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Adding a heavy box on top of a crate makes it harder to push. Why?
AThe contact area has been halved
BThe friction coefficient has doubled
CThe crate has become more elastic
DThe normal force has increased
Answer & Solution
Correct answer: D. The normal force has increased
1. Piling mass on the crate presses it more firmly against the floor.
2. That raises the normal force between the crate and the floor.
3. Friction is proportional to the normal force, so the friction rises with it.
4. The coefficient depends on the pair of materials and does not change when load is added.
_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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