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Homework Help: Every day examples of Newton's 3rd Law?

  1. Jan 16, 2009 #1
    Hello all
    Is it possible to provide examples of life (I mean things that we have had a day) as a kind of applications to the Newton's third Law
    with all my thanking
    Last edited by a moderator: Jan 16, 2009
  2. jcsd
  3. Jan 16, 2009 #2


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    Jumping...you exert a force on the ground, the ground exerts a force on you which causes you to move upwards.
  4. Jan 16, 2009 #3


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    You can begin walking across the floor because you push on the floor and the floor pushes back on you.

    An airplane pushes back on the air and the air pushes forward on the plane.

    Jump off a skateboard - it is pushed one way and you are pushed the other way.
  5. Jan 16, 2009 #4
    Try punching someone in the face, did it hurt your hand? (note not suggesting this seriously I'm not a psycho). :smile:

    Now try punching the wall, who/what came off worse? Why?
  6. Jan 16, 2009 #5


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    Nasty! I would say this one has more to do with impulse - the delta t is so small that the F will be large.

    As an example of the 3rd law, better to ask "what stopped your hand?"
    Or lean on the wall and ask what stopped you from falling over.
  7. Jan 16, 2009 #6
    Very true but I wasn't being serious as you no doubt knew? :biggrin:

    What stopped it: electromagnetism perhaps?

    My hand is neutral and so is the wall.
  8. Jan 16, 2009 #7


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    The wall pushed back on the hand . . . because the hand pushed on the wall.

    Interesting that you mention electrical forces. How exactly does one object push on another through contact? Apparently it DOES all come down to electrical forces between molecules in the microscopic view. It might even be correct to say your electrons repel the electrons in the surface of the wall. But the main thing is, its electrons in turn push back on yours!
  9. Jan 16, 2009 #8
    And that goes to show Newtons laws apply classically and microscopically.

    If my hand wasn't neutral and the wall wasn't then I'd bond with the wall and that would not be pretty. :smile:
  10. Jan 17, 2009 #9
    Thank you very much for all
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  11. Jan 26, 2009 #10
    If I am not mistaken, you are new to english. And you wish to convey the following:
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    And if they dont prefer that you stay with us at PF, you would leave.

    And here is ur answer. You are welcome. The language of physics overlooks all barriers. I think everyone else will also welcome you.
    Have a nice time.
  12. May 11, 2010 #11


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    This example really needs expanding to explain the everyday application.

    When standing on the ground, you exert a force on the ground [down] and the ground exerts a force [up] on you. [that's one force on the ground, one force on you]
    But that's not all.
    Through the effect we call gravity, the Earth is attracting you [down] and you are attracting the Earth [up]. [that's a second force on you and a second force on the Earth]

    In that case all those forces have the same size, so both you and the Earth have a net force of zero - neither you nor the Earth moves.

    If you alter your stance so that you are in a squat position, it is possible for you to activate your leg muscles and exert a greater force on the Earth [down] than in the case above. The Earth will of course exert a greater force [up] on you.

    Gravity has not changed, so the attracting forces mentioned above have not got any bigger.

    The force attracting you down is now smaller than the force pushing you up, so the UP force wins - you accelerate up.

    Note: By the time your legs are straight, you will have gained some upward velocity and will thus continue to rise with your feet leaving the ground - you jump!!
    Note Also: Once your feet leave the ground there is no upward force acting on you, so the remaining force (the attraction force [down] due to gravity) is now unopposed and first stops you rising [after a short distance] then brings you back to earth - where balance can again be achieved.

  13. Jul 27, 2011 #12
    I have a question. It might be a stupid question. Please do not mind as I am not so good at Physics.

    Why does Newton's Third Law does not say about angle?

    For instance consider a ball thrown to the wall with an angle, it come back, but makes an angle. It is not necessary that ball come back with the same angle everytime we throw the ball against the wall.
  14. Jul 27, 2011 #13


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    Newton's 3rd doesn't say that the ball should bounce at any angle in particular, all it says is that the force exerted on the wall by the ball is equal in magnitude and opposite in direction as the force exerted on the ball by the wall.
  15. Jul 27, 2011 #14
    So you mean it is limited just to force and cannot predit any angle what so ever?

    But I heard that coming to Nano Technology, Newton's Laws don't work is it True?
  16. Jul 27, 2011 #15


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    The direction of force on a moving object, and the direction the moving object travels are often different.
    When an object travels in a circle at constant speed, the direction of travel is tangential at any instant, but the force is radial - IN towards the centre.
    When the ball is thrown at a wall, on contact a force occurs meaning the "towards the wall" component of the balls motion will be changed to an " away from the wall" component.
    There may also be a "parallel to the wall" component of the ball's motion. Depending on the friction between the ball and the wall, this may also be entirely changed, slightly changed or not changed at all.
    It is the interaction of the changes to the two components that determines the angle at which the ball will bounce of the wall.

  17. Jul 27, 2011 #16


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    Don't forget that any force has magnitude and direction - it is a vector. It is just that the force causes the object to accelerate in the direction of the force, but if it already has some velocity, the acceleration won't necessarily cause the object to reverse its direction of travel.

    When you throw an object into the air [but not fast enough to create a significant air resistance to complicate things], the force acting on that object during "flight" is vertically down. The object does not [immediately] move vertically down. If that were the case, a basket baller could never score from the free-throw-line, and a golfer could never get the ball off the end of the Tee.

  18. Jul 27, 2011 #17


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    3rd law does say the two forces are exactly opposite in direction, so the force on object A is 180 degrees from the force on B. When a ball hits a wall, the wall pushes only perpendicular to it's surface and the angle of bounce can be determined from that. If the ball is spinning that will also have an effect on the angle.

    Newton's laws give way to quantum mechanics as things get very small. But the laws are good for anything larger than an atom in most circumstances.
  19. Jul 27, 2011 #18


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    Force is a vector - it has magnitude and direction so Newton's law does say something about direction - but only the direction of the force, not the direction of the motion resulting. Clearly for a ball to collide with a wall, it is already moving. Its direction of motion will change after the contact force has acted.
    That contact force can be considered as two components: the normal [ie at right angles to the wall] reaction force and the friction force parallel to the wall. It is the mystery of the relative size of those two components that leads to variation in bounce. If you can throw a ball with and without spin, you will see the affect of having different values of friction while having the same reaction force. But, you would have to be EXTREMELY skillful to keep the reaction force the same with each throw, and the angle of approach the same - but you can at least imagine those cases. [Physics = Natural Philosophy]

    [I am assuming that when you say "it come back" you are meaning "it bounces off"]

    As outlined above, just as it is probably not possible for us to throw the ball with exactly the same direction and spin each time, so there will be variations in the direction of bounce. If the wall is not perfectly smooth and flat, and the ball is not smooth and spherical you will get variations as well.

  20. Jul 27, 2011 #19


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    Newton's laws are derived for "our" world. We are unlikely to find ourselves in the sub-atomic world, nor traveling at extremely high speed, so Newton's laws serve us well.

    The few people who end up working with the sub-atomic world, or traveling very fast [in a space capsule?] can learn to adapt.

    The rest of us [most of the world] can be happy to use Newton's laws to predict "what happens next".

    I introduce/summarise Newton's Laws to my High School students more in plain language.

    1: What happens if no Force acts? [an object will continue ...etc]

    2: What happens if a force does act? [ eventually quantify as F = ma]

    3: Well how do you get a force anyway? [Forces result from an interaction between two bodies - they come in pairs, each equal in magnitude, opposite in direction and acting on opposite bodies]
  21. Jul 27, 2011 #20
    Thank you PeterO, Delphi51. That was great Explaination :approve: I appreciate it :smile:

    Actually it clears my doubt 99.9%. As Peter said "It does not say about motion resulting."

    Probably he never bothered about the resulting motion.

    Thanks for your time guys :smile: I will come up with more questions.

    Thanks to Disconnected for replying.
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