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What are the components of an applied force?

  1. Mar 12, 2016 #1
    Something said in my physics textbook really confused me.
    The following was written : "Applied Force is our label for a contact force that a person exerts. When an applied force acts at an angle, it is actually a combination of two forces : normal and friction. The component of the applied force that is perpendicular to the surface is a normal force, and the component parallel to the surface is a friction force."

    I always thought an applied force acting at an angle was made up of Fa[y] and Fa[x] for the vertical and horizontal components respectively. I questioned my teacher about this and he said that the forces of friction and normal are between the surface of the object and the hand, not between the object and the ground, however that didn't help much at all. Could anyone provide some clarification with regards to the statement above please? Thank you.
     
  2. jcsd
  3. Mar 12, 2016 #2
    Consider this example - pushing a book kept on a table, at an angle, say, ##θ## with the vertical. You are now pushing the book against the table, ##and## parallel to the table. For simplicity, let us assume that there exists no friction between the book and the table, and friction can exist beteen the book and the man, only.

    Suppose the book moves horizontally. Which force is responsible for this?
    How is this force generated? Do you get it now?
     
  4. Mar 12, 2016 #3

    haruspex

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    Your confusion comes from intent versus consequence.
    Consider an extreme case of your thought experiment, a massless block jammed between two frictionless plates. The plates exert a normal force on the block. Now you intend to apply a force on the block parallel to the plates. But since there is no friction and the block is massless, there is no resistance. In consequence, you are simply not able to apply a force.
    Now back to the original example. If you try to push down at an angle on a frictionless horizontal surface, you cannot actually apply such a force. The force you apply will be purely normal to the surface.
     
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