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About Newts First Law of motion

  1. Apr 21, 2003 #1
    About Newts First Law of motion....

    U kno the first law of motion..."Object has a netforce of zero when its at rest or moving at constant velocity..." I dont get how you could say an object has a total netforce of zero when its moving...If its moving in one direction then isnt a force acting on it to go in that direction?
  2. jcsd
  3. Apr 21, 2003 #2
    Um ... not exactly.
    This is something that find hard to understand at first, but eventually it is understandable.

    First you have to understand that this is a law of nature, so we cannot really say 'why', we only put laws to understand nature.

    What may seem illogical for you now is that you need a force in order to make an object moving in a constant velocity.
    Well, you do not need the force to maintain its speed, as long as the net (or resultant) force is zero.

    In our everyday life experience, there is always friction and air drag acting on moving objects, therefore we always need a force to make an object move at a constant velocity.
    What this force that we provide actually does is that it works against the resistance forces (friction and air drag and maybe other forces) to make the net force zero, which will make the object move at a constant velocity.

    Tell us if you still feel there is something you don't understand.

    I hope i helped.
  4. Apr 21, 2003 #3
    I think I understand a little better, its just hard to imagine a object in a vacuum I guess...
  5. Apr 21, 2003 #4


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    Consider that a tiny moving object has two properties because of its motion. It has a speed and a direction. Now Newton saw that neither one of these properties could just change without some cause acting. The kind of causes that act to change the speed or direction (or both) of a moving object are called Forces.

    An ordinary object moving through the air will feel two kinds of forces: the air pressure which tends to decrease its speed in the direction of motion, and gravity which continually adds a certain speed change in the down direction.
  6. Apr 21, 2003 #5
    Re: About Newts First Law of motion....

    Dock, Please restrict your misinformation to the Theory Development fourm Integral
    Last edited by a moderator: Apr 21, 2003
  7. Apr 21, 2003 #6
    Re: About Newts First Law of motion....

    Hi PiRsq,
    well the fact that you ask this question means IMO that you really look at physics from its very birth-place. Yes, for thousands of years it was generally accepted that a force is needed to keep a body in motion. Everyday experience says so, and even the scholars (e.g. Aristotle) wrote this down in their doctrines. But astronomers, since long, have known that there is 'eternal' motion in the sky, as exhibited by the Sun, Moon, and planets circling about. IIRC, even Copernicus and Kepler (who laid the foundations of Physics) assumed that the planets are 'brushed' around the sun by quasi-mechanical devices impeding some tangential force on them (=pushing them). It was Newton who postulated that there is no such pushing force, just gravitational attraction. He postulated that the only force acting on a planet (if you neglect other planets) is directed towards the Sun but not in the direction of the planet's motion (well, for a circular orbit...). This means the planets go on forever without anything actually pushing them. That is maybe the sort of 'motion in vacuum' which you (among many people) find so hard to imagine. Hope this helps a bit :wink:
  8. Apr 21, 2003 #7
    Example: say you're flying in a plane. You toss a ball up in the air. It acts normally -- goes up and comes straight down. If it needed a force to keep it moving, it ought to stop moving forward with the plane as soon as it left your hand, and get slammed backwards. Obviously this doesn't happen. :)
  9. Apr 21, 2003 #8


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    This is a post made by FZ+ which I erronously deleted in an effort ot clean out the post he is reffering to. Sorry FZ!

    It is important to know that force is defined in physics as being the rate of change of momentum, which is m*v. So, if the force is zero, then all it means is that m*v is constant. Further, reality is based not on absolute motion, but on relative motion. So, it is irrelevant without a reference point whether you are at rest or moving at steady velocity. We usually imply relative to the earth when we say this.
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