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I get that forces are what accelerates an object but if the tension force was equal to the force of gravity and the object was already in motion, would it really just move up? Hard to picture.

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  • $\begingroup$ This is Newton's first law in action. You say that you understand that forces are what cause acceleration, so I am confused about what you are asking. $\endgroup$
    – jm22b
    Nov 1, 2016 at 12:32
  • $\begingroup$ I'm just confused because the cables will need to pull up the elevator and thats always a force so even if the tension is keeping the elevator from falling you have another force with the pulling of the rope by the mechanism on the top. $\endgroup$ Nov 1, 2016 at 12:49
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    $\begingroup$ If the tension in the cable is balanced by the weight of the elevator, there is no net force and thus no acceleration. It is important to consider all of the forces acting upon the elevator. $\endgroup$
    – jm22b
    Nov 1, 2016 at 12:56

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That's exactly how Newton's first law works. No net force = no acceleration. Note that "no net force" means that the cable has a tension equal to the force of gravity - and moving that cable under tension does require work to be done. So the elevator doesn't move "for free".

Think about a puck sliding on an air hockey table. Almost no net force, it glides along...

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  • $\begingroup$ I guess what im not picturing is what keeps it moving. So I assume the force is the tension of the rope, but what abt the pulling up of the elevator isnt that counted as a force? I get the first law but to get the puck moving there has to be a force applied to it first and then we can say theres no net force in the "ideal" situation $\endgroup$ Nov 1, 2016 at 12:33
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    $\begingroup$ You apply a net force to get the elevator moving initially - after that it keeps going at constant speed - that's the idea of inertia. If you were in a parallel elevator moving up you would see the elevator appearing to be stationary and you would it be surprised - as you saw the tension in the cable balancing the force of gravity. $\endgroup$
    – Floris
    Nov 1, 2016 at 12:36
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    $\begingroup$ @GiancarloGatti You need to think about two different time intervals during the motions: 1) an interval during which there is a non-zero net force so that the object accelerates, followed by 2) an interval in which the individual forces have changed so that there is zero net force and the acceleration ceases and the velocity is constant. $\endgroup$
    – Bill N
    Nov 1, 2016 at 13:30
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The answer to your questions is ..... Yes it will go up with uniform velocity if the tension force is equal to the force of gravity given that it already has an upward velocity. As the lift gradually rises it increases the tension force slowly and then decreases it to the previous value (that it had when it was at rest). Yes force of gravity changes with height but that little change is little enough to be neglected. And after that just like @Floris said its Newton's first law. When both the forces are equal there is no force regulating it's velocity and hence its value does not change. It rises up with a constant velocity.

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Even if there is no friction, we have to keep pushing on the elevator to stop if from falling down, pulled by gravity. But if it were far out in empty space, away from all gravity, the elevator would keep moving with constant speed in a stright line when we stop pushing.

What keeps the elevator moving is inertia. This is difficult to picture because it doesn't happen in real life, only in ideal situations such as empty space. In real life everything slows down and stops when we don't keep pushing it. That is because there is always some other small force like friction or air resistance which is present whether we push or not.

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