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Forces .

  1. Sep 28, 2007 #1
    Forces .....

    this was one question i've always thought upon right from the time i have learnt abt forces and velocity .. a bus movin with a uniform velocity has zero acceleration ... then how, in such a case does a person get injure or even die when a bus collides with him ?? if the bus is moving with uniform velocity, there should be zero force and if the force is zero, the person shouldnt be injured or die...
    can anyone please help me in this question.??
  2. jcsd
  3. Sep 28, 2007 #2


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    Assume that the bus doesn't lose any significant velocity after impact. Now look at the CHANGE in velocity of the person. He sent from zero velocity to the same velocity of the bus in less than 1 second! So there's a change in velocity in a very short time, and that means a very large acceleration for him. Consequently, a large force must have been applied to this person. Guess where the force came from?

  4. Sep 29, 2007 #3
    is it true to say that everything in the universe has a force acting on it at all times....?

    in some cases these forces are balanced therefore the object is in equilibrium and appears to stay still?

    is it correct to say everything in the universe is in a state of motion? even an object on the earth... its accelerating down towards the earth... but because the earth is providing an equal and opposite accelerating force back at it it appears to not be moving?
  5. Sep 29, 2007 #4


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    True. Gravity is pretty much the weakest of the forces, but it's the only one that has no known counter-force. That's why it dominates on macro scales; the others cancel each other out.

    Everything is in a state of motion relative to something else. An object on the Earth, however, is not accelerating downward nor being accelerated equally upward by the Earth. They are at rest relative to each other. The term 'acceleration' indicates a change of velocity, which neither is experiencing.
  6. Sep 29, 2007 #5
    isn't it experiencing 'g' by virtue of the fact it has weight? - and g is a measure of acceleration?

    F=ma? W=mg?
  7. Sep 29, 2007 #6
    and surely the forces are only cancelling each other out if something appears to stay still (in your frame of reference).... if something appears to be moving then the forces aren't balanced - i.e thrust > air resistance for a moving vehicle...?
  8. Sep 29, 2007 #7


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    Actually, it's experiencing weight because of 'g'. Weight is just the effect of gravity upon mass. I'm not sure of the proper way to phrase this, since I'm not a scientist, but 'g' is a measure of potential acceleration, as well as actual. If you stand at the edge of a cliff, you feel gravity attracting you but you have no acceleration because the ground is holding you up. If you step off, then there's acceleration (followed by an unpleasant negative acceleration a few seconds later).
    As for the forces cancelling, what I mean is that for every positve electrical charge (proton), there's a negative one (electron) to counteract it. For every north magnetic pole, there's a south. As far as has yet been verified, there is no 'antigravity'.
    Last edited: Sep 29, 2007
  9. Oct 1, 2007 #8
  10. Oct 1, 2007 #9
    Maybe something to do with the energy transfer, K.E.=1/2* m*v^2
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