My sister Coral and I were doing some push-ups and it made me think of physics. Every time Coral pushes herself up she feels her normal force pushing against the force of the earth. The earth is pulling all 25 kg of her at a force of 245 N. Thus she has to push up with a force of 245 N to move up at all and if she moves up accelerating then she is pushing with a force greater than 245 N. After about 200 push-ups the force the earth was pulling on her was just too much and gravity defeated coral.
