrams be used if friction at pivot is not negligible?

    Are we allowed to use free body diagrams in dynamic systems where there is non-negligible friction in say the rotational pivot between two linkages? If so, how to incorporate friction into the equations of motion? If not, what method allows friction to be accounted for?
  27. Lotto

    B Rope wrapped around a rod - belt friction

    Ideally, it should look circa like this: But in reality, it looks often like this: Is the formula valid also in this case?
  28. A

    Plate sliding on ice with friction (Physics competition question)

    I reason the frictional force on the plate from the ice is doing work first 3 meters (while the motorbike is moving on top) and then an "x" distance after the motorbike has left it. Does anybody have an idea of how one might solve this problem?
  29. enigmaticbacon

    Capybaras and Stonehenge

    I’m writing a presentation for younger students explaining how science can help them satiate their curiosity in a fun way. This presentation’s goal is to apply as much science as possible to a ridiculous question in hopes of promoting STEM fields. My example is, how many capybaras --the world’s...
  30. 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...
  31. E

    Work is 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...
  32. 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...
  33. M

    Damping and friction in syringe equation of motion

    Hello Everyone I want to model forces affecting on syringe plunger , but I do not know how to calculate terms like friction and damping coefficient. What I imagine is that : F_driving = ma + cv + f ----------------(1) where: f: friction c: coefficient of viscous damping m: mass of plunger (is...
  34. Volt582

    Distance traveled by a Ball affected by friction after t seconds

    Hi everyone, i have been trying to find an answer to this problem I have but couldnt find any good answers... (I dont know much about this stuff, but need a formula for a Project I am currently working on). So The problem goes as follows: Assuming we have a ball with a mass of m which is...
  35. 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...
  36. Argonaut

    Find the Coefficient of Kinetic Friction

    Young & Freedman 13th ed, Exercise 7.81 Starting with the crate, here is its free-body diagram: In accordance with Newton's First Law: $$ \Sigma F_y = 0 = n+(-w_c \cos{\alpha}) $$ Thus ## n = w_c \cos{\alpha} ##. And according to Newton's Second Law: $$ \Sigma F_x = m_c a_x = w_c...
  37. BurpHa

    Static Friction Between a Box and the Floor

    I don't understand part (b) In part (a), I need to calculate the coefficient of the static friction: mg * \mu_static = 35 58.8 * \mu_static = 35 \mu_static = 35 / 58.8 \approx 0.6 So from part (a) I know that the force applied is equal to the static friction, meaning that the box cannot...
  38. 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...
  39. haruspex

    Insights Subtleties Overlooked in Friction Questions: Object Slides Down Ramp

    Continue reading...
  40. nav888

    Friction and an object stopping

    Initially I tried to explain it via kinetic energy of the object and how the frictional force can only do as much work on the object as the object has kinetic energy but I got confused. Could someone here please explain why if I have an object with a net force due to friction (and no other...
  41. 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...
  42. 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
  43. Alaraity

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

    None yet
  44. 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...
  45. F

    I I would like a better understanding of friction and hysteresis

    Hello, In reviewing friction, I realized a couple of things: the coefficient of static friction can be larger than 1 (always thought it would be smaller than 1), that the surface area does not matter for static and kinetic friction ONLY for simple solid, rigid objects ( for materials like...
  46. S

    Distance travelled by a car considering only air friction?

    TL;DR Summary: Distance traveled by a car considering only air friction? How much distance would a 3-ton car travel if its initial speed was 17 km/h and we only take into account air's friction? (Assume that the car has an airfoil-like shape, so that the resistance against the air is very low)...
  47. 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...
  48. 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...
  49. M

    B Matter and Friction: Adhesion, Deformation & Hydraulic Press

    Friction happens because of adhesion between high points of the pertubrances of the two surfaces. The pertubrances deform. More the force between surfaces more deformation. Is the deformation elastic or inelastic? Will the surface of one body become smoother if pressed with hydraulic press?
  50. F

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

    Hello, When we consider a block sitting on a surface, the gravitational force ##W## and the normal force ##F_N## are applied to the block. Both equal i magnitude and opposite in direction. We call the normal force the reaction force exerted by the surface on the block. Now we consider the...