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How does a body have zero weight in free fall?

  1. Jul 8, 2009 #1
    I might sound like an idiot asking this question, but, how does a body have zero weight in free fall??
     
  2. jcsd
  3. Jul 8, 2009 #2
    I don't get your question completely. What do you mean with a "zero weight"???
     
  4. Jul 8, 2009 #3
    don't people say that a body's wight is equal to zero during free fall?
     
  5. Jul 8, 2009 #4

    Doc Al

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    It doesn't. The true weight--the gravitational force that the earth exerts on the object--remains. What is zero is the apparent weight, which is the magnitude of the contact force supporting an object. That's what is meant by the term "weightless"--the apparent weight is zero, not the "true" weight.

    When standing on the ground, both your true and apparent weight equal mg. When you jump off the roof, your true weight is mg while your apparent weight goes to zero (ignoring air resistance).
     
  6. Jul 8, 2009 #5
    Classically speaking, we've defined weight as a force. It happens to be a force that is applied to every particle of an object. So if the object is sitting on the ground, there is a force applied to the ground which is the sum of the forces on each particle, and some strain is noticed in the object (if its your body, you feel that strain). If the object is in free fall, the force is still there but it isn't felt by any strain in the object because it is applied to every particle of the object.
     
  7. Jul 8, 2009 #6
    When a body sets in motion inside a fluid(like atmosphere, water) the viscous force of the fluid which retards its own motion starts retarding the motion of the body until all the force the body exerts on the fluid(like gravitational force) becomes zero so the body moves with a constant velocity called terminal velocity.It is commonly applied to falling under gravity but then it is not called"free fall under gravity" because the body does not accelerate under gravity.
     
  8. Jul 8, 2009 #7
    If you stand on the ground (or whatever), you exert a force onto the ground, but the ground also exert an exact force (opposite) to your body through the feet. So there must be strains within your body. You feel that. In free fall, there no strain inside the body, all the parts are imposed the same force.
    I myself experienced once a free fall for about 15 sec when the 747 plunged into a turbulence. Very scared !
     
  9. Jul 9, 2009 #8
    I kind of get it... kind of....
    I understand that the apparent weight becomes zero, but where is the contact force? And how does the apparent weight (which is a downward force) become equal to the contact force (which is acting in the upward direction)?
    If the body is moving under free fall, we neglect air friction, right? So how will it ever reach a terminal velocity, when there is no force of friction to deccelerate the body?
    Also, what do you mean by 'no strain in the body'? What relation does it have with the weight of the body. Does having 'no strain on your body' mean that the net of all the forces acting on you are equal to zero?
     
  10. Jul 9, 2009 #9
    The apparent weight becomes equal to contact force by newton's third law, and I had a great discussion on it yesterday.
    If the air is stationary (no wind) according to a non-moving observer on the earth then we do not neglect air friction because it is the viscous force slowing the person down.
    If there are forces acting in opposite directions on the same body it feels strain.So you did an exact mistake.No strain simply means the body does not exert any force on anything(and of course nothing exerts forces on the body by newton's third law)
     
  11. Jul 9, 2009 #10

    Doc Al

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    What we call "apparent weight" is just a measure of the support force. There's no separate downward force on the body, other than its true weight. The contact force is produced by whatever is supporting the body. It's this contact force which creates the stresses in your body that gives you the feeling of having weight--when you remove that support, and thus the stresses, you feel "weightless".
    We often ignore air resistance when talking about free fall, but obviously you cannot ignore it if you want to talk about terminal velocity.
    As I said above (and others have explained) it is the contact force that produces the strain in the body (gravity alone acts uniformly and thus produces no stresses). You feel such stresses, for example, when you are sitting in your chair--but the net force on you is zero. When you are in free fall (the only force acting on you being gravity) you experience no stresses and the net force on you is your weight.
     
  12. Jul 9, 2009 #11
    Yes, we ignore air resistance when talking about free fall because it does not make a difference, right?
     
  13. Jul 9, 2009 #12
    Wow, thanks!
    You really simplified the whole concept about the 'no stress on the body'. If a body has no normal force acting on it (or any two opposite forces), it feels no stress.
    But, during free fall you say that the only force acting on the body is its weight, yet, its apparent weight is zero... how is that so?- there is no opposite force acting on it, because it feels no stresses...?
     
  14. Jul 9, 2009 #13

    Hootenanny

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    Free-fall is defined as motion without any external force except weight. Therefore, free-fall on earth is not technically possible.

    I am unsure of what you mean by "does not make a difference". For relatively small objects undergoing low and slow falls, drag will not have a significant impact. However, if one say jumps out of an aeroplane, then drag certainly plays an important role.
     
  15. Jul 9, 2009 #14

    Hootenanny

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    Imagine you are standing on a bathroom scale whilst in free-fall. Both you are the scales are accelerating at the same rate, what would the reading be on the scales?
     
  16. Jul 9, 2009 #15
    So it does make a difference and is ignored because it would not be the typical free fall. Then I must say skydiving is misinterpreted as free fall and statements like "free-fallers reach their terminal velocity when.." are wrong.
    That's all right.
     
  17. Jul 9, 2009 #16

    Doc Al

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    The apparent weight is zero because there is no supporting force. As Hootenanny points out, your apparent weight is what a bathroom scale would read.
     
  18. Jul 9, 2009 #17

    Doc Al

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    Yes, in physics the term "free fall" means (by definition, as Hoot says) that the only force acting is gravity. In everyday usage, "free fall" has a less strict meaning. If you wish to consider the effect of air resistance, then it's no longer "free fall".
     
  19. Jul 9, 2009 #18
    Considered in the traditional inappropriate way, when on the earth, a person sees everything on the earth to be stationary and an object in the air freely falling to the earth.
    When the person is himself falling with some of his belongings, he sees himself and the objects to be stationary and the earth with all its "gravitational belongings" to be freely falling towards itself but the definition of free fall doesn't go with that.
    Weight can be defined as the force with which the earth pulls objects or from the object's reference, the force with which it pulls the earth towards itself. Earth's gravity or the object's gravity, it's the same. The moon has lesser mass, so the same object attracts the moon towards itself witth lesser foce, by the same newton's equation.
    I have the answer myself,right.
     
    Last edited: Jul 9, 2009
  20. Jul 10, 2009 #19
    The concept of the scale accelerating along with you at the really clarified things up.
    Thanks
     
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