I've seen the movie 3 times now and always wondered, did Prime hold back when fighting Sentinel a little bit or did he go full on? Because Prime was actually hit by an explosion right before he fought Sentinel, maybe he was weakened already? Since Sentinel didn't fight until the end and Optimus fought the whole time. The reason I ask is because there is no way Sentinel is a better fighter than Optimus based off the cartoons, and he ripped his arm off just like that and almost killed him if it wasn't for Megatron. Or maybe they did that just to introduce the Megatron Sentinel betrayal.