# What is Friction forces: Definition and 35 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. ### I A question on momentum conservation of bodies in combined motion

Frictions are neglected in the system pictured. Here, a 1kg object released from above moves the car underneath. When this object falls off the car, what is its speed in m/s? This question was asked from momentum conservation, but I didn't want to do it from there. Can you solve the question...
2. ### 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?
3. ### 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...
4. ### What determines the direction of static friction forces?

My understanding of static friction is that it is a resistive force in response to an applied force. I recently read the following example. A rope of uniform density, length L, is fastened to a plane with incline angle “theta” along its length. The question asks for the tension at the top of...
5. ### When Friction the block m2 start to move?

Homework Statement A block m2 is placed on a surface where the coefficient of friction is μ2 and another block m1 placed over the block m2 where the coefficient of frictio is μ1. A force F=k*t(Force as a function of time),is acting on the block m2. Now my question is that when would the...
6. ### How to find the coefficient of kinetic friction

Homework Statement The speed of a 4.0-N hockey puck, sliding across a level ice surface, decreases at the rate of 0.61 m/s2. The coefficient of kinetic friction between the puck and ice is: A) 0.062 B) 0.25 C) 0.41 D) 0.62 E) 1.2 Homework Equations I'm not too sure but i know that : If the...
7. ### Finding Minimum Force Needed to Hold Object In Place

Homework Statement A woman holds a book by placing it between her hands such that she presses at right angles to the front and back covers. The book has a mass of m = 0.75kg and the coefficient of static friction between her hand and the book is μs = 0.59. What is the minimum force she must...
8. ### When do friction forces stop acting?

Another classic case of "I understand it in reality, but on paper it just makes no sense and I'm confused". There is an object sliding north on a surface where the normal is 10N. The co-efficient for kinetic friction for the surface is 0.5. Simple math will tell me that the force of friction...
9. ### What are the equal-and-opposite forces of friction forces?

Homework Statement A block rests on a table, and a second block, connected to the first with a rope, hangs over the side of the table. See the illustration. (Assume the pulley is massless and frictionless.) All objects pictured are stationary. Identify every force involved in the pictured...
10. ### The applied and friction forces of a particle

Homework Statement A particle of mass 14kg, slides along a straight wire in a horizontal plane. The coefficient of dynamic friction u=0.6 The equation of the line of the wire is y=3√x so that the angle between the wire and the X-axis is 60 degrees. The particle accelerates with a constant...
11. ### Motion with gravitational, elastic, and friction forces

Homework Statement A 2000-kg elevator with broken cables in a test rig is falling at when it contacts a cushioning spring at the bottom of the shaft. The spring is intended to stop the elevator, compressing 2.00 m as it does so. During the motion a safety clamp applies a constant 17,000-N...
12. ### Effects of inertia forces and friction forces on a piston in engine

Hello, I am curious regarding the forces other than the pressure created by combustion process in IC engine.Whether inertia and friction affect worth considering or not, I want to know magnitude and nature of these forces and their effects On efficiency and performance.Thank you...
13. ### Coefficient of kinetic friction forces

Homework Statement Find the applied force for a object if the coefficient of kinetic friction equals 0.30 and the acceleration equals 5m/s^2. The applied force makes a 40 degree angle with the horizontal. The mass of the object is 10kg. Homework Equations Fnet=ma The Attempt at a...
14. ### Friction Forces, find the coefficient of kinetic friction

Hello, I am stuck on this question, I don't understand what to do. Any help would be great! I know that I have to use Newton's Second Law in the x and y directions. Greezy is pushing a very large crate of 50 kg across the floor at a constant velocity with a force of 200N. He is pushing on...
15. ### Solving Friction Forces: Finding Coefficients for Block & Surface

1. A 25.0-kg block is initially at rest on a horizontal surface. A horizontal force of 75.0 N is required to set the block in motion, after which a horizontal force of 60.0 N is required to keep the block moving with constant speed. Find (a) the coefficient of static friction and (b) the...
16. ### Calculating friction forces

Homework Statement A 1326kg car lies on a flat surface. To make it move very slowly (a~0), one needs a force of 160N parallel to the surface. To stop it, the force must be reduced to 120N. Homework Equations Calculate the friction forces of the car On Tuesday I have an exam regarding...
17. ### Line integral of friction forces

I was just performing the usual wine glass experiment where I rub my wet finger across the rim and I hear the resonant frequencies from the water column-due to the friction produced from my finger rubbing. Today I learned about "conservative" forces from my professor.Since,frictional forces come...
18. ### Friction forces playing the role of centripetal force

Can you describe( graphically,like a diagram,or a picture) how a friction force plays the role of the centripetal force?For example, a car performing an uniform circular motion on a plane in a idela situation.The car will perform the turn because of friction forces between tyres and...
19. ### Solving a Pulley System: Mass M & Friction Forces

Homework Statement An object of mass M=10kg is being pulled up by a system of pulleys. a)What force do you need to exert on point A for the object to move uniformly ? b) What force do you need to exert on point A, if the friction in each of the pullies is 0,5N and the pulleys are not massless -...
20. ### 3 blocks no friction forces question

Homework Statement [PLAIN]http://img834.imageshack.us/img834/6920/captureqt.jpg Homework Equations f=ma The Attempt at a Solution Fsum=Fl + Fr=(m1+m2+m3)a 26+-8=(2+8+4)a a=18/14=1.28571 f=ma f=4(1.28571)=5.14268N I know this is incorrect. The force of 4kg block pushes...
21. ### Friction forces on ball hitting wall

Homework Statement Suppose I throw a ball at a wall parallel to its x-axis. At the moment it hits the wall, what forces are acting upon the ball? Homework Equations None. Relevant assumptions: ignore any spin on ball. The Attempt at a Solution I think they'll be: -...
22. ### Friction Forces (Static Friction)

Homework Statement A boy has stacked two blocks on the floor so that a 5.0 kg block is on top of a 2.0 kg block. 1. If the coefficient of static friction between the two blocks is 0.40, and the coefficient of static friction between the bottom block and the floor is 0.22, with what force...
23. ### Do Friction Forces Exist Between a Rod and a Shaggy Bowl?

Homework Statement http://picasaweb.google.com/lh/photo/ChOBWTwVEO3hI0wtftmdmw?feat=directlink A rigid rod is put in a global bowl horizontally.The bowl is shaggy.Do friction forces exist where the rod and the bowl touch? Homework Equations The Attempt at a Solution If the bowl is...
24. ### Static and Kinetic Friction Forces (Finding Net Force)

Homework Statement The coefficient of static friction between a block and a horizontal floor is 0.414, while the coefficient of kinetic friction is 0.138. The mass of the block is 5.24 kg. A horizontal force is applied to the block and slowly increased. What is the value of the applied...
25. ### Horizontal and Friction Forces

Homework Statement A horizontal force F pushes a block of weight 5.0 N aganist a vertical wall. The coefficient of static friction between the wall and the block is 0.25. The force F is the minimum required to hold the block in place. Find the magnitude of the net force on the block by the...
26. ### What is the Normal Force on a 16.2 kg Object Being Pulled with 10.2N Force?

Question: A 16.2 kg object is pulled along a frictionless horizontal surface by a horizontal net force of 10.2N. What is the normal force acting on the object? Answer: Fn= 16.2*9.8 =158.8 I am sure do i have to ignore the 10.2N force... Thank you for helping
27. ### Friction Forces: Maximum Intensity & Acceleration

http://www.digitaluploader.com/1/2a578af8cd660754014ea75dbe966588.GIF Hi, I need some help with this exercise because I can't figure out where the friction forces point to.Find the maximum intensity possible for F before any of the blocks start moving, and the acceleration that they acquire...
28. ### Static friction forces on ramp

An 55.0-N crate of apples sits at rest on a ramp that runs from the ground to the bed of a truck. The ramp is inclined at 30.0° to the ground. I sketched the problem using a popular paint program. I seem to be having problems visualizing components of the normal force for some reason. I know...
29. ### Friction Forces and max braking force

1. The coefficient of static friction between the tires of a car and a dry road is .62. The mass of the car is 1500 kg. What is the maximum braking force is obtainable: a. horizontal to the road. b. on an 8.6 degree downgrade? 2. F=mA f(s)=M(s)N N=mg F=f(s)-mgsin(x) 3. a...
30. ### Friction forces to keep board from slipping

The board sandwiched between two other boards weighs 95.5 N. If the coefficient of friction between the boards is .663, what must be the magnitude of the comrssion forces (assume horizontal acting on both sides of the center board to keep it from slipping? I have no idea how to do this...
31. ### Friction Forces on rock climber

A 52 kg rock climber is climbing a “chimney” between two rock slabs. The coefficient of static friction between her shoes and the rock is 1.33; between her back and the rock it is 0.89. She has reduced her push against the rock until her back and her shoes are on the verge of slipping. (a) What...
32. ### Blocks, Friction Forces and Acceleration

I am having trouble with this problem: A block whose weight is 50.0N rests on a horizontal table. A horizontal force of 25.0 N is applied to the block. The coefficients of static and kidnetic friction are 0.510 and 0.350 respectively. Will the Block move under the influence of the force and if...
33. ### The work of friction forces - and - the total work of systems

I've been given two conceptual problems that I can't quite crack, at least, I'm not sure that I'm cracking them: Problem 1 When a box is moving across a rough surface the friction force does negative work. Can a friction force ever do positive work? If yes, give an example, if no, explain...
34. ### Equilibrium & Friction Forces: What's the Answer?

I need help with these 2 force questions. My answers have ***** next to them. 1) An object in equilibrium Choices: a) does not move. b) has no forces acting on it. c) has 0 accelleration. *************** d) two of the above e) all of the above An object in equilibrium...
35. ### Need help with friction forces please

Hi, I was giving a problem by my teacher that will most likely be on the exam. A block, m1, is ontop of another block, m2 and there is a constant friction, mu, between the two blocks. Between block m2 and the ground there is no friction. What is the largest force, P, that can be put on m2 and...