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Homework Help: How to find Normal Force?

  1. Jun 16, 2010 #1
    I know that normal force usually equals mg on a flat surface. but how do i find Fn on a slanted surface? And also, how do i make superscripts and subscripts with alt codes (windows 7)?

    Also, is ma=mg

    i know that a=acceleration, and that's usually because of gravity. But in the book i'm reading (physics for dummies), it gives me a long example for finding vectors and comes to the conclusion that:

    a=g sin [tex]\theta[/tex]

    this is one of my first posts, cool website, im thankful i joined. thank you for the help. One last thing, I will probably be having more problems like this involving what im reading from my book. I was wondering if there was someone (you) that i could just email directly about problems (just for the sake of saving time).
  2. jcsd
  3. Jun 17, 2010 #2
    What is the objects acceleration? If the acceleration is zero then the sum of the forces acting on the object is zero, and therefore the sum of the horizontal and vertical components of the forces are 0, so you can split each force into horizontal and vertical components using trigonometry and then use these to find the Normal Force.

    F=ma only applies to the resultant force, ie the sum of all the forces acting on an object, which would include the weight, which is equal to mg.

    The example given by the book, where a=g sin [tex]\theta[/tex] sounds like a projectile question, where the object is travelling at some constant horizontal velocity, whilst it is falling, and [tex]\theta[/tex] is the angle between the objects motion and the horizontal plane.

    Generally a is the acceleration due to the resultant force, this may or may not be due to gravity.
    Last edited: Jun 17, 2010
  4. Jun 17, 2010 #3

    Doc Al

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    Staff: Mentor

    ma = mg only when the only force acting on the object is gravity--if it's in free fall. But an object sliding down a ramp is not in free fall--the ramp exerts a normal force on the object.

    To find the normal force, analyze force components perpendicular to the ramp. You know the acceleration in that direction must be zero, so the net force perpendicular to the ramp must be zero.

    The weight can be separated into two components: one component parallel to the ramp; another component perpendicular to the ramp. The normal force balances out the component of the weight perpendicular to the ramp, so all you're left with is the component of gravity down the ramp. That happens to give you a net force of mg sinθ, thus the acceleration is g sinθ.
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