Human Weight, Work and Gravity

by agilis1
Tags: gravity, human, weight, work
 P: 2 Hello there, My name is Rick and a couple of my friends are having a discussion at a local rock climbing gym about the weight of a person and whether or not a person who is heavier, works harder than someone who is lighter. Neither one of us are gurus when it comes to physics but would love an answer. I am under the impression that if 2 people, one lighter than the other, who have identical proportions of muscle and fat, the heavier person will be working harder than someone who is lighter when it comes to lifting their own body weight. I believe the force of gravity acting on the heavier person makes lifting their bodyweight harder. Thank you in advance.
 P: 15,294 How do you define "working harder"? Total calories burned? Calories per unit mass? (To take it to an absurd degree, it's pretty obvious that a 60ft tall creature would burn more total calories than a 2" tall creature.)
 PF Patron P: 919 To expand a bit on Dave's post: A physicist might answer: With the two persons being physiologically and "metabolically" equal except for the differences you mention, then yes, the heavier person will be doing more total physical work, than the lighter person since he has to lift more mass up the same distance. This also means the heavier person must on average have a greater calorie intake to climb the same distance. A (sports) engineer might add: However, I believe metabolically differences between persons can make a huge difference. Also, as recent studies have revealed, when tasked with doing the same low-activity work (like resting, sitting) two persons can have vastly different muscular activity. Of course such effect probably have less significance when climbing vertical wall on full power, but it just go to illustrate that there still are individual variations in human metabolism that are not fully understood yet. An exercise physiologist may add something else, but I'm not qualified to guess what that might be.
Mentor
P: 5,226

Human Weight, Work and Gravity

It really depends on their physique. The heavier person may have a higher muscle/weight ratio or the lighter person might. It really depends on that ratio more than anything else.

EDIT: thinking about it when I go rock climbing I tend to do well because I am light but quite strong. One of my friends who usually comes with me is stronger and a lot heavier (he's very tall) but he always seems to have an easier time than me. Conversely I have another friend who is heavier and has a harder time of it. So to conclude the same thing again, it's not all about weight :P
 HW Helper P: 2,959 The energy that must be expended to gain gravitational potential energy is mass*9.8*vertical distance (in meters, kilograms). So a heavier person will use up more energy to climb to the same height. But probably more important is how physically fit a person is, since most of the energy used by a person climbing is actually wasted (not turned into gravitational potential energy).
 P: 2 Wow! Thanks so much for everyone's input. It's really appreciated. I actually understand everyone's response which his hats off to you all :) Ryan's response is the only one that was the odd ball. Ryan, in my example, we were thinking that the ratios were identical but the weight was different. Again thanks so much and if anyone wants to add anything else I'll be sure to read it and/or respond. ----- Ryan I wasn't calling you an oddball. Your post just said the opposite than the other replies. I think in your example, your friend, being heavier / stronger may have better technique when climbing than you, so it's easier. Because I don't know you, this is just a guess.
 HW Helper P: 2,959 no problem, all the best
 P: 948 Another interesting thing to think about is this: mass varies cubically with size (taken to mean one length, height, or width) while muscle strength varies according to the cross-sectional area of muslce, as a squared term. The end result is that people (and animals) who are smaller typically can lift more relative to their mass. This is why ants, for example, can lift ten times their weight. It's actually really not that special.

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