University of North Carolina at Chapel Hill
Normal Forces - Overview


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Video Transcript

welcome to our first example Video Will investigating normal forces. In this video, we'll consider one of the most common problems in introductory physics, which involves you riding in an elevator with a scale beneath your feet. Okay, so we did this before in a previous video. So let's review really quickly when we draw our free body diagram for you. There are two forces on you. There's the force due to gravity. And then there's the force due to the scale, which is actually a normal force due to the scale. Okay. And that is the force essentially, that the scale is reading. Now this is going to be influenced by the motion of the elevator in particular. If it is accelerating up or accelerating down, then we will need to take that into account. Because when we write this, we say okay, have to forces fn minus F G. Because F G is going down. FN is going up. That's going to be equal to the mass of you times your acceleration which will match the acceleration of the elevator. Hopefully. So, looking at this, then if I wanted to solve for the force of the scale, the normal force here that would be equal to F G plus mass times acceleration. If we're accelerating up notice, that means the force of the scale will actually be greater than your normal weight than your normal F g here. But if we're accelerating down than the normal force of the scale on you or the force of the scale, that is, reading is going to be less than because we'll have a negative second term instead of a positive second term. So the right hand side will be smaller than your normal weight. So we could also rewrite this because F G is equal to M G. We could write. This is equal to mass time.

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