Static and Kinetic Friction
Static friction is a self-adjusting contact force up to a maximum fs(max) = μs N; kinetic friction acts on sliding surfaces with fk = μk N.
What is Static and Kinetic Friction?
Static friction is a self-adjusting contact force up to a maximum fs(max) = μs N; kinetic friction acts on sliding surfaces with fk = μk N.
Key formula / rule: fs ≤ μs N with equality only at the point of slipping.
Key points
- Distinguish static from kinetic friction using the limiting condition.
- Compute the friction force for a body on a horizontal surface under applied force.
Common exam trap
Writing fs = μs N when the body is not on the verge of slipping.
Definitions
- Term
Static and Kinetic Friction
- Meaning
Static friction is a self-adjusting contact force up to a maximum fs(max) = μs N; kinetic friction acts on sliding surfaces with fk = μk N.
- Term
Static and Kinetic Friction — explanation
- Meaning
Static friction takes whatever value is needed to prevent relative sliding, up to its limit; only then does the body slide and the smaller kinetic value take over. This is why fs = μs N is a limit, not a formula for every situation.
Learning objectives
Distinguish static from kinetic friction using the limiting condition.
Compute the friction force for a body on a horizontal surface under applied force.
Formulae
- Key point
fs ≤ μs N with equality only at the point of slipping.
- Key point
fk = μk N and μk < μs in general.
- Key point
Friction is independent of contact area for rigid dry surfaces.
Prerequisites
PHY-U1-NLM-T1-S2-C1
Common mistakes
Writing fs = μs N when the body is not on the verge of slipping.
Assuming friction always opposes motion rather than relative sliding tendency.
Keywords
Static
Kinetic
Friction
Practice preview
A block of mass 5 kg is placed on a horizontal surface. The coefficient of static friction between the block and the surface is 0.5, and the coefficient of kinetic friction is 0.3. What is the minimum horizontal force re…
medium
A block of mass 10 kg is placed on a rough horizontal surface. The coefficient of static friction is 0.6, and the coefficient of kinetic friction is 0.4. If a horizontal force of 50 N is applied to the block, what is the…
hard
Which of the following statements is generally true regarding the coefficients of static friction (μ_s) and kinetic friction (μ_k) for a given pair of surfaces?…
easy