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A crate of mass m rests on a level surface that is not accelerating. What is the magnitude of the normal force on it?
AEqual to zero, since the crate does not move
BEqual to half of mg for a level surface
CEqual to mg, the weight of the crate
DEqual to twice mg for any level surface
Answer & Solution
Correct answer: C. Equal to mg, the weight of the crate
1. The crate is stationary, so the net external force on it is zero.
2. Only two vertical forces act, the weight downward and the normal force upward.
3. For those to cancel, the normal force must equal the weight in magnitude.
4. On a non-accelerating horizontal surface the normal force is therefore mg.
5. A zero normal force would leave the weight unbalanced and the crate would fall.
_Source: OpenStax College Physics (CC BY 4.0), Ch 4 "Dynamics: Force and Newton's Laws of Motion", section 4.5 Normal, Tension, and Other Examples of Forces_
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