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Stop and Go Friction
Static vs Kinetic Friction
Related Labs
Friction is the force that resists motion between surfaces in contact, and it plays a major role in everyday actions like walking, braking, and pushing furniture. In stop and go situations, friction does not behave the same way all the time. An object at rest can resist a growing pull without moving, but once it starts sliding the friction usually changes. Understanding this difference helps students predict when motion begins and how objects move afterward.
Static friction acts when two surfaces are not sliding relative to each other, and it adjusts up to a maximum value to prevent motion. Kinetic friction acts once sliding begins, and its magnitude is often smaller than the maximum static friction. This is why an object can be hard to start moving but easier to keep moving. The basic friction model uses the normal force and coefficients of friction to describe both cases.
Key Facts
- Static friction satisfies 0 <= fs <= fs,max
- Maximum static friction: fs,max = musN
- Kinetic friction: fk = mukN
- Static friction matches the applied force until fs,max is reached
- Motion begins when Fapplied > fs,max
- For horizontal motion, N = mg if there are no other vertical forces
Vocabulary
- Static friction
- The friction force that prevents relative motion between surfaces that are in contact and not sliding.
- Kinetic friction
- The friction force that acts between surfaces that are sliding past each other.
- Normal force
- The support force exerted by a surface on an object in contact with it, acting perpendicular to the surface.
- Coefficient of friction
- A dimensionless number that describes how strongly two surfaces interact through friction.
- Net force
- The total force on an object after all individual forces are added together.
Common Mistakes to Avoid
- Assuming friction always equals muN, which is wrong because static friction can take many values up to its maximum and only kinetic friction is usually modeled as fk = mukN during sliding.
- Using the coefficient of kinetic friction before the object starts moving, which is wrong because the object is still under static friction until the applied force exceeds the maximum static friction.
- Forgetting that friction opposes relative motion or attempted motion, which is wrong because the friction direction must be chosen opposite the sliding or the tendency to slide.
- Setting the normal force equal to weight in every problem, which is wrong because ramps or extra vertical forces can change N from mg.
Practice Questions
- 1 A 12 kg crate rests on a horizontal floor with mus = 0.50 and muk = 0.30. Find the maximum static friction force and the kinetic friction force after the crate starts sliding. Use g = 9.8 m/s^2.
- 2 A 20 kg box on a level surface has mus = 0.40. A horizontal pull of 60 N is applied, then increased to 90 N. For each pull, determine whether the box moves and find the friction force.
- 3 Explain why a heavy box may be difficult to start moving but easier to keep moving once it is already sliding, using the ideas of static friction and kinetic friction.