Why is the coefficient of static friction greater than the coefficient of kinetic friction?
The correct answer and explanation is :
The coefficient of static friction (μ_s) is greater than the coefficient of kinetic friction (μ_k) because more force is required to initiate motion than to maintain it. This happens due to the microscopic interactions and surface asperities (roughness) between two objects in contact.
Explanation (300 Words)
Friction arises from the interaction between two surfaces. When an object is at rest, the microscopic irregularities on its surface interlock with the irregularities on the surface it rests upon. These interlocking asperities create a resistance that must be overcome to initiate movement. This resistance is known as static friction (F_s).
The force required to break these interlocking asperities and set the object in motion is greater than the force required to keep it moving. Once the object starts moving, the surfaces are no longer fully interlocked; instead, they glide over each other with less resistance. This results in kinetic friction (F_k), which is generally lower than static friction.
Mathematically, the relationship is expressed as:
[
F_s = \mu_s N
]
[
F_k = \mu_k N
]
where N is the normal force, and (\mu_s > \mu_k).
Key Reasons Why Static Friction is Greater
- Interlocking Surface Irregularities
- When stationary, surfaces settle into each other’s microscopic grooves, requiring extra force to detach.
- Stronger Molecular Adhesion
- At rest, molecular forces between the surfaces are stronger, requiring higher force to separate them.
- Reduced Contact Time in Motion
- Once moving, there’s less time for strong intermolecular bonds to reform, reducing friction.
Conclusion
Since initiating motion involves overcoming stronger interlocking forces, static friction is always greater than kinetic friction.