From practice (pls don't call police), I know that it is much easier to knock down short people. A tall person of equal mass is more difficult to knock down on the floor. Theoretically, the taller person should be easier because there's this whole center of gravity thing. But in practice, one can feel another factor playing in: Elasticity. Taller people tend to be more 'elastic', so to say, so it's foolish to expect them to be on the floor even after your mightiest punch. Even if they go off balance, they quickly recover. Shorter people? umm not so much. But I have a feeling there's something more. I cannot reconcile how the defense of elasticity and the weakness of center of gravity combines to make it more difficult for me to knock down tall people. The inverse applies to short people. So what is the physics behind it? Assuming that force is applied on the chest area, what happens between the moment I shove and the moment they fall?