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Weight -- the very basics

  1. Aug 11, 2015 #1
    Are the following definitions of weight valid;

    The force of gravity pulling on a mass?
    The force between two bodies due to the force of gravity?

    Q. Does weight only apply to the force due to gravity on a body in relation to a planet (or satellite, asteroid etc), eg, a man standing on Earth or on The Moon?
    Or can we talk about the weight of, say, a very small object (eg, a pebble) in relation to a relatively larger object (eg, a person)?
  2. jcsd
  3. Aug 11, 2015 #2
    Is it sensible to talk about "my weight" or your "weight", or even "my weight on The Moon" etc, given that the weight is not mine as such?
  4. Aug 11, 2015 #3
    Given that weight is a vector quantity / measurement should it be represented by an arrow (pointing downwards) as well as a magnitude in N (Newtons)?
  5. Aug 11, 2015 #4
    Does weight only apply to a body in contact with another body, eg, a man standing on Earth? Or does it apply also to the man falling to earth? I'm guessing the latter given that the falling man would be accelerating at 10/m/s^2 ; the reason I'm asking is that I'm wondering if Earth has weight in relation to the Sun. I know that there is a gravitational pull on Earth towards the Sun, or there's gravitational attraction between Earth and the Sun (keeping it from 'flying off' at a right angle, in other words, keeping it in orbit) but I don't know if there is (technically) weight involved here.
  6. Aug 11, 2015 #5
    Yes , it is a vector , and is represented by both magnitude and direction . The arrow you refer to only tells you it is a vector , and doesn't have to point downwards .
    No , it essentially counts as a non - contact force .

    Another point - All objects exert a force of gravity on another , but in most cases , this force is negligible .
    For example , force of gravity on a rock by me might be in the range of 10-9 N , and thus plays a more significant role when heavenly / larger bodies are involved .

    Hope this helps .
    Last edited: Aug 11, 2015
  7. Aug 11, 2015 #6
    Thanks. If weight is represented by direction as well as magnitude why do I only ever see the N (for Newtons) and no arrow in answers to weight questions online? And if the arrow doesn't have to point downwards what would be an example of it pointing otherwise (bodies fall to Earth, or fall to the Moon, ie, they fall 'down', no?)?

    Also; so I pull the Earth towards me as well as it pulling me towards it? If so, is that force, although extremely negligible, technically calculable?
  8. Aug 11, 2015 #7
    To your first point - I meant you would write the force as :
    1d133119bde0ace20718ce274db8bf84.png ( Vector form ) .

    To your second point - Yes , according to Newton's third law .
    Please note , I said it is negligible in many cases , but not when larger bodies like the earth are involved .

    Thus , force between you and the earth is not negligible - look at the formula , F doesn't remain negligible ( Weight of the earth is 5.972 × 10^24 kg ) .

    The force on the earth by man is equal to force on man by the earth = mg , where ' m ' is mass of the man .
  9. Aug 11, 2015 #8
    The word "weight" is usually reserved for the gravitational pull on an object caused by a heavenly body.

  10. Aug 11, 2015 #9
    "The force on the earth by man is equal to force on man by the earth = mg , where ' m ' is mass of the man ."

    But when talking about "my" weight on Earth the 'm' in the equation is my mass; 900N = 90kg (my mass) x 10kg/N.
    Wouldn't talking about Earth's weight in relation to me use, w = m (of Earth) x g (my 'gravitational field strength', ie, my equivalent of Earth's 10N/kg, and therefore make Earth's weight in relation to me different from my weight in relation to it?
  11. Aug 11, 2015 #10
    "The word "weight" is usually reserved for the gravitational pull on an object caused by a heavenly body."

    Thanks. But can it (technically) be used in terms of the relationship between the pull on, say a pebble, by a man?
  12. Aug 11, 2015 #11
    No , the resultant force would be the same in both the cases .

    If you do use -
    Then your gravitational field strength would be Gm/r2 , where m is your mass .

    This would be the same as w = m × Gm1/r2 .

    Hope this helps .
  13. Aug 11, 2015 #12
    Why is the answer to this question so important to you? Do you think you will ever encounter it in practice? As far as I'm concerned, you can call it whatever you want.

  14. Aug 11, 2015 #13
  15. Aug 11, 2015 #14
    Chet, do you have a source for that convention?
    Your "g" is so tiny that the two forces come out equal & opposite.
    Sure, if you like. But it sounds more "normal" if you just talk about force. The name "weight" doesn't add much information.
  16. Aug 11, 2015 #15
    Thanks. It's not that I'm concerned with encountering it in practice, it's more an attempt to understand what weight really means.
  17. Aug 11, 2015 #16
    The earth's mass is 6 x 10^24 kg; is this 6 billion quadrillion kg? Or would we say 6 trillion trillion kg?
  18. Aug 11, 2015 #17
  19. Aug 11, 2015 #18
    No. Just personal experience. If I were writing about the gravitational force that the man exerts on the pebble, I would always refer to it "the gravitational force that the man exerts on the pebble," and not "the weight of the pebble relative to the man."

  20. Aug 11, 2015 #19
    Thanks. Septillion. Nice. I like that. The mass of Earth is a septillion kg.
  21. Aug 11, 2015 #20
    Five septillion, nine hundred and seventy-two sextillion, one hundred and ninety-something quintillion...
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