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Homework Help: Normal force of weight on the ground

  1. Aug 16, 2012 #1
    1. The problem statement, all variables and given/known data[STRIKE][/STRIKE]
    If we are simply standing on the ground will our normal force be always equal to our weight besides when we are jumping or when we land on the ground from a fall?
    When we are in an elevator, will the normal downwards force on the floor be of our weight (cancelling out our the extra force acted to allow us the gain an acceleration?
    Eg I am 10kg and I move up with acceleration of 5m/s^2. Net force=upwards-normal downwards force=upwards-100N=50N. So upwards force is 150N and hence normal downwards force is 150N as well? In other words, the normal downwards force is always equal to weight?

    2. Relevant equations

    3. The attempt at a solution
    I think so, because if we look at the free body diagram (FBD) of me. I have 100N of force acting downwards (weight) but since I have a net force of 50N, so I need 150N of upwards force to counter the 100N of force. So in essence, the normal downwards force can be evaluated by the object's (in this case myself) motion? So for another example, when jumping I experience 10N net force, so my normal downwards force will be my weight plus the additional 10N of force. Totalling to 110N? Thanks for the help!
  2. jcsd
  3. Aug 16, 2012 #2
    Your logic is correct. When you accelerate upward, the floor gets your normal weight plus ma. You could verify all of that with a bathroom scale. When jumping off it, though, be gentle, because the acceleration during a jump is rather big, but brief. Having a video camera capturing the indicator of the scale would be helpful.
  4. Aug 16, 2012 #3


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    On flat ground yes, but if you are on an inclined slope, then your normal force will balance some component of gravity. When you land on your feet onto flat ground from a fall, you experience a relatively large stopping force upwards from the ground. If you could measure this with sensitive equipment, your weight would appear to increase.
    Last edited: Aug 16, 2012
  5. Aug 19, 2012 #4
    Thanks, I guess we can just use the forces shown on our own body to answer the forces acting on the stuff below us if the plane of contact is the ground eg jumping. Is this right? Thanks again :)
  6. Aug 19, 2012 #5


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    For clarification, the upwards force on your body from the elevator floor (the normal force) is 150 N, and the downwards force on the elevator floor from your body is also 150 N , from Newton's 3rd Law. So why do you say the the normal downwards force is always equal to your weight? Your weight is only 100N. The 150 N is sometimes called your 'apparent' weight, but your true weight is still 100N.
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