# Friction Definition and 998 Threads

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. ### Friction force on a ball falling through a body of water

I first attempted to solve the problem by ##A_{ext} = \Delta E_{mech} = F_{ext}s##. Here, ##F_{ext} = F_{friction}## and ##\Delta E_{mech} = E_{k2} + E_{p2} - E_{k1}##. We obtain then the following equation: $$F_{friction} = (m((v_2)^2 - (v_1)^2))/2h + mg,$$where ##v_1## is the velocity at time...
2. ### Ball revolving around a massive disc and a cube placed at centre

I thought that the maximum force on the block in the x direction would be the point where the ball crosses the plane of center and thus frictional force would be maximum, and if the block does not slip in that case then it never will slip as the value of force in x direction only decreases...
3. ### A cylinder rotating on a plane with friction and then moving across a frictionless plane to a collision

I think the angular velocity keep increasing on the plane with friction and the translational velocity keep decreasing due to friction while the total kinetic energy is conserved. When it moves to the frictionless plane, all energy converts to translational kinetic energy and it stop rolling...
4. ### Why is the direction of kinetic/static friction up a ramp?

I've been studying rolling motion for a bit and I realized that there's another hole in my knowledge. Let's say that we have a wheel rolling down an inclined plane. Let's assume that the wheel accelerates down the inclined plane. We can look at the motion of the wheel as if it is moving on a...
5. ### I Determine if the block will slide or flip over after contact

I have a problem like above. A is fixed, B is fixed in Y direction and C is placed on B. Theta, h, w and m are known input parameters. If an increasing force is applied on B towards -ve x direction, I want to determine whether C is slide or flip over. Before slide or flip happened, I believe I...
6. ### Friction problem of a block sandwiched between 2 surfaces

I don't know how to post a picture so I tried my best to describe my question in words. X is horizontal, Y is vertical and there is no gravity effect. There is a block is sandwiched by 2 surfaces (top surface and bottom surface). Both surfaces are fixed in X direction and the bottom surface is...
7. ### I Will This Friction Clamp Hold the Weight?

Hi guys, I am trying to design a brake/locking mechanism to hold up a weight, here is some background; I have a weight 18kg, 170mm away from the pivot point on an arm. A brake compresses the arm at the pivot point. This consist of a M10 bolt compressing a surface area of 1500mm^2 on each side of...
8. ### B Rolling Without Slipping on an Inclined Plane

I have asked this question last year (on discord; IPhO server) and I believe I wasn't satisfied by the answer at that time, but I let it go. Today, as I was going through some physics videos on YouTube a video about it popped up. So, I would like to address this issue now. Let's imagine an...
9. ### A Boundary condition of limiting friction in continuum mechanics

Suppose I have a block of deformable material on a rough surface. I want to have the boundary condition for the stress tensor that takes into account of friction. If the mass of my block is m, and of density \rho and the coefficient of friction is \mu as well as gravity g. The resultant force is...
10. ### I Question about friction in situation where interface slip occurs

Hello, I have question about friction at inteface between two component slip occurs at interface when external force bigger than friction force(= normal reaction force * static friction coefficient) After slip occured, component motion stopped and got new static equlibrium state In this...
11. ### Morin 3.7 -- Block sliding sideways on an inclined plane

TL;DR Summary: In Morin 3.7 sliding sideways on a plane I used a completely different method than he did and got the correct answer is my method right The problem statement is as follows I split up the friction force into x and y components derived a diff eq for v_y in terms of v_x then took...
12. ### Block on top of moving slab, with a rough surface - When does v_b=v_s?

For the first question I thought of using an energy balance, there is friction ##\Rightarrow \Delta E_m = -W_f##. Both at the start and at the end, the block has no velocity. Therefore ##E_{\text{initial}}= \frac 1 2 m_s v_{s,i}^2## and ##E_{\text{final}}= \frac 1 2 m_s v_{s,f}^2##. This means...
13. ### Is it friction or tension acting on the person hanging on a rope?

The first question statement was under the chapter ##Newton's Laws Of Motion (Without Friction)##. Whereas, the second question was under ##Friction##. The free body diagram for the first question is given as: And the free body diagram for the other question is given as: In the first...
14. ### Energy analysis of this system (curved ramp up and a spring)

For point one it's clear that I have to use energy => ##ΔE_{AB} = W_{friction}## ; ##\frac 1 2 mv_0^2 - \frac 1 2 mv_1^2 = mgμ_d d## After that there is the path BC, but I don't know how to analyze it from an energetic standpoint. Then after BC the block will now have a different velocity, I...
15. ### I Shouldn't the pressure difference along a pipe be a negative number?

Across a horizontal pipe with fluid flowing inside, the pressure will drop due to the friction between the fluid and the pipe walls So if for example the fluid flows from left to right and we take 2 points: P1 (pressure on the left) and P2 (pressure on the right) then P1 should be bigger than...
16. ### I How does friction in a pipe occur, with the "no slip condition" ?

Hello everyone 😊 Let's say, we are having laminar flow in a cylindrical pipe. The fluid in direct contact with the pipe doesn't move (no slip condition), so there is no sliding between the surface of the pipe and the surface of the water. The friction that occurs is actually between this...
17. ### How can I determine the instant the block overcomes static friction?

I know that the block will move only if the force that pushes him is greater than μs mb g. The only force that can act on the block is the elastic force (Fs) generated by the expansion of the spring, caused by the rolling sphere, that rolls because a force F is acting on the sphere. Then can I...
18. ### Find the coefficient of friction between block and surface

Ff.x= mv2/2 - kx2/2 umgx= mv2/2-kx2/2 u= mv2-kx2/2mgx =
19. ### B How will a spring stretch in the following cases?

1) I have a spring on the ground with no friction and the spring is not attached from one end. If I apply a force ##F## and the spring is massless, will it stretch? I think that it won't. But if it has a mass ##m##, will it stretch now? Will it be ##x=\frac{F}{k}##? I don't know, but I imagine...
20. ### Calculate how fast the car was travelling when brakes were applied

My query is on highlighted. This was straightforward. I thought that coefficient of friction ought to lie between ##0## and ##1## maybe i need to check that again anyway; ##F_{lim} = 1.5 ×15000= 22, 500 N## Using ## v^2 = u^2 +2as## and ##F=ma## ##22,500 = 1500a##...
21. ### Help with Friction Forces: Box resting on a board on a frictionless surface

Hello, I'm struggling understanding why my answer to this question is incorrect. I know the right answer, but why is this way wrong? I have attached my solution and would really appreciate the help. Thank you!
22. ### I Looking for a formula for damage when sliding to a stop

The context is going down a hill in rollerblades: when people fall and they come to a quick stop (due to not having protective gear and there being lots of friction), they always end up with major injuries, while people with protective pads can sometimes fall and slide for 60+ feet and often be...
23. ### Tidal friction and global warming

Ignoring global warming, the Earth will spin progressively slower. But how will global warming affect this? THEY say, LESS water, LESS slowing. > slowing spin is all about the FRICTION from the action of WATER on the LAND, when the water is gone the slowing of the spin will ceased (or at least...
24. ### B I have a question about energy & ignoring friction losses

If the energy itself ignores loss resistance friction and the energy moves forward, can infinity go far at a constant speed?
25. ### Normal force at the base of a ladder

Hello, I was recently tested on finding the normal force at the base of a ladder leaning against a wall as well as its friction force. So this is the question from memory. Given: θ the acute angle between the ground and the ladder μ as the coefficient of friction between the ground and the...
26. ### Loss of atoms from the surface of the material: How does it work?

Good morning! A question from a mere layman, so I hope you won't eat me :D Some time ago I read that every time one solid body makes some contact with another solid body, several billion atoms are lost from the surface of both the first and the second. On the Internet, I once read a post...
27. ### I How Does Friction Function in the Movement and Turning of a Car?

How can friction be responsible for all of these: stopping, moving and also turning a car? Does friction actually exist or is it something we assume because we don't know something about motion of objects? I have read a lot of discussions about friction and now it is a cloud of mess in my...
28. ### Pull-off force: hose, pipe, clamp

Was surprised by a study recently where we tested 6 samples for pull-off force at a 10% reduction in clamp Torque and noticed negligible shift in results. Is it possible that at higher torque the clamp is deforming the pipe, reducing the surface contact (friction) between the hose and pipe? The...
29. ### Vertical Mass Spring System | Analyzing work

Is there a typo in this question? Supposing there was no friction, the block would fall until the force of the spring was equal to ##mg = 2 * 9.8 = 19.6##, taking the upward y direction as positive. Since ##F_{spring} = -200y## and ##19.6 = -200(-0.098)##, the block would fall 9.8 cm. It's not...
30. ### Work done by friction on a hockey puck

The answer key claims the answer is -0.9 J. By my math it should be -0.501 J, is that incorrect?
31. ### Find the coefficient of friction and acceleration with masses given

I tried to use the energy route but the acceleration was not a plausible one. It was just way too big.
32. ### I Transmission of torque using friction

I am trying to obtain the expression for the potential transmission of torque using friction. I could derive the formula assuming constant pressure between planar surfaces. To have it in LATEX so it is easier to read, this is the expression for the torque transmission using friction...
33. ### Fluid Dynamics: Maximizing Downforce Between a Robot and the Floor

Good evening, all! Some quick background: I am working on a robot for a competition that strictly limits entry weights to 3lb. Robot speed & acceleration are highly desirable in this competition, so I have been focusing on ways to maximize my entry's performance. Drive motors for these...
34. ### Friction between tractor tyres and the ground

So in (a) the answer is just mgsintheta. When doing (b), I got 8.44x10^3N to be the NET force the tractor must exert. Does the tractor not need to overcome its own friction as well? If so, shouldn't the equation be F_tractor = F_exerted - F_friction, and the question is asking for F_exerted...
35. ### Pushing a block against the wall of an elevator that is accelerating

Easier case: Elevator is at rest. We need to prevent box from free fall so friction should be bigger than "mg".(And they can be equal) When we push with force F we know that the maximum static friction is ##u_sF##. "mg" should be smaller than ##u_sF## or should be equal to it so the minimum...
36. ### Static vs Kinetic Friction on an Unbanked Ramp

I used kinetic friction and did mgμ_k=mv^2/r. However, the solution is mgμ_s=mv^2/r. I am confused on why we consider static friction and not kinetic friction, thanks!
37. ### Friction Forces on 2WD vs 4WD Vehicles

My answer for this question is d as every car has the same result for the force of friction since the normal and coefficient of static friction is the same. I cannot find an answer online so can anyone help verify this? Thank you.
38. ### I An ID-card sliding on a low friction table

I hold my identification card on a low-friction surface by one of its edges. I slightly lean it, and it starts to fall. Before it falls over, I place my finger against the card, and this prevents it from falling all the way over. Then, I withdraw my finger without pushing or pulling the card and...
39. ### Relationship between horizontal force and friction in an exercise

The thing with this exercise is that I don't think that the question makes sense at all (or, at least, is incomplete). First of all, we don't know if the mass moves with any of those tensions, therefore I cannot know which coefficient apply. Second of all, even if we suppose that the mass is...
40. ### Oscillation with friction - Analytical mechanics

Hi, I had those exercises and want to know if they're correct. Also, feedback/tips would be great from you, professionals. $$A$$ 1. Let's consider the oscillator with a friction parameter... $$m \ddot{x}+\alpha \dot{x}=-\kappa x$$ but with ...
41. ### Capstan Equation, Friction and Normal Force

We have a following setup (see below). A plastic rod is placed inside a plastic ring. A wire is wrapped around a plastic rod from which it goes to the outer surface of the plastic ring. The friction coefficient between the wire and the plastic ring is about 0.1. We have several different sizes...
42. ### 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...
43. ### 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...
44. ### 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##?
45. ### 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...
46. M

### 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...
47. P

### Filmmaker and inventor fascinated by Physics

Hi, my question is about electricity, hydrostatic pressure, potential energy and friction. Using an electrically powered compressor (such as compressors used to fill diving bottles) if I fill a tank of volume 10 cubic meters to a pressure of 100psi, 1. what is the potential energy stored in that...
48. ### 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...
49. ### 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...
50. ### 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...