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Not able to understand weight on the surface of earth

  1. Jul 28, 2013 #1
    When i am standing on the surface of earth, there are two forces acting on me
    1)the gravitational force towards the center of the earth
    2)normal force radially oppt to gravity

    as the normal force equals gravity, (that is why i dont sink into the ground or start floating in air), so they cancel each other.
    now no net force is acting on me.
    as gravity force is cancelled by normal force, why do we consider weight rather than considering only mass?
  2. jcsd
  3. Jul 28, 2013 #2


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    Science Advisor

    When you stand on a scale, the scale measures the normal force only and records this as your weight (this is why your weight as read on a scale can be different if you are for example in an elevator that is accelerating upwards vs accelerating downwards vs in free fall).
  4. Jul 28, 2013 #3
    So whenever we measure the weight of any object, we just measure the normal force that is been acted on that object.
    it has no direct relation with gravity.
    Now is my statement correct "When an object is experiencing a free fall, it is getting accelerated due to gravitational force and it also has mass, still it is weightless since there is no normal force acting.
    Thus since weight is only a measurement of normal force, it can not give proper idea about (only gravitational force) that is present, but only give a idea about the net force acting on the object.
  5. Jul 28, 2013 #4


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    For example, if I am in an elevator accelerating upwards with acceleration ##a## then ##N - mg = ma \Rightarrow N = m(a + g)## so the normal force is the sum of the gravitational force as well as the ##ma## term due to the net acceleration of the elevator. This ##N## is what is read as my weight. Your statement about free fall and weightlessness is correct.
  6. Jul 28, 2013 #5


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    Staff: Mentor

    Hi RohitRmB. It would be best to avoid the term "normal force" as that is not an applicable term in physics. Weight is the force attributable to gravity acting on a mass. If there is no force opposing your weight, then you accelerate in free fall. If you are held in a fixed position (e.g., by the platform on a set of bathroom scales), then the dial on the scales registers the force it's exerting to hold you up. If there is a force partially opposing your weight, such as air resistance, then your acceleration towards the centre of the earth is slowed because of the smaller nett force to accelerate your mass.

    There is an equation that defines weight: weight = m x g
  7. Jul 28, 2013 #6


    Staff: Mentor

    Sometimes the term "apparent weight" refers to the weight that would be measured by a scale, i.e. the normal force and "weight" refers to the force of gravity on the object. Using this terminology, an astronaut in the international space station has an "apparent weight" which is 0, but the astronaut's "weight" is about 88% of his/her weight on earth.
    Last edited: Jul 28, 2013
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