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Does a simple pendulum come to stop due to the action of gravity ?

Consider : No air drag, No change in place.

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    $\begingroup$ See physics.stackexchange.com/q/69013 $\endgroup$
    – user43617
    Nov 4, 2014 at 16:04
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    $\begingroup$ No. It has $EP$ at the height from which it is dropped, and this $EP$ keeps getting converted to $EK$ and then back to $EP$ at the other extreme end. $\endgroup$
    – user49111
    Nov 4, 2014 at 16:04
  • $\begingroup$ Is the air the only "agent" that can slow the motion of the pendulum? $\endgroup$
    – DJohnM
    Nov 4, 2014 at 16:05
  • $\begingroup$ Cant we state this with the fact that "horizontal velocity of an object will not get affected by gravity acting perpendicular to it" ? $\endgroup$
    – Vinayak
    Nov 4, 2014 at 16:07
  • $\begingroup$ @Jun-GooKwak I actually want to know will there be an effect of gravity causing any dissipation that I couldnt find there. Anyway, Thanks for conveying the message. $\endgroup$
    – Vinayak
    Nov 4, 2014 at 16:13

1 Answer 1

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Say you release the pendulum, then on the other side, there is a period of time where it is motionless where $E_k = 0$. In addition, this is the point where $E_{p,max}$. At the next instant, the pendulum will swing back increasing velocity due to the acceleration of gravity pulling at the weight at the end of the pendulum. Following the Law of Conservation of Energy, $E_{p,max}$ is converted into $E_k$. At the pendulum's lowest point, $E_p = 0$ and $E_{k,max}$. The pendulum begins to decelerate due to the force of gravity as it swings back up, and it will reach the same height as the opposite side in an ideal system.

In reality, the two forces, friction and air resistance cause small losses of momentum for every period back and forth. Air resistance is zero when the pendulum is stationary at the two moments of time where it is at its maximum height, and the greatest when the pendulum reaches it's fastest velocity at it's lowest point.

There is also a frictional force in the pivot point of the pendulum. When the pendulum reaches its highest points, the static friction must be overcome by the gravitational force for it to move back down, however, there is no kinetic friction as the object is not in motion. And then as the pendulum starts to move again, static friction increases from air resistance.

It is the force of gravity that keeps the pendulum moving.

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