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Homework Help: What makes us feel heavy?

  1. Feb 11, 2007 #1
    1. The problem statement, all variables and given/known data
    A guy is riding a bike down a hill. He feels "lighter" when he accelerates than when he's braking. Why?

    3. The attempt at a solution
    I thought that a higher normal reaction force than weight was responsible for feeling "light". But this can't be right. Is it something to do with the fact that less friction is acting on him when he's not braking?

    Thanks :smile: I just need a really *basic* answer because the physics we're doing at the moment is just very basic.
    Last edited: Feb 11, 2007
  2. jcsd
  3. Feb 11, 2007 #2


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    You are correct in thinking about reaction force. There are two things you need to consider here - apparent weight (which we feel) and actual weight. The actual weight is the downward force of gravity. Usually apparent weight and actual weight are the same, but when we are accelerating or some other force other than gravity and normal force is acting on us, the two are different. Also since this is crudely similar to free-fall (for which we feel no apparent weight). Can you now tell why he feels lighter?
    Last edited: Feb 11, 2007
  4. Feb 11, 2007 #3


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    You are in the right direction although you have it backward. A normal force higher than the weight means that the person is feeling heavier than normal (and vice versa). Now, with a simple free body diagram it should be clear why the person feels heavier when braking (in what direction is the acceleration? That will be the direction of the *net* force, which will tell you something about the normal force exterted by the seat on the person).
  5. Feb 11, 2007 #4
    Ah so is it because when he's braking, there is friction from the bike acting up the slope, which has a vertical component that adds to normal reaction force to give a higher upward force? Sorry, I know that's very wordy.

    I hope I have this right :biggrin:
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