AP Physics 1- 2.7 Kinetic and Static Friction- Study Notes- New Syllabus
AP Physics 1-2.7 Kinetic and Static Friction – Study Notes
AP Physics 1-2.7 Kinetic and Static Friction – Study Notes -AP Physics 1 – per latest Syllabus.
Key Concepts:
- Kinetic Friction Between Two Surfaces
- Static Friction Between Two Surfaces
Kinetic Friction Between Two Surfaces
Kinetic friction is the resistive force that opposes the relative sliding motion between two solid surfaces in contact. It acts parallel to the surface and opposite to the direction of motion.
Key Features:
- Kinetic friction acts only when there is sliding between surfaces (unlike static friction, which prevents motion).
- It depends on the nature of the surfaces (roughness, texture, etc.), not on the contact area.
- The force of kinetic friction is approximately constant for a given pair of surfaces under steady motion.
Mathematical Expression:
\( f_k = \mu_k N \)
- \( f_k \): Kinetic friction force
- \( \mu_k \): Coefficient of kinetic friction (dimensionless, depends on surface materials)
- \( N \): Normal force exerted by the surface on the object
Properties:
- \( f_k \) is independent of velocity (at moderate speeds).
- Usually, \( \mu_k < \mu_s \), meaning kinetic friction is less than maximum static friction.
- Direction: Opposite to relative sliding motion.
Example:
A 10 kg block slides across a horizontal floor. If the coefficient of kinetic friction is 0.3, find the force of kinetic friction.
▶️Answer/Explanation
Normal force: \( N = mg = 10 \times 9.8 = 98 \, \text{N} \)
Kinetic friction: \( f_k = \mu_k N = 0.3 \times 98 = 29.4 \, \text{N} \)
Answer: The kinetic friction force = 29.4 N (opposite to motion).
Static Friction Between Two Surfaces
Static friction is the force that opposes the initiation of relative motion between two solid surfaces in contact. It keeps objects at rest until an applied force exceeds its maximum value.
Key Features:
- Acts only when there is a tendency for motion, but no actual sliding.
- Adjusts itself to balance the applied force up to a maximum limit.
- Depends on the nature of surfaces (roughness, texture, etc.), not on the contact area.
Mathematical Expression:
\( f_s \leq \mu_s N \)
- \( f_s \): Static friction force
- \( \mu_s \): Coefficient of static friction (dimensionless, depends on surface materials)
- \( N \): Normal force exerted by the surface on the object
Properties:
- Self-adjusting: increases with applied force until the maximum limit is reached.
- Maximum static friction is usually greater than kinetic friction: \( f_{s,\text{max}} > f_k \).
- Direction: Opposite to the impending motion (the force trying to cause motion).
Example:
A 5 kg block rests on a horizontal surface. If the coefficient of static friction is 0.4, find the maximum static friction that can act on the block.
▶️Answer/Explanation
Normal force: \( N = mg = 5 \times 9.8 = 49 \, \text{N} \)
Maximum static friction: \( f_{s,\text{max}} = \mu_s N = 0.4 \times 49 = 19.6 \, \text{N} \)
Answer: The maximum static friction = 19.6 N.