Figure: The trajectory of a high jumper doing a Fosbury flop clearing the bar even though their center of mass does not

Caption: The net external force determines the motion of the center of mass of a body per Newton's 2nd law of motion (AKA F=ma). However, internal motions can relocate the center of mass relative to the various parts of the body. Recall a center of mass is the mass-weighted average position of a body.

In the image, the high jumper (doing a Fosbury flop) clears the bar, but NOT their center of mass---they wiggle over the bar. The center of mass follows a parabolic trajectory as dictated by Newton's 2nd law of motion (AKA F=ma) and gravity. This happens to any thrown object if air drag is negligible. For millennia, no one understood this---even though it's obvious from jet fountains.

So why do you need to know about center of mass? To clear the bar.

High jumping is like being in orbit---once you leave the ground, your center of mass has to go with gravity.

Credit/Permission: © User:AlanSiegrist, 2016 / Creative Commons CC BY-SA 4.0.
Image link: Wikimedia Commons: File:Fosbury Flop English.gif.
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