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Theory/Dynamics

Dynamics · Chapter 11

Basics of Friction

Basics of Friction detailed theory study guide for Physics.

12 min read · 4 topics

01

Key Concepts

Key concepts will be added here.

02

Examples

Examples will be added here.

03

Introduction to Basics of Friction

Welcome to Basics of Friction. Content to be added.

04

Friction Force - Basics

Introduction to Friction

Friction is a force that opposes the relative motion or tendency of relative motion between two surfaces in contact. It arises from the electromagnetic interactions between the atoms and molecules of the surfaces. On a microscopic level, surfaces are not perfectly smooth; they have irregularities like hills and valleys. When two surfaces are in contact, these irregularities can interlock. Adhesions can form between the atoms of the two surfaces.

Microscopic view of surfaces in contact, showing irregularities.
Microscopic view of surfaces in contact, showing irregularities.

When one surface moves or attempts to move over another, these interlocked points must be broken, and the adhesions must be overcome. This resistance to motion is what we experience as friction. There are two main mechanisms that contribute to friction: adhesion (the attractive forces between the surfaces) and ploughing (one surface deforming the other).

Types of Friction

Friction can be broadly classified into three main types:

1. Static Friction (fsf_sfs​)

Static friction is the force of friction that acts on an object when it is at rest. It is a self-adjusting force, meaning its magnitude is equal to the applied force until it reaches a maximum value, known as the limiting friction.

fs≤μsNf_s \le \mu_s Nfs​≤μs​N

where μs\mu_sμs​ is the coefficient of static friction and N is the normal force.

Block of mass 5 kg on a rough horizontal surface, subjected to applied force F and friction f.
Block of mass 5 kg on a rough horizontal surface, subjected to applied force F and friction f.

2. Kinetic Friction (fkf_kfk​)

Kinetic friction is the force of friction that acts on an object when it is in motion. Its magnitude is approximately constant and is usually less than the limiting static friction.

fk=μkNf_k = \mu_k Nfk​=μk​N

where μk\mu_kμk​ is the coefficient of kinetic friction.

Block stationary on an incline; static friction balances mg sin θ.
Block stationary on an incline; static friction balances mg sin θ.

3. Rolling Friction

Rolling friction is the resistance that occurs when a round object (such as a ball, tire, or wheel) rolls on a surface. It is caused by the deformation of the object and the surface. Rolling friction is much weaker than static or kinetic friction, which is why it is easier to roll an object than to slide it.

Laws of Friction

The behavior of friction is summarized by the following empirical laws:

Typical behavior of friction force f versus applied force F. Static friction increases until maximum, then kinetic friction remains constant.
Typical behavior of friction force f versus applied force F. Static friction increases until maximum, then kinetic friction remains constant.

  1. Friction is proportional to the normal force. The maximum static friction and the kinetic friction are directly proportional to the normal force between the two surfaces.

    fs,max=μsNf_{s,max} = \mu_s Nfs,max​=μs​N

    fk=μkNf_k = \mu_k Nfk​=μk​N

  2. Friction is independent of the apparent area of contact. As long as the normal force is the same, the friction force does not depend on the area of the surfaces in contact.
  3. Kinetic friction is independent of the relative velocity of the surfaces. The kinetic friction force is approximately constant and does not change with the speed of the object (for low to moderate speeds).
  4. The coefficient of kinetic friction is generally less than the coefficient of static friction. It takes more force to start an object moving than to keep it moving. (μk<μs\mu_k < \mu_sμk​<μs​)

The force of friction is a...

It is easier to roll a barrel than to pull it along the road. This statement is correct because:

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