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🔋college physics i – introduction review

Fs

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

fs, or force of friction, is the force that opposes the relative motion between two surfaces in contact. It arises due to the microscopic irregularities on the surfaces and acts to resist the sliding or rolling of one surface over the other.

5 Must Know Facts For Your Next Test

  1. The force of friction is proportional to the normal force between the surfaces, with the constant of proportionality being the coefficient of friction.
  2. Friction can be either static (before motion occurs) or kinetic (during motion), with the kinetic friction force generally being less than the static friction force.
  3. The direction of the frictional force is always opposite to the direction of the relative motion or the intended direction of motion.
  4. Factors that affect the force of friction include the nature of the surfaces in contact, the presence of lubricants, and the roughness of the surfaces.
  5. Friction is a dissipative force, meaning it converts mechanical energy into thermal energy, which can be useful (e.g., in braking) or undesirable (e.g., in machinery).

Review Questions

  • Explain the relationship between the normal force and the force of friction.
    • The force of friction, $f_s$, is directly proportional to the normal force, $N$, between the two surfaces in contact. This relationship is expressed as $f_s = \mu N$, where $\mu$ is the coefficient of friction, a dimensionless quantity that depends on the properties of the surfaces. The normal force determines the magnitude of the frictional force, and changes in the normal force will result in corresponding changes in the frictional force.
  • Describe the differences between static and kinetic friction, and how they impact the motion of an object.
    • Static friction, $f_s$, is the force of friction that acts on an object at rest, preventing it from moving. Kinetic friction, $f_k$, is the force of friction that acts on an object that is already in motion, sliding relative to the surface. Generally, the kinetic friction force is less than the static friction force. Static friction must be overcome to initiate motion, while kinetic friction acts to oppose the continued motion of the object. The difference between these two types of friction can significantly impact the acceleration and motion of an object.
  • Analyze how various factors, such as surface roughness and the presence of lubricants, can affect the force of friction.
    • The force of friction, $f_s$, is influenced by the nature of the surfaces in contact. Rougher surfaces tend to have higher coefficients of friction, leading to greater frictional forces, as the microscopic irregularities on the surfaces interlock more strongly. Conversely, the presence of lubricants, such as oil or grease, can reduce the coefficient of friction by creating a smooth, low-friction interface between the surfaces. This reduction in the force of friction can have significant implications for the efficiency and performance of mechanical systems, where minimizing friction is often a key design consideration.