Re: Understanding "decoupling"
If the ground doesn't push back, you sink into the ground and gravity wins. It's called the "ground reaction force." When you jump, you may provide the energy (ok, ATP does - ok glucose does - ok, the sun makes the photosynthesis that makes the starches that get turned into ATP that gets used to produce muscular contractions which you use to cause your legs (lever system) to push on the ground and jump. When the force created by you against the ground is greater than the force holding you on the ground (gravity) you jump. But if the ground didn't push back, your feet would sink. If you jump on thin ice, it's not able to push back enough, and your feet go through (brr).