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Big kids fall hard Thu, Jan 29, 2009
Gravity tells us that all objects, regardless of weight, when dropped from the same height, travel at the same speed and thus hit the ground at the same time (or rather, this is what would happen if air resistance didn't decelerate objects that weighed less and/or had a larger cross-sectional area, hence why a bowling ball would hit the ground before a feather if dropped from the same height). 

The problem with this idea (not that it's incorrect) is that it only accounts for speed, while ignoring mass.  This becomes painfully obvious as we humans grow taller/larger.  It's fun to watch little kids ski down a mountain or tackle an opponent on the football field, and then get up as if nothing happened.  When we adults try this, we end up with big bruises on our hips and knees, and we're left blaming it on age.  I don't think age is the culprit.  And I don't think it's gravity.  I think it's mass. 

This can be proved with one of two physical quantities, (1) force and (2) kinetic energy.  Force is defined as (mass × acceleration).  Since acceleration is the same for everybody because of gravity, the difference is mass.  Let's say I'm ice skating, and when I fall, I fall on my hip/butt.  Let's also say that I weigh three times as much as that little kid who just zoomed past me (e.g. 150 lbs vs. 50 lbs).  Assuming all my weight falls on that one body part, the force of my hip bone hitting the solid ice is exactly three times the force of that little kid's hip bone hitting the same ice.  Similarly, kinetic energy is the energy associated with a moving object, and it's defined as (½ × mass × velocity²).  If we assume the velocity of a falling child (that's a funny thought) is equal to the velocity of a falling me, that leaves the same relationship:  I have three times the kinetic energy as that little kid.  Maybe that's why it takes longer for me to get back up. #science


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