Use the ground for power
[embedplusvideo height="296" width="474" editlink="http://bit.ly/1DiuM7O" standard="http://www.youtube.com/v/sYmirjWuGdQ?fs=1&vq=hd720" vars="ytid=sYmirjWuGdQ&width=474&height=296&start=&stop=&rs=w&hd=1&autoplay=0&react=0&chapters=¬es=&lean=1" id="ep6995" /] I have recently had a few students asking about ground reaction force...