Friction

This page discusses the concept of friction and how it relates to both moving and static objects.

Claimed by drao31

The Main Idea

Friction is caused by the interactions between the atoms that make up objects. When objects interact, the atoms that make up the objects also interact. Temperatures of interacting surfaces rise because of such interactions. Friction is a force that resists movement. Generally, friction makes it harder to move objects. There are two main types of friction that are covered in Physics 2211: Static Friction and Kinetic Friction. Static friction can be described as the friction acting on resting objects. Kinetic friction is the friction acting on objects in motion.

Frictional forces depend on a few factors: the normal force and the coefficient of friction.

In simple terms, the normal force [math]\displaystyle{ {F}_{N} }[/math] or [math]\displaystyle{ {N} }[/math] is the equivalent force between objects. For example, when an object of mass M is placed on a table, there is a gravitational force Mg on the table. If the table can support the object without collapsing, the table exerts an equivalent force Mg on the object. According to the momentum principle, since there is no change in momentum, the net force must be 0. This is also apparent since there is no movement if the table can support the object.

[math]\displaystyle{ {p}_{f} - {p}_{i} = {F}_{net}{dt} = 0 }[/math]

OR

[math]\displaystyle{ {F}_{g} = {M}{g} }[/math]

[math]\displaystyle{ {N} = {M}{g} }[/math]

[math]\displaystyle{ {F}_{net} = {F}_{g} - {N} = {0} }[/math]

The coefficient of friction is usually specific to each object or material. The coefficient of static friction is denoted by [math]\displaystyle{ {μ}_{s} }[/math] and the coefficient of kinetic friction is denoted by [math]\displaystyle{ {μ}_{k} }[/math]

Static Friction:

[math]\displaystyle{ {F}_{s} \le {μ}_{s}{N} }[/math]

Static friction is the frictional force that must be overcome to begin moving an object at rest. The static friction can be no more than the normal force multiplied by the coefficient of static friction. Therefore, the maximum static friction an object can have is [math]\displaystyle{ {μ}_{s}{N} }[/math].

Kinetic Friction:

[math]\displaystyle{ {F}_{k} = {μ}_{k}{N} }[/math]

Kinetic friction is the frictional force that exists between objects already at motion.

A Computational Model

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Examples

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Simple

A 100Kg box is placed on a flat table. The box does not move. The coefficient of static friction [math]\displaystyle{ {μ}_{s} = {.25} }[/math] and the coefficient of kinetic friction [math]\displaystyle{ {μ}_{k} = {.15} }[/math]. Find the force of friction [math]\displaystyle{ {F}_{s} }[/math].

Solution:

Since the box does not move, there is only static friction. The normal force is equivalent to the force due to gravity: [math]\displaystyle{ {F}_{N} = {F}_{g} = {100}{g} }[/math]. The static friction can be found by: [math]\displaystyle{ {F}_{s} = {μ}_{s}{F}_{N} }[/math]

[math]\displaystyle{ {F}_{s} = {(.25)}{(100)}{(9.8)} = {245.25} }[/math]

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