Friction
First: Summary:
Friction force(𝒇): A resistive contact force of electrostatic origin (i.e., arising from the attractive and
repulsive electric forces between the atoms of the two contacting bodies), and it is always directed
opposite to the direction of motion.
► The friction force depends on two factors:
① Normal force (𝑭⊥ ): The greater it is, the closer the atoms of the two bodies come, increasing the
electric attractive and repulsive forces.
② Nature of the surfaces (coefficient of friction "𝝁"): The electric forces between atoms vary depending
on the type of atoms and the types of chemical bonds between them.
f (N)
Static friction force (𝑓𝑠 ):
𝑓𝑠𝑚𝑎𝑥 = 𝜇𝑠 𝐹⊥ f sₘₐₓ
fₖ
► friction force
Kinetic friction force (𝑓𝑘 ): State State of
of Rest Motion
𝑓𝑘 = 𝜇𝑘 𝐹⊥
FApplied (N)
► Notes:
1. The force of friction does not depend on the apparent contact area.
2. The static friction force increases with the applied force on the object, reaching its maximum value
(𝑓𝑠𝑚𝑎𝑥 ) at the moment motion begins.
3. The kinetic friction force is approximately constant and is always less than the maximum static
friction force (𝑓𝑠𝑚𝑎𝑥 ).
4. The coefficient of friction (𝜇) has no unit because it is a ratio between two forces.
5. The solution technique depends on the given data; either the friction law is used directly, or the net
force equations are applied.
First: Summary:
Friction force(𝒇): A resistive contact force of electrostatic origin (i.e., arising from the attractive and
repulsive electric forces between the atoms of the two contacting bodies), and it is always directed
opposite to the direction of motion.
► The friction force depends on two factors:
① Normal force (𝑭⊥ ): The greater it is, the closer the atoms of the two bodies come, increasing the
electric attractive and repulsive forces.
② Nature of the surfaces (coefficient of friction "𝝁"): The electric forces between atoms vary depending
on the type of atoms and the types of chemical bonds between them.
f (N)
Static friction force (𝑓𝑠 ):
𝑓𝑠𝑚𝑎𝑥 = 𝜇𝑠 𝐹⊥ f sₘₐₓ
fₖ
► friction force
Kinetic friction force (𝑓𝑘 ): State State of
of Rest Motion
𝑓𝑘 = 𝜇𝑘 𝐹⊥
FApplied (N)
► Notes:
1. The force of friction does not depend on the apparent contact area.
2. The static friction force increases with the applied force on the object, reaching its maximum value
(𝑓𝑠𝑚𝑎𝑥 ) at the moment motion begins.
3. The kinetic friction force is approximately constant and is always less than the maximum static
friction force (𝑓𝑠𝑚𝑎𝑥 ).
4. The coefficient of friction (𝜇) has no unit because it is a ratio between two forces.
5. The solution technique depends on the given data; either the friction law is used directly, or the net
force equations are applied.
