1). Air resistance
2). Friction in the pivot.
These two effects rob energy from the pendulum. Without air resistance or
friction in the pivot, a pendulum, once set in motion, would not stop.
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In measuring the period of a pendulum's motion with a stopwatch, you can best minimize the influence of reaction time by measuring period at the maximum swing point, i.e. the point where the pendulum stops and then reverses. It is at that point that the pendulum is moving the slowest. You should also make your line of sight be perpendicular to the swing, in order to minimize parallax error; and you should have a mark of some kind that you can move around to the peak value, then recording the period between successive peak values.
You know how when you pull a pendulum to the side and let it go, and then it swings away from you to the other side, but then it stops and turns around and swings back to you ? The period of the pendulum is the length of time it takes, after you let it go, to go away from you and then come back to your hand.
No it is a rational number because it stops.
It is a rational number because it stops.
If we understand what you mean when you say "straight", then you could be describing a "line". -- But if it ever stops in one direction, then it's actually a "ray". -- And if it eventually stops in both directions, then it was a "line segment" all the time.