The maximum frictional force which comes into play when a body just begins to slide over the surface of another body is known as:
Try it first: pick an option, then read the explanation.
Asked in SSC JE 03/03/2017 (Morning)
Detailed Explanation
Push gently on a heavy box. At first it does not move: friction grows to match your push exactly. As you push harder, friction keeps growing — but only up to a limit. The moment you exceed that limit, the box starts to slide.
\[ F_{lim} = \mu R \]
where \(R\) is the normal reaction and \(\mu\) the coefficient of (static) friction.
Why “Limiting friction” Is Correct
The maximum friction force, reached just as sliding is about to begin, is called limiting friction. Below it, friction is static and adjusts to the load; once sliding starts, friction drops slightly to the kinetic value.
| Term | Meaning |
|---|---|
| Static friction | Friction while there is no sliding; it adjusts to match the applied force |
| Limiting friction | The maximum static friction, reached when sliding is just about to start: F = μR |
| Kinetic (dynamic) friction | Friction during sliding; slightly less than limiting friction |
| Coefficient of friction μ | Limiting friction ÷ normal reaction |
| Angle of friction φ | Angle between the normal reaction and the resultant reaction at limiting friction: tan φ = μ |
| Angle of repose | Steepest slope on which a body rests without sliding; equals φ |
Why the Other Options Are Wrong
- A. Static friction: Static friction is any friction before sliding; its maximum value is specifically the limiting friction.
- C. Dynamic friction: Dynamic (kinetic) friction acts after sliding has started and is slightly smaller.
- D. Coefficient of friction: The coefficient of friction is a ratio, not a force.
Key Point
Limiting friction = maximum static friction = μR.
Key Concepts for Students
- Laws of dry friction: F ∝ R, independent of contact area, depends on surfaces.
- Kinetic friction is roughly independent of speed at low speeds.
- Rolling friction is much smaller than sliding friction.
Practice more: this is question 2 in Friction MCQs – Part 1. Try the whole set and check every answer instantly.