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Centripetal force

  1. Jan 3, 2007 #1
    Well, I was just browsing through this website:http://www.physicsclassroom.com/mmedia/circmot/rcd.html

    it explains stuff about the centripetal force, but how come for a rollercoaster, you add the normal force and gravity force to come up with the net force or the centripetal force?

    Could you also take the tangential velocities of the cart and subtract V_f by V_i to get the inward acceleration that the cart experiences?

    I know that they want to find the net force, but why don't they consider the tangential veolcities vectors also? Why do they only add the normal force and gravity? Doesn't the rollercoaster cart have a constantly changing velocity also?

    Wait, velocity is not a force right? Are the only forces the cart experiences the norm and gravity? I think I made a mistake in the last paragraph.. I remember from newton's laws that objects do not require a force to cause it to move, is this why there isn't a x component force acting on the cart? Thanks for your help! :smile:
  2. jcsd
  3. Jan 4, 2007 #2


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    Staff: Mentor

    Just remember that F=ma and it's hard to go wrong. When you are on or near the surface of the Earth, then you have to list the force due to the acceleration of gravity. And you then add in any forces that are causing changes in velocity (centripital or otherwise). Does that help clear things up some?
  4. Jan 4, 2007 #3
    Think about the net force in the vertical direction.
    V_f - V_i? That's a velocity.

    Here is an analogy that may help you understand what is going on. Say that you are walking in a circle. You are walking at constant speed. You are accelerating in the radial direction (centripetal acceleration).

    Say that you break into a run, but you're still running into a circle. As you start running faster and faster, you are accelerating tangentially. Once you stop running faster, though, it's back to just radial acceleration.

    The formula for finding centripetal acceleration is [itex]\frac{v^2}{R}[/itex], so you can find the centripetal acceleration from the linear (tangential) velocity.

    Most of this circular motion stuff is just geometry, so if you can understand the geometric arguments everything makes more sense.
    Velocity is not a force.
  5. Jan 4, 2007 #4
    I think I was getting the centripetal acceleration (it's a vector right? when you subtract V_f from V_i and divide it by t)<--[wait of course it is a vector because V is a vector lol] confused with the net force which causes the centripetal acceleration right?

    Does Newton's second law state that an object with an acceleration has to be experiencing a net force in the same direction as the accleration? Yeah I think thats what confused me, the net force compared to the centripetal accelerationg. So what you do to get the centripetal acceleration if you have the net force of an object moving in a circular motion is divide the force by the mass of the object?

    Hmm.. I think this is all making sense now. So if you have two forces, the normal and gravity force, you can determine the net force. Therefore, you can also determine the centripetal acceleration right? By dividing the net force by the mass?? Thanks again! :smile:
  6. Feb 10, 2007 #5
    Centripetel force is the one which acts in the radial direction to a circle
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