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What causes things to BEGIN moving in a field?

  1. Feb 28, 2010 #1
    so i know that things accelerate in a field (like gravity) because they move through a potential gradiant, gaining energy as they go. but why do they start moving in the first place? Bear with me.

    Ok so i throw an apple up in the air. It has motion and energy that i gave it. It travels up away from the earth and against the potential gradient untill its kinetic energy is "used up" and it is motionless at a time t. Since it has absolutly no motion at this time t, up or down, what gives the apple the innitial nudge back toward the earth. At the time of motionlessness it is not crossing the equipotental lines and therefore it is not gaining or loosing energy. therefore its speed should remain the same, zero.

    this confuses me. Things accelerate in a field because they are passing from places of higher potential energy to lower potential energy and the difference in pot. energies is converted into k. energies. if an object has no kinetic energy it is without motion. if it is without motion it cannot pass from a point of higher potential energy to a point of lower potential energy, thereby ganing k. energy and speed.

    Ok so thats my question. i said it a couple of different ways trying to avoid confusion.
  2. jcsd
  3. Feb 28, 2010 #2


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    Acceleration is the change in velocity with time. So, it doesn't really matter what the initial velocity is; if there is a non-zero force on a particle, then it will have a non-zero acceleration, and thus its velocity will change over time. So, if a particle at rest (zero velocity) feels a force, it will start to move, because its acceleration dictates that its velocity must change at a certain rate. Clear?
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