I am a person. I have a solid mass. This mass is capable of exerting Force upon other solid masses and is completely incapable of passing through other bodies of mass. This should be a pretty obvious concept.
And yet every day I encounter people who seem unable to grasp the very basics behind this scientific truth. As I come running down the sidewalk towards them there seems to be a willful and conscious decision made to not move even the slightest bit to the side to allow me to pass. Now I know they are capable of doing this, for they often do so with other walkers, but for some reason they can't figure out that someone running at them can cause a significant amount of bodily damage if they were to be struck by that runner. Do they not see me as a corporeal being? I make every effort to stay out of their way, returning the favor would be nice.
As we all know, Force is equal to Mass times Acceleration; F=mxa. I weigh 74.84 kilograms (I know, I'm working on it) so if I were to be leisurely running down the sidewalk at an 8min/mile pace (7.5mph or 3.35meters/second) I would be exerting the equivalent of 250.7 newtons [N]. (1 kilogram force = 9.7 newtons) All of this Force would be transferred into you if I was to run into you due to your inability to move a couple of inches to the side.
Now I can hear what you are saying: "But doesn't the Earth's gravity exert approximately 689N on you at all times? So 250.7N is nothing!" And you are right! Except that my force is being exerted laterally and in the case of a collision will flow through the opposing person due to the lack of a sufficient mass to stop me. A wall can easily absorb my Force but a standing person can not. Not only will the person be knocked to the ground and potentially suffer an injury, my momentum will most likely cause me to continue forward and land on the person who was just knocked over. Now we have a situation in which both gravity and the Force I am exerting is brought down on top of said person. If the person is flat on the ground we have introduced an immovable platform (the ground) on which they are laying and therefore all of the Force I am exerting will be brought to bear directly on the person, unlike the initial collision in which some of that Force was transferred into momentum. This is where serious injury can occur.
So basically, if you see me running down the sidewalk make an effort to move even the slightest bit out of my way. After all, I come armed with physics.