Friction: Contact Forces, Models, Inclines, and Microscopic Origins

All flashcards

What is friction?

Friction is a contact force that opposes relative motion or attempted relative motion between systems in contact.

What is static friction?

Static friction acts between contacting surfaces that are not sliding relative to one another. It adjusts to resist a tendency to slide, up to a maximum.

What is kinetic friction?

Kinetic friction acts between contacting surfaces that are sliding relative to one another and opposes their relative sliding.

How is sliding friction oriented?

It acts parallel to the contact surfaces, opposite the direction of relative sliding.

How does friction help a person walk?

Static friction between the shoe and ground provides grip, allowing the person to push against the ground without the shoe slipping.

Quiz questions

1. A 50.0-kg crate accelerates with a truck at $2.00\,\text{m/s}^2$ on a level bed without slipping. What horizontal static-friction force acts on the crate?Answer: $100\,\text{N}$ in the direction of the truck's acceleration.
2. An object slides downhill on an incline of angle $\theta$, with kinetic-friction coefficient $\mu_k$. Taking downhill as positive, what is its acceleration under the simple friction model?Answer: $g(\sin\theta-\mu_k\cos\theta)$
3. An object slides down an incline at constant speed. If gravity is the only force component driving it along the slope, which relation gives its kinetic-friction coefficient?Answer: $\mu_k=\tan\theta$
4. Why can friction often increase when two surfaces are pressed together more strongly?Answer: The stronger pressing can increase the real area of microscopic contact.
5. Which statement best describes the simple empirical friction equations?Answer: They are useful approximations that may be inaccurate for lubricated surfaces or high-speed sliding.