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Question about Third Newton Law

  1. Nov 16, 2014 #1
    If I jump, I accelerate away from Earth.
    However, if I drop a human on the ground, he doesn't accelerate away from the Earth.

    In both cases the force used on the Earth is about the same, but acceleration away from the Earth exists in one case and doesn't exist in another case. Why so?

    (not a homework question but one I thought of myself)(English is my 2nd language)
    Last edited: Nov 16, 2014
  2. jcsd
  3. Nov 16, 2014 #2


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    Only while still in contact with the ground, using your muscles.
    It is not.
  4. Nov 16, 2014 #3
    "It is not."
    I understand that they're different. But they're not that much different - both are not negligible and not huge. When you jump, you use your mass + a bit more. When you fall, you use your mass + a bit more.
  5. Nov 17, 2014 #4


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    No, when you jump your feet push earth quite hard (with more than your gravitational force), when you fall your feet do not push earth at all.

    Please do not remove your posts, others might have the same question and be happy to find an old thread about it (also via google).
  6. Nov 21, 2014 #5
    You're right that acceleration exists away from the earth in both cases. But, when you fall, you start out with a negative velocity, so that when you are finished accelerating away from the earth, you end up with a zero velocity. When you jump, you start out with zero velocity, you accelerate away from the earth, and you end up with positive velocity.
  7. Nov 21, 2014 #6


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    Thanks, Lsos. I think that you have decoded the OP correctly. The distinction is not between jumping and falling. It is between jumping and landing.
  8. Nov 21, 2014 #7
    Errr..... you can start with 0 velocity, nothing seems to prevent it. That is 0 velocity downward towards earth's core. 0 velocity is not the same as negative velocity. 0 is non-negative. Sorry to nitpick... I apologize for rudeness.:s

    Imagine if you are Felix Baumgartner and balloon stays at constant altitude.Then you jump out of the balloon, or walk out of the balloon rather. Ground vanishes beneath your feet, when you take step outside the cabin of the balloon. Then begins the air drag and gravity effects. Essentially the lift force from balloon, stops at this point indeed.

    With regards to jumping upwards, from the ground...

    Certainly your own mass stays the same during jumping and standing still, on earth.

    But when you jump upwards, you basically squat down first (bend your knees)... Then your muscles engage and you are able to jump upwards. Does your weight, change?

    Is it simply so that the weight doesn't change? Does there become increased net force towards earth, when you push hard from your bent knees? (increased net force when compared to simply standing still on your legs).

    Increased net force toward earth, would mean that there exist equally great counter-force away from the earth? Certainly it feels like your body has more momentum outward from earth, when you jump with the squatting technique... :eek:

    Weight happens because of gravity, right? Gravity is fundamental force in universe.

    But gravity strength, depends on mass? My mass stays the same, earth mass stays the same, when I jump upwards from earth. ?:)
  9. Nov 21, 2014 #8
    Technically speaking, the following happens because of Newton's third law.

    You accelerate away from earth, AND EARTH accelerate away from YOU.

    Earth acceleration is really small, though. Because earth has massive inertia, it is difficult to move earth from orbit, by humans jumping off from earth.
  10. Nov 21, 2014 #9
    I understood this question right after putting it on this forum and I erased everything in the post (before anyone posted anything) because there is no 'delete post' button on this forum, but for some reason mods decided to recover the post.... just ignore this post....
  11. Nov 21, 2014 #10
    I'm not sure I follow this question clearly. This seems a very simple matter - a human being is not a rigid object. When a person lands after falling, he/she will absorb the impact with their body, so the force is actually spread through the body. But there is still an upward force. The earth will tend to move away from you, and you will tend to move away from the earth. It's just not apparent from how your flexible body reacts. If you replace the human body with a rigid object, say a steel ball of same mass and drop onto a hard surface, the object doesn't just stop. There will most definitely be an acceleration away from the earth. Several times until the force is dissipated. Have I misunderstood the question and the forces involved?
  12. Nov 21, 2014 #11
    Like I said, ignore this post..
  13. Nov 22, 2014 #12


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    I think this thread is done.
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