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Reason for Velocity

  1. Jul 23, 2012 #1

    I have a question, what is the reason for the velocity of a rigid body?

    I know the force is the reason for movement status changing of a rigid body. And also this can be said, the forces and the movements are reciprocal to each other.

    now, what is the reciprocal part of the velocity of a rigid body?

    thank you
  2. jcsd
  3. Jul 23, 2012 #2


    Staff: Mentor

    Hi ldeshusheng, welcome to PF!

    There is no physical reason for velocity. It is purely a consequence of the arbitrary choice of your coordinate system.
  4. Jul 23, 2012 #3
    thank you.
  5. Jul 23, 2012 #4
    Do you mean why some objects get higher velocity than others? Velocities are closely related Movements.

    I think movements depend on space through which objects move. The more we learn about empty space, the more we'll understand how/why objects move. Until then I take it as a law of nature. The higher the force, the higher the velocity, the higher the acceleration.
  6. Jul 23, 2012 #5


    Staff: Mentor

    This is not true in general. For example, consider an automobile's brakes. In that case, the higher the force the lower the velocity wrt the ground.
  7. Jul 23, 2012 #6
    The force is the reason for the acceleration based on Newton's second lawm, not the velocity.
  8. Jul 23, 2012 #7
    Assume there is a revolute joint, a torque (just caused by a couple, not by a force) can make it rotate around the axis.

    why there are two parasitic velocities along x-axis and y-axis?
  9. Jul 23, 2012 #8
    velocity describes the state of movement of an object, i think there's no actual reason why something moves at the speed they do. its change can be described by acceleration which is linearly related to F so yeah, you can argue that force is the reason for the change of velocity but you don't have to bond velocity with something else since it's in fact a relative thing that may vary from one observer to another, while F doesn't.
  10. Jul 23, 2012 #9
    Come on guys, keep it simple.
    You apply a force on an object at rest, it moves, meaning object changes its velocity from zero to a non-zero value. Change in velocity is acceleration. This is why acceleration comes in Newton's law.
  11. Jul 24, 2012 #10
    And in a frictionless environment after a force is applied and then removed you have velocity. "a body in motion tends to stay in motion unless acted upon by an outside force." ... "an object at rest tends to remain at rest unless acted upon by an outside force."
    So you have velocity because at some point a force acted upon an object and accelerated it for a finite time. (or continues to act on the body to overcome external forces working to slow it down)

  12. Jul 24, 2012 #11


    Staff: Mentor

    A complete history of all the forces acting on an object can account for the objects change in velocity from it's initial velocity, but it cannot explain the initial velocity. That depends entirely on the choice of coordinate system.
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