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Newton's laws (conceptual)

  1. Oct 5, 2006 #1
    I am a bit confused, I thought I understood it, but the way the questions are worded seem to suggest I am wrong, which is usually a good indicator...
    I keep putting things as equal, but it seems a bit of repetitive for everything to be equal. I am using Newton's third law for this.

    1. Situation: there is an elevator hanging from a cable at rest.

    a. Compare the force exerted by the cable on the elevator to that of the elevator on the cable.
    They are equal. (1st law)

    b. Compare the tension in the cable to the weight of the elevator.
    The tension is equal to the mass of the elevator times the gravity.

    c. The elevator begins accelerating upwards, compare the force exerted by the cable on the elevator, and vice versa.
    I know that this is F = m (g+a) for the elevator. The F(net) needs to be upward. T-mg > 0. So it would mean the tension is greater than the downward force. However, the force exerted on the elevator by the cable and the force exerted on the cable by the elevator is still equal due to Newton's third law?

    d. Compare the tension in the cable to the weight of the elevator.
    Now, now I know that the tension is greater than the weight or it wouldn't accelerate.
    -----
    2. Similar question, a truck tries to pull a float (no movement), connected by a chain. Compare the force exerted by the bumper on the rope to the rope on the bumper, and rope on the float, to the float on the rope.
    ... They are all equal ...
    The float begins to move...
    ... Aren't they all still equal? Rope to bumper, bumper to rope; float to rope, rope to float
     
  2. jcsd
  3. Oct 5, 2006 #2
    If they all had the same mass they would be equal. The way you have it seems right.
     
  4. Oct 5, 2006 #3

    PhanthomJay

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    For part (a), the force of the cable on the elevator and the force of the elevator on the cable are equal and opposite because of Newton's 3rd law. His 1st law concludes that the force of the cable on the elevator upward is equal to the force of gravity on the elevator downward (its weight) , as per your response to part (b).
     
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