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Force question

  1. Oct 23, 2011 #1

    I understand that force is mass x acceleration. What I don't understand is why acceleration and not velocity. Can someone explain why exactly
  2. jcsd
  3. Oct 23, 2011 #2
    well mass times velocity is actually something else..known as momentum
    and force is actually the rate of change of momentum..you could perhaps try and read Newton's own writing on the matter to see how it was developed..I don't really know how to prove it :'(
  4. Oct 23, 2011 #3
    Well that would explain it. My physics teacher never would explain why and he skipped momentum for some odd reason
  5. Oct 23, 2011 #4
    You should get to momentum afterwards I think
  6. Oct 23, 2011 #5
    Ill dive into my college physics book when I have the chance. Thanks I appreciate the fast replies
  7. Oct 23, 2011 #6
    Suppose you are travelling in an elevator in gravity free space. If the elevator is moving with a constant velocity, you wouldn't even know that you are moving. But if it is accelerating, you would know, because you will be exerting a force on the floor of the elevator.
  8. Oct 23, 2011 #7
    Well, I understand the acceleration aspect. Just wasnt sure why Force was mass by acceleration. Couldnt we say that an object traveling at a constant velocity that hits a motionless object imposes a force upon impact?
  9. Oct 24, 2011 #8
    Very true. That's because when the object moving with a constant velocity hits the motionless object, it decelerates and the motionless object accelerates from rest. Check this out: http://en.wikipedia.org/wiki/Impulse_(physics)
  10. Oct 24, 2011 #9
    Wow, its the simple things I seem to not think of.... Lol
  11. Oct 24, 2011 #10


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    Staff: Mentor

    Er - I wouldn't say that. Impulse is not force. A collision does not have one single force associated with it, so it is far too simplistic to associate force with velocity.
  12. Oct 24, 2011 #11
    mvan: smart people have developed very exact definitions, such as force and momentum, acceleration and velocity, and refined them over many years...maybe even several thousand years. So no one should be surprised as they study that accumulated knowledge that it is not always obvious....especially at initial exposure.

    Force is related to acceleration because a fixed unbalanced force acting on a mass causes a uniform acceleration, not a uniform velocity. A uniform velocity is associated with a fixed unbalanced force: zero.

    One relationship between force and velocity: FT = MV
    Last edited: Oct 24, 2011
  13. Oct 24, 2011 #12
    If you send something at a velocity, it will stay at the same speed if not exerted on by a force.
  14. Oct 24, 2011 #13
    This may be an off question, but what is Earth's movement classified as?

    I'm guessing momentum: The sun being (mass x acceleration) creating the force (distance of mass). Then that force times the velocity of the mass to give our momentum.

    Then you would basicly repeat this for the sun to the galaxy's center, and so on down to the center of space. Going from smallest body of mass to the largest.
  15. Oct 24, 2011 #14
    I guess the suns force wouldnt be times velocity because it doesnt go at a uninterupted pace. The solar systems force/mass must be impacted by another force to create that momentum.

    Maybe an insight to anti-matter? It wouldnt be pushed out and compressed or anything. Since the mass of the solar system is accelerating and spreading out It would actualy be bringing more of the anti-matter/space closer to its core. In other words a displacement of mass and space, the distance between mass.

    So the velocity would be the suns force constant rate of speed, then times the force of the anti-matter/space to equal our momentum.
  16. Oct 26, 2011 #15
  17. Oct 26, 2011 #16
    Why not if a collision involves changing the sate of ones motion?
  18. Oct 26, 2011 #17
    If you like you can say it is a result of Newton's first law, that stuff which is already moving is going to just keep moving that way unless something makes it change, so a force causes a change in velocity rather than velocity itself, but I think it's fair to say that the reason something will just keep on moving the way it is without a force is at the moment unknown.
  19. Oct 26, 2011 #18
    Three reasons:
    1) Impulse is not a force, it is a measure of change of momentum. Divide it by the time period it changes in, and you get the average net force.
    2) Notice I said net force. The components of this force can be many different things.
    3) Notice I said average force. In a collision, the net force often changes during the duration of the collision.
  20. Oct 26, 2011 #19
    I guess I am still speaking simplistically then. If impulse is the measure of change in momentum and you get the average change (some non-infinitesimal time interval), you get a average net force, correct? If so, I guess going back to a simplistic mode - ignoring what could be the components of the net force - it is, on that level of an ignorant mind - a force, right? Maybe a am being a little to philosophical? Nonetheless, I think I understand where russ was getting at now with it being simplistic from what you have said. :)
  21. Oct 26, 2011 #20
    If you mean average change per unit time, then yes, you are right, if you are looking at it in a simplistic way. Remember, though, some materials will change the momentum faster than others, e.g. a metal ball will change the momentum faster than a pillow, so you will get different forces with the same impulse depending on the material.
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