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Homework Help: Roller Coaster physics- Newton's Law of motions

  1. Aug 18, 2008 #1
    1. The problem statement, all variables and given/known data
    The Following text is taken from a roller coaster webpage:

    "You reach the top of the hill, and finally realize that being in the front seat wasn't the best idea. You find yourself hanging over the ledge for what seems eternity, and finally you feel the foce of the back cars push you down the steep first drop into the most terrifying two and a half minutes of your life."

    What wrong physics do you see in this text?

    3. The attempt at a solution

    well one mistake is that you feel the force of gravity not the force of the back cars. all the carts are connected and gravity applies equal force on them. So the person sitting on the front will feel the force of gravity applied on him/her and since gravity pulls all object irrelevant of size and shape, the person cannot feel the weight of the other carts.

    Please correct me if i am wrong

    Thank You
  2. jcsd
  3. Aug 19, 2008 #2


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    Homework Helper

    You are correct in that you will not "feel the force of the back cars" (whatever that's intended to mean). In fact, you don't even feel the force of gravity acting on you. Since you, the car you're in, and the rest of the train of cars are accelerating together down the slope, there is no net force on you from any of the cars: you are nearly in "free-fall".

    The reason you feel something in that situation is because of the way the vestibular canals in your inner ears (your sort-of onboard accelerometer) respond to free-fall. You are normally "adjusted" to the sensation of a normal force pushing up on you from the ground and the way the fluid pressure in your v-canals behaves in one-gee. When those physical effects are removed for more than perhaps a second, the signals are interpreted by the brain (either from "adaptation" or experience) as abnormal, leading to the unsettling sensations one has when falling for long. (For orbiting astronauts in "microgravity", these sensations actually cause nausea and motion sickness in at least half* of all those sent into space, and long-term physiological changes due to the brain-body response to lack of "body weight".)

    *the percentage is very large, but not precisely known: many astronauts in the history of space flight were reluctant to admit to having been "space-sick"

    What you do tend to feel on a roller coaster are the effects of air resistance and irregularities in accelerations owing to the rails the cars ride on being less than perfectly smooth and even.
  4. Aug 19, 2008 #3
    thx dynamics

    really cleared my doubts
  5. Aug 19, 2008 #4
    But how does the roller coaster link with Newton's law of motion?
  6. Aug 19, 2008 #5


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    I'm not entirely clear on the intent of the problem, given what the question is. I think what they might want you to consider is the Second Law. The net force acting on you is Mg while you're in free-fall, which can be accounted for by the action of gravity (force of attraction from mass of Earth). So there is no additional force involved from something like, say, the rear cars of the train pushing on you (it's not even clear how they would do that).
  7. Aug 19, 2008 #6
    Mg is mass x gravity rite?
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