normal forces

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V4viet

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hey guys i was wondering if anyone can tell me about normal forces and why is it that in an elevator, when one goes down meaning (accelerating down)..normal forces decrease and when one goes up (accelerating up) normal force increase?
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HippocratesX

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Where did you read that? I don't think think the normal force decreases or increases during acceleration. Although, there can be a force (increasing or decreasing), either up or down, that makes the elevator move, but I believe that the normal force does not change. As long as your feet are touching the ground, your normal force will always be Mg. Correct me if i'm wrong here people?
 

DrPalindrome

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if you're accelerating down, there must be a net force going down. if you're accelerating up, there must be a net force going up.
 

cmm26red

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if you're accelerating down, there must be a net force going down. if you're accelerating up, there must be a net force going up.

It all depends on your frame of reference; I am taking it that you just mean a person inside the elevator. If the elevator is not moving, there are 2 forces acting upon the occupant inside; 1 the gravitational force, which is acting downward. Let’s say you weigh 100 kg, then there is a gravitational force of roughly 980 N acting down. However, there is a normal force equal to this gravitational force acting up. This normal force is being exerted by the floor beneath you. Now lets say the elevator is accelerating downward at 5 m/s^2. The gravitational force will be cut in half (almost); this is why you weigh less in an elevator that is accelerating down. With the frame of reference in mind, you are static; the elevator is accelerating, so the normal force is equal to the gravitational force acting in the opposite direction (up). Since the gravitational force is reduced when the elevator is accelerating down, the normal force is therefore reduced, so in that context you are right.
 
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