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Oiling a concrete floor makes a crate easier to move. What has changed?
AThe mass of the crate has fallen
BThe weight of the crate has fallen
CThe friction between surfaces is less
DThe normal force has become zero
Answer & Solution
Correct answer: C. The friction between surfaces is less
1. Oil changes the nature of the two surfaces in contact, not the crate itself.
2. With oil between them, the friction resisting motion becomes smaller.
3. So a smaller push starts the crate and a smaller push keeps it going.
4. Mass, weight and the normal force are all unchanged by oiling the floor.
_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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