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Friction: Static vs Kinetic
Maximum Static Friction, Kinetic Friction, and Coefficients
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Friction is a contact force that resists motion or attempted motion between surfaces. It matters because it affects everything from walking and driving to machines and sports. Two important types are static friction, which acts when surfaces are not sliding, and kinetic friction, which acts when they are sliding. Understanding the difference helps students predict when objects start moving and how they behave once in motion.
Static friction adjusts its size to match an applied force up to a maximum value, so it can keep an object at rest. Kinetic friction usually has a nearly constant magnitude once sliding begins, and it is often smaller than the maximum static friction for the same surfaces. This is why an object can be hard to start moving but easier to keep moving. Both types depend on the normal force and the nature of the surfaces in contact.
Key Facts
- Static friction prevents slipping and acts only as large as needed until a limit is reached.
- Maximum static friction: f_s,max = mu_s N
- Kinetic friction while sliding: f_k = mu_k N
- Usually mu_s > mu_k for the same pair of surfaces.
- Friction acts parallel to the contact surface and opposite the actual or impending motion.
- If applied force exceeds f_s,max, the object starts moving and static friction changes to kinetic friction.
Vocabulary
- Static friction
- The friction force that prevents two surfaces from starting to slide relative to each other.
- Kinetic friction
- The friction force that acts between surfaces that are already sliding past each other.
- Coefficient of friction
- A number that describes how strongly two surfaces resist sliding, written as mu.
- Normal force
- The support force exerted by a surface on an object, acting perpendicular to the surface.
- Impending motion
- The condition in which an object is just about to start moving but is still at rest.
Common Mistakes to Avoid
- Assuming static friction always equals mu_s N, which is wrong because static friction can be any value from zero up to its maximum depending on the applied force.
- Using kinetic friction before the object starts sliding, which is wrong because kinetic friction applies only after relative motion begins.
- Forgetting that friction opposes relative motion or attempted motion, which is wrong because friction does not always point opposite the applied force.
- Treating the coefficient of friction as a force, which is wrong because mu is unitless and must be multiplied by the normal force to find friction.
Practice Questions
- 1 A 12 kg box rests on a horizontal floor with mu_s = 0.50. Find the maximum static friction force if g = 9.8 m/s^2.
- 2 A 15 kg crate slides across a floor with mu_k = 0.30. What is the kinetic friction force if g = 9.8 m/s^2?
- 3 A student pushes a heavy box gently and it does not move, but when pushed harder it starts sliding and then seems easier to keep moving. Explain this using static and kinetic friction.