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Projectiles (bullets), energy, force

  1. Jul 4, 2003 #1
    Question regarding bullets and such.

    Consider a 4.5kg mass at 20m/s has 900 Joules, 90 Newtons. And a 2kg mass at 30m/s has 900 Joules, 60 Newtons. Perhaps I've got the figures wrong, but it doens't make a difference to my question really.

    On webpages dealing with ballistics, when discussing the bullet's energy, they give the Joules from the kinetic energy formula 1/2m*v^2. Now, it seems to me inappropriate to give the answer in Joules, when discussing what a bullet does. You might as well give it in BTUs or such. It's simply an amount whihc doesn't really describe much. Seems to me it should be given in Newtons.

    Why would they give the Joules rather than Newtons when discussing the matter? Shouldn't Force be used, to show what the bullet is actually doing?
     
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  3. Jul 4, 2003 #2
    Adam,
    it makes no sense to say that an object moving frictionless at constant speed has a certain 'force'.
    I think you arrived at this concept, because you maybe argued like this: "If I put down a stone on the sand, it makes a crater. If I drop it from a height, it makes a bigger crater. So a falling stone is heavier than a resting stone. So the motion gives the stone some etra weight. So motion is force".
    This is pre-scientific, naive thinking (sorry, no offence meant!). Aristotle might have argued like this.
    But: If the bullet hits something soft, it hits with only a small force. But it will travel a long way before it stops in the soft medium. If it hits something hard, it hits with a big force, and stops after a very short distance.
    What is really constant, is the product force*pathlength, and this is called 'work'. Your bullet has the ability to do a certain amount of work, and this is called 'energy'. The unit is the same as that of work, namely 1Newton*1meter, and that's called 1 Joule. OK?
     
  4. Jul 4, 2003 #3
    Yes, I know what a Newton is. But it seems an inappropriate way to describe what the bullet is doing. It says nothing more than that there is, about the object X Joules. Force includes motion. Joules could be anything: it could be a point radiating lots of energy, could be any dang thing. Force, however, gives me a description of mass being moved. I dunno, it just seems more descriptive to me, to use Force.
     
  5. Jul 4, 2003 #4

    ahrkron

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    Adam,

    I think part of the problem comes from the fact that many of the words we use in physics (like force, energy, work, etc.) already have some meaning in everyday live language. For example, the ammount of (physical) work needed to move a big package to a certain height is the same regardless of the path taken (in a frictionless situation), while somebody can argue that "hey, I work much more to lift it directly than if I used an incline"). To some extent, it is a matter of familiarity.

    Also, for a bullet, imagine it is flying. What force is being applied? How does that tell you the potential effect of the bullet?

    As arcnets mentioned, the right quantity to use here is energy, since you will have the same effect with a 90kg bullet at 20 m/s than with a 10kg bullet at 60 m/s (mv2/2 is the same).

    If you want to get a feeling for this, you may want to "translate" various energy values into the speed needed for a 10kg mass to produce the same effect. For example:
    4500J would be the same as 10Kg hitting at 30 m/s,
    18000J will deposit as much energy as 10Kg at 60 m/s.
    Four times as much energy only needs doubling the speed (for the same 10Kg).
    (many more details can affect this, but this is a good first step).
     
    Last edited: Jul 4, 2003
  6. Jul 4, 2003 #5
    So, stating it in Joules gives you an IF answer. An answer which only applies once you ALSO state the speed. Why not use Force then, which incorporates mass and speed?
     
  7. Jul 4, 2003 #6

    ahrkron

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    No, it's not an "IF answer". I was suggesting a way for you to understand the quantity better.

    Tell me, how would you assign a force to a projectile? Say, 10 Kg and 30 m/s. What would be the corresponding force?
     
  8. Jul 4, 2003 #7
    Wouldn't it be 300 Newtons?

    The thing is, if you simply say the bullet has 1000 Joules, or whatever amount, that doesn't tell you anything about rate of energy use. It could be a thousand Joules over two years, in which case it won't matter at all to the target. Maybe Newtons isn't the best form either, but it at least includes motion, by Joules*metres.
     
  9. Jul 5, 2003 #8

    pmb

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    The question/answer is incomplete. What is it you set out to find? If the question was "What is the kinetic energy?" then the answer must be in units of energy. If the question was "What constant force produces a kinetic energy after so many seconds?" then the answer must be in units of force.

    The statement "Consider a 4.5kg mass at 20m/s has 900 Joules, 90 Newtons"

    Doesn't have that much meaning to me.

    Can you state the *exact* question that you were seeking the anwer to? Or tell us where one of those web pages is that you were reading?

    What is useful depends on what you want to find out. Energy is just something that is constant in a conservative system, i.e. a number which remains constant.

    But if you have a particle moving in an inertial frame of referance with no force acting on it then the "force" has nothing to do with it. I.e. a particle moving at so many meters per second has nothing to do with a force.
     
  10. Jul 5, 2003 #9
    You appear to be talking about several different quantities, none of which actually go by the names you're calling them. The "300 Newtons" to which you refer are actually 300 Newton-seconds, which are units of momentum (a Newton is 1 kg*m/s^2, whereas the units you gave were kg*m/s). Too, the rate of energy use is power, which is given as watts, or joules/second. And, to the best of my knowledge, joules*meters is not a standard unit of anything at all.
     
  11. Jul 6, 2003 #10

    selfAdjoint

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    In classical physics like this, energy is defined as "The ability to do work". Work like tearing a hole in the target or blowing up a can of beer.

    A definition of work is a force multiplied by the distance through which the force acts. This is perfectly good to think of the interaction of the bullet with a target. It doesn't just push on the surface, it continues to push into the interior. And as we always read about in combat fiction, the exit wound is much bigger than the entrance one.

    So if you think about the (gory!) physics, Joules (energy) is really the right descriptor.
     
  12. Jul 6, 2003 #11
  13. Jul 6, 2003 #12
    i think momentum is the vital quantity here. although surely different quantities tell you different (all useful) things at different points. otherwise what would be the point in calculating them, and creating units for them.
     
  14. Jul 6, 2003 #13
    I do realise what people mean about the energy involved. I just wonder if it is the best thing to say about the passage of a bullet.
     
  15. Jul 25, 2003 #14
    Well, sorry to dig up an old thread, but I found a better way to ask my question.

    How much do you weigh? Say, 80 kg? Obviously you don't, since that is mass. At Earth's surface, you will weigh (if your mass is 80 kg) 784 Newtons. Mass is the unchanging factor, it won't alter, but it says NOTHING about the motion of the body. Weight, or Newtons, is the term to use, to show what is actually happening.

    In the same way, it just seems to me that Joules is the wrong term to use for describing the action of a bullet. It remains constant, and does nothing at all to describe what is actually going on.

    Can someone clear this up for me please?
     
  16. Jul 25, 2003 #15

    enigma

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    But weight isn't telling anything about the motion of the body either.

    I weigh the same standing on a scale as I do jumping out of an airplane.

    Force is the rate of change of my speed (for constant mass).

    F= d/dt (mV)

    Energy and Impulse are ways to tally up how much force has acted on me and for how far or how long.
     
  17. Jul 25, 2003 #16

    enigma

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    That is a problem with the colloquial definitions. Scales are calibrated to give you your mass in kg. They 'feel' the weight in Newtons and then 'divide' out the acceleration to give you your mass.
     
  18. Jul 30, 2003 #17

    jeff

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    Re: Re: Projectiles (bullets), energy, force

    Good.

    Oh I get it. It's okay for you to say what energy "is", but not for anyone else.

    You've framed your claim about what energy "is" in terms of it's conservation. Since you're so interested in fundamental issues, why did you invoke energy conservation in this way?
     
  19. Jul 30, 2003 #18

    pmb

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    Re: Re: Re: Projectiles (bullets), energy, force

    Since when did I say that it's not okay for someone to say anything?

    I didn't intend that to be taken as a definition. It's a property which characterizes energy.

    Is there as reason why you're still on this subject?

    Pmb
     
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