Why does the aglet of laces in my jacket hood swing like a pendulum when I walk? I have observed that the aglet in the laces of my jacket hood swing like a pendulum when I walk. I cannot think of any forces which are the cause of its movement in this way.
So why is it like this.
 A: The aglet, on its lace, is performing forced oscillations. The periodic movement of your body as you walk causes a periodic force on the top of the lace, making the aglet swing like a pendulum.
The oscillations are usually noticeably large only when the frequency of the periodic force from you is quite close to the natural frequency of the aglet and lace. We are then said to have resonance. [This is the same thing that makes a badly-designed suspension bridge sway and even collapse when an army marches across it, marching at the 'wrong' frequency of foot-fall.] What do I mean by the natural frequency of the aglet and lace? I mean the frequency (number of cycles per second) with which it would swing all by itself when displaced from the vertical and released. [It won't swing like this for very long, owing to air resistance and so on, but this doesn't affect the main argument.]
If you walked at a different pace, either considerably faster or considerably more slowly, I expect you'd find that the a-and-l swung through a smaller angle: you'd no longer have resonance, because the periodic force that you are applying would have a frequency well away from the a-and-l's natural frequency. Alternatively, you could alter the natural frequency of the a-and-l by altering the length of the lace!
[Note that the aglet and lace might be responding to your up-and-down movement as you walk, or to your side to side movement. The frequency of the first is twice that of the second!]
A: When you walk normally at a steady pace, the motion of your body is fairly periodic.
The motion of your arms and upper body for example, closely resembles harmonic motion, and so any object hanging from your body as in this case or a necklace, will move like a pendulum in rhythm with your body's motions.
