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Once a heavy crate is finally set into motion after a hard push, why does it become easier to keep sliding than it was to start it moving?
ABecause the normal force disappears once the crate is sliding
BBecause gravity stops acting on the crate once it starts moving
CBecause kinetic friction is weaker than the static friction had been
DBecause the crate becomes lighter as it slides across the floor
Answer & Solution
Correct answer: C. Because kinetic friction is weaker than the static friction had been
1. The source draws this conclusion directly from ordinary experience with pushing a heavy crate.
2. It states that once in motion the crate is easier to keep moving, which indicates that kinetic friction is less than static friction.
3. Gravity and the normal force do not change simply because the object starts sliding, so neither of those options explains the effect.
4. The crate's mass is unchanged by sliding, so it does not become lighter either.
_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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