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Which phenomenon correctly describes the direction and magnitude of normal forces?
  1. electromagnetic attraction

  2. electromagnetic repulsion

  3. gravitational attraction

  4. gravitational repulsion

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OpenStax College Physics for AP® Courses Solution, Chapter 4, Problem 29 (Test Prep for AP® Courses) (1:54)

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This is College Physics Answers with Shaun Dychko. Which phenomenon correctly describes the direction and magnitude of normal forces? So a normal force is a force that is perpendicular to a surface and away from a surface. So because it is away from a surface, it's a repulsion so that narrows our possibilities down to only B and D which are repulsions. Now, gravity is always an attractive force, there are no exceptions to that. So in fact any surface and thing on that surface are always attracting each other due to gravity. So that's not what's at work here for normal force. The normal force is a repulsion. So B is going to be our answer and this is the force on the block due to the surface and then there's the Newton's third law counterpart force on the surface due to the block. These are repulsion forces and it's due to electromagnetic forces. So this is on the atomic scale, the same sort of thing t hat keeps the electrons from going into the nucleus. There's a certain -- it's not really -- it's a bit difficult to explain isn't it because there's an attraction between the electrons and the negatively charged and the positively charged nucleus. But there are certain energy states in which the electron likes to stay that keeps it in its orbit and not going into the center. But it's things like on that scale that are causing repulsion between this block and the surface.