- #1
oneiron
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Hello Everyone.
I was teaching a physics class the other day on basic momentum and kinetic energy when I decided, for fun, to work out how fast the kilojoules from a Quarter Pounder could propel you. A Quarter Pounder (according to the box) gives you roughly 2000 kilojoules, and my mass is about 80kg. so...
E = 1/2 m v^2
2000kj=.5*80*v^2
sqrt(50) = v = 7ish ms^1 = 25ishkph
Which is fast, but is not THAT fast. Certainly well slower than the records.
So I tried to look at things another way.
If the average person can run 100m in about 20seconds then they're running at 5ms^1.
So, 1/2 mv^2 = .5*80*25=1000kj.
When one is running, chemical energy is being turned into kinetic energy to propell me up to that speed. When I slow down, I can't see how the kinetic energy could be transferred into chemical potential energy in my body...
so this math would say that to burn off a burger, I only need to do two short sprints, reaching 5ms^-1. This seems far far far too easy in my mind.
Where is the mistake in my thinking?
Cheers,
I was teaching a physics class the other day on basic momentum and kinetic energy when I decided, for fun, to work out how fast the kilojoules from a Quarter Pounder could propel you. A Quarter Pounder (according to the box) gives you roughly 2000 kilojoules, and my mass is about 80kg. so...
E = 1/2 m v^2
2000kj=.5*80*v^2
sqrt(50) = v = 7ish ms^1 = 25ishkph
Which is fast, but is not THAT fast. Certainly well slower than the records.
So I tried to look at things another way.
If the average person can run 100m in about 20seconds then they're running at 5ms^1.
So, 1/2 mv^2 = .5*80*25=1000kj.
When one is running, chemical energy is being turned into kinetic energy to propell me up to that speed. When I slow down, I can't see how the kinetic energy could be transferred into chemical potential energy in my body...
so this math would say that to burn off a burger, I only need to do two short sprints, reaching 5ms^-1. This seems far far far too easy in my mind.
Where is the mistake in my thinking?
Cheers,