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A block sits still on a bench while nobody pushes it. What is the friction on it?
AKinetic friction at full value
BStatic friction, and it is zero
CKinetic friction pointing upward
DStatic friction at its maximum
Answer & Solution
Correct answer: B. Static friction, and it is zero
1. Static friction is the type that acts when surfaces are not sliding.
2. It is a responding force, taking whatever value is needed to balance the push applied.
3. With no push applied there is nothing to balance, so the friction is zero.
4. Its maximum value only matters at the instant the block is about to slip.
_Source: OpenStax College Physics (CC BY 4.0), Ch 5 "Further Applications of Newton's Laws: Friction, Drag, and Elasticity", section Kinetic Friction_
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