Question

Which of the following statements is true regarding static friction?

• A. It always equals the maximum static friction.
• B. It is always less than kinetic friction.
• C. It adjusts up to a maximum value to prevent motion.
• D. It acts only when the body is moving.

Hint:

Static friction prevents motion and adjusts its value to match the applied force up to a limit:
\[ f_s \leq \mu_s N \] It acts only when the body is at rest relative to the surface.

Answer 1

The Correct Option is C

Static friction is the frictional force that resists the initiation of motion between two surfaces in contact. Unlike kinetic friction (which acts when there is motion), static friction varies depending on the external force applied.
Key property:
Static friction is self-adjusting. This means it increases as the applied force increases, but only up to a limit — the maximum static friction given by:
\[ f_{\text{max}} = \mu_s N \] where \( \mu_s \) is the coefficient of static friction and \( N \) is the normal force.
Let’s analyze the options:
- (A) It always equals the maximum static friction: Incorrect — it only equals the maximum when motion is about to occur. Otherwise, it's less.
- (B) It is always less than kinetic friction: Incorrect — static friction is usually greater than kinetic friction.
- (C) It adjusts up to a maximum value to prevent motion: Correct — this is the defining feature of static friction.
- (D) It acts only when the body is moving: Incorrect — that's kinetic friction, not static.