# What is Contact forces: Definition and 27 Discussions

Contact mechanics is the study of the deformation of solids that touch each other at one or more points. A central distinction in contact mechanics is between stresses acting perpendicular to the contacting bodies' surfaces (known as the normal direction) and frictional stresses acting tangentially between the surfaces. This page focuses mainly on the normal direction, i.e. on frictionless contact mechanics. Frictional contact mechanics is discussed separately. Normal stresses are caused by applied forces and by the adhesion present on surfaces in close contact even if they are clean and dry.
Contact mechanics is part of mechanical engineering. The physical and mathematical formulation of the subject is built upon the mechanics of materials and continuum mechanics and focuses on computations involving elastic, viscoelastic, and plastic bodies in static or dynamic contact. Contact mechanics provides necessary information for the safe and energy efficient design of technical systems and for the study of tribology, contact stiffness, electrical contact resistance and indentation hardness. Principles of contacts mechanics are implemented towards applications such as locomotive wheel-rail contact, coupling devices, braking systems, tires, bearings, combustion engines, mechanical linkages, gasket seals, metalworking, metal forming, ultrasonic welding, electrical contacts, and many others. Current challenges faced in the field may include stress analysis of contact and coupling members and the influence of lubrication and material design on friction and wear. Applications of contact mechanics further extend into the micro- and nanotechnological realm.
The original work in contact mechanics dates back to 1881 with the publication of the paper "On the contact of elastic solids" ("Ueber die Berührung fester elastischer Körper") by Heinrich Hertz. Hertz was attempting to understand how the optical properties of multiple, stacked lenses might change with the force holding them together. Hertzian contact stress refers to the localized stresses that develop as two curved surfaces come in contact and deform slightly under the imposed loads. This amount of deformation is dependent on the modulus of elasticity of the material in contact. It gives the contact stress as a function of the normal contact force, the radii of curvature of both bodies and the modulus of elasticity of both bodies. Hertzian contact stress forms the foundation for the equations for load bearing capabilities and fatigue life in bearings, gears, and any other bodies where two surfaces are in contact.

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1. ### A question about contact forces and friction

Homework Statement Two blocks of masses m1 and m2 are placed on a table in contact with each other as shown in the figure below. The coefficient of kinetic friction between the block of mass m1 and the table is μ1, and that between the block of mass m2 and the table is μ2. A horizontal force of...
2. ### Why is the normal reaction force zero in the absence of any external force?

Homework Statement My question is not a homework problem. I am having difficulty in understanding the concept oof contact forces. Homework Equations What I don't understand is that when two bodies A and B are in contact and left on a frictionless surface, they apply normal reactions on each...
3. ### Frames of Reference: A Skateboarder's View - Comments

kuruman submitted a new PF Insights post Frames of Reference: A Skateboarder's View Continue reading the Original PF Insights Post.
4. ### B Quantum Model of Contact Forces

I'm curious about contact forces, such as the normal reaction force on a book resting on a table. I've read that it's actually the electromagnetic force, but atoms are electrically neutral and there is no ostensible repulsive force between electrons and the nucleus, so I have some doubts. Has...
5. ### Understanding Fermi Contact forces

Normally, you think about SM forces being conveyed by gluons, weak force gauge bosons, or photons (ignore that troublesome gravity thing for the moment) between point particles. There is also a property of fermions (particles with total angular momentum Q=1/2, 3/2, etc.) that they can't occupy...
6. ### How Does Pushing Force Affect Contact Force Between Two Boxes?

Homework Statement Alex is asked to move two boxes of books in contact with each other and resting on a rough floor. He decides to move them at the same time by pushing on box A with a horizontal pushing force FP = 9.3 N. Here A has a mass mA = 10.6 kg and B has a mass mB = 7.0 kg. The contact...
7. ### Exploring Contact Forces: Understanding Collisions and Launching Objects

I have recently studied collisions, in which one or two bodies are already in motion. I have tried to apply them to cases in which the bodies are initially at rest, like launching or kicking a ball etc, but they do not work. Can you tell me how to google to find them, 'contact force' gives...
8. ### How can teeth be bent without contact forces

Hi, so by no doubt you came here because of the bold phrase of the question. So to clear things up, this is a serious question,an no ido not believe in supranomral mind powers. This is a pragmatic question, and I am not a delusional person trying to get attention. it concerns my teeth which...
9. ### What are contact forces, really?

The four non-contact forces are explained with the help of different fields and their interactions. What about contact forces? Why does the door shut when I push it? Is it a simple transfer of momentum? If it is, do the electrons collide?
10. ### Calculating Contact Forces- Three objects

Homework Statement If you have a system with three boxes, two side by side and one on top of the first box and there is a horizontal force acting on the boxes, do you have to account for the third box in calculating the contact force between two of them? Say if box 1, box 2, and box 3 (three...
11. ### Finding Magnitude of Contact Forces in Boxes 1, 2 and 3

Homework Statement A force of magnitude 7.50N pushes three boxes with masses m1=1.3kg, m2=3.2kg, and m3= 4.9kg as shown in the figure. I want to find the magnitude of the contact forces between boxes 1 and 2, and between boxes 2, and 3. Now, the force, 7.5N is being pushed from right to...
12. ### What Is the Coefficient of Kinetic Friction in Brenda's Wallpaper Project?

Homework Statement Before hanging new William Morris wallpaper in her bedroom, Brenda sanded the walls lightly to smooth out some irregularities on the surface. The sanding block weighs 2.0N and Brenda pushes on it with a force of 3.0N at an angle of 30.0 degrees with respect to the vertical...
13. ### What is the Contact Force between Boxes in a Horizontal Force System?

Homework Statement Three boxes rest side-by-side on a smooth horizontal floor. Their masses are 1.45 kg, 3.10 kg, and 5.20 kg, with the 3.10 kg one in the center. A horizontal force of 24.0 N pushes on the 1.45 kg mass which pushes against the other two masses. What is the contact force...
14. ### Contact Forces HW Problem with Three Boxes

Correct? Please let me know if my logic is off. =] Given Problem As shown in the figure, a force of magnitude 7.50N pushes three boxes with masses m1 = 1.30 kg m2 = 3.20 kg m3 = 4.30 kg (a) Find the magnitude of the contact force between boxes 1 and 2. (?N) (b) Find the magnitude...
15. ### Newton's laws multiple objects contact forces

Homework Statement Three crates are arranged. Their masses are m1=29.8, m2= 15.3, m3= 27.3. The applied force of 910 N is directed horizontally and the coefficient of friction is u= .277 WHAT IS THE MAGNITUDE OF THE CONTACT FORCE BETWEEN M1 and M2? WHAT IS THE MAGNITUDE OF THE CONTACT...
16. ### How Much Weight Must Block C Have to Prevent Block A from Sliding?

blocks A and B have weights of 45 N and 25 N, respectively. block A is sitting on a table and block B is attached to block A by a piece of string on a frictionless pulley and block B is hanging off the table. also block C is on top of block A Figure 6-48 (a) Determine the...
17. ### Calculating Contact Forces for Pushing Boxes

I've looked everywhere to find how do you calculate contact forces, like if you were pushing boxes. Don't want the answer just need to know how to do the calculations.
18. ### How Do You Calculate Tension in a Wire at a 45 Degree Angle?

Homework Statement A body of mass m = 10kg is suspended by a fixed support through a perfect wire. The body is resting on the vertical support to another disk, so that the thread formed with a vertical angle α = 45 °, as shown. The tension from the wire has the value: a. T = 41N b. T =...
19. ### Contact Forces with a Massless String

Homework Statement Three blocks of equal mass (m = 0.88kg) are placed in a row onto a rough ramp (μ=0.77) whose angle of inclination is 35 (degrees). Each block is connected to the other by a massless string. Suppose you pull upwards on the top block with a force of 34.9N. Answer the...
20. ### Contact Forces Question(s) part 2

Homework Statement Three blocks are placed onto a frictionless horizontal surface. These blocks are side by side and are in contact with each other. Let m1=2m2=5m3. Answer the following exercises. (Part 1) a)If a force is applied to m1, then show that i)Fa = 17/2m3a ii)F12 = 7/2m3a...
21. ### Contact Forces in Three-Block System

Homework Statement Three blocks are placed onto a frictionless horizontal surface. These blocks are side by side and are in contact with each other. Let m1=2m2=5m3. Answer the following exercises. a)If a force is applied to m1, then show that i)Fa = 17/2m3a ii)F12 = 7/2m3a where...
22. ### Calculating Contact Forces between Blocks

Homework Statement Three blocks are in contact with each other on a frictionless, horizontal surface, as in Figure P5.54. A horizontal force F is applied to m1. Take m1 = 2.00 kg, m2 = 3.00 kg, m3 = 4.00 kg, and F = 22.0 N. (c)What is the magnitude of the contact force between m1 and m2...
23. ### Three Blocks with Forces: Calculating Contact Forces

Three blocks are in contact with each other on a frictionless horizontal surface with m1=1kg m2=2kg m3=3kg A horizontal force of 24N is applied to M1. a) Find the acceleration of the three blocks b) Find the net force on each block c) Find the magnitude of the contact forces...
24. ### Verifying Newton's Laws: Finding Acceleration & Contact Forces

Hi I have a problem and a doubt regarding Newton's laws of motion. Consider three blocks of 5,2,5kg respectively placed one over the other and dropped from the top of a tower. (the 5 kg if at the top, 2kg in the middle and the 5kg at the bottom). i) Find the acceleration of each block...
25. ### Contact forces homework problem

Hello, I have a problem and I would like to check my answers: A mass m1 rests on top of another mass m2 , which in turn rests on a frictionless horizontal surface. A light cord is attached to m2 ,which is used to pull on it witha force F. a) Find the acceleration ofeach object when...
26. ### Contact forces and action-at-a-distance forces

Contact forces and action-at-a-distance forces... Are all contact forces the result of action-at-a-distance forces? After all, surely it all boils down to electrostatic repulsions, etc? Thanks in advance.
27. ### Calculating Contact Forces Between Blocks on a Frictionless Surface

This problem seemed very straightforward but I've somehow missed something... Three blocks (M1, M2, M3) are in contact with each other on a frictionless, horizontal surface. A horizontal force is applied to M1. If M1= 1.67kg, M2= 2.67kg, M3= 3.78kg, and F= 16.1 N: a) what is the...