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Homework Help: Newton's third law and a motor

  1. Oct 8, 2012 #1
    hey people,greetings to everyone

    i'm an freshman in physics so i have some problems understanding newton's third law.
    for example how can a motor accelerate an elevator upward,or downward?i know newton's third law acts on different objects,i understood the house and the cart problem.
    horse is pulling cart if the force from the ground that acts on the horse is greater than the force of the cart that is pulling a horse,but i don't see two pair of forces in a situation between a motor and an elevator??
    also,how can we pull a bucket of water from the watter well,if the forces that act on a bucket and on us are the same?how can a weightlifter push the weight over his head?
    please answer me to my questions through newton's third law,i'm really confused
    i'm sorry if i have some grammar mistakes,english is not my native language

  2. jcsd
  3. Oct 8, 2012 #2


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    Homework Helper

    In a Newton third law pair of forces, the reaction force due to acceleration, if any, is included. If there's acceleration, then the net force on the accelerating object(s) is non-zero. So in the elevator case, the tension in the cable equals the weight of the elevator + the mass of the elevator times the acceleration of the elevator. So the magnitude of force = weight + mass x acceleration, and the Newton third law pair is the upwards force exerted by the cable onto the elevator, and the equal and opposing downwards force exerted by the (accelerating) elevator onto the cable. In the case of a rocket in space, both the exhaust and the rocket accelerate, and both could be considered reaction forces.
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