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Two surfaces are in contact and sliding past one another right now. What is the friction between them called?

AStatic friction
BKinetic friction
CRolling friction
DNormal friction
Answer & Solution
Correct answer: B. Kinetic friction
1. The source names friction between surfaces already sliding relative to each other. 2. When two surfaces are in contact and moving relative to one another, that friction is called kinetic friction. 3. Static friction instead applies when the surfaces are not yet moving relative to each other. 4. Rolling friction and normal friction are not the terms the source uses for this sliding case. _Source: OpenStax College Physics (CC BY 4.0), Ch 5 "Further Applications of Newton's Laws: Friction, Drag, and Elasticity", section 5.1 Friction_
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