4 ms·

To put these numbers in context, 8000 joules is about 2 kilocalories (or about half an M&M candy). It's about $0.0003 of wall electricity. It's about the energy

by nonce42 13y ago

To put these numbers in context, 8000 joules is about 2 kilocalories (or about half an M&M candy). It's about $0.0003 of wall electricity. It's about the energy in an AA battery. It's the energy from exploding 2 grams of TNT. 1.7 megajoules is about 400 kilocalories, or a king size Kit Kat bar. This is the energy of a car going 140 miles per hour. This is about 7 cents of AC power.

PhasmaFelis 13y ago
"...about 400 kilocalories, or a king size Kit Kat bar. This is the energy of a car going 140 miles per hour." Wow. How can those two things be equivalent? I know there's a lot I don't know about physics...
- Florin_Andrei 13y ago
  Kinetic energy is the speed, squared, times the mass, halved. 140 mph is not a small number; to the power of two it's rather large. Now multiply the result by the mass of a car, which is still pretty huge even halved. A racing car barreling down the speedway packs a hell of a punch.
  - StavrosK 13y ago
    I think the GP's question was more like "how the hell can a single Kit Kat bar get a whole car to 140 mph?".
    - Florin_Andrei 13y ago
      I see. Well, a kitkat bar actually has a decent amount of energy in it. Very roughly, the same energy as the same mass of wood, or gasoline. Chemical fuels don't vary too much in terms of energy density. They stay within the same order of magnitude, at most. And yes, food is chemical fuel.
      - maaku 13y ago
        No, a Kit Kat bar has way more energy per gram than wood, or even TNT. A king size Kit Kat bar packs basically the same energy as 0.5kg of TNT. That's a lot of power. Probably only a straight-up chemical fuel like gasoline or methane packs more punch per gram.
        gus_massa 13y ago
        Strange. I did some research and found some numbers. The energy density of Kit Kat is 22 MJ/Kg (5.2 Kcal/g) http://www.wolframalpha.com/input/?i=%28Kit+Kat+calories+to+megajoules%29+%2F+%28Kit+Kat+weight+to+kilograms%29 http://www.wolframalpha.com/input/?i=%28Kit+Kat+calories+to+... TNT has 2.8 MJ/Kg in an explosion and 4.2 MJ/Kg in a combustion. It has too much Carbons. In an explosion, it doesn’t react with the oxygen in the air, because it’s too fast. There is an interesting discussion in http://en.wikipedia.org/wiki/Trinitrotoluene#Energy_content http://en.wikipedia.org/wiki/Trinitrotoluene#Energy_content In general, the explosives have to explode without using the Oxygen in the air, so they have a lot of Oxygen and Nitrogen atoms inside. The fuels for slow combustions can have almost only Carbon and Hydrogen atoms to store the energy more efficiently and get the Oxygen from the air. Nitroglycerin has less Carbons, so has more energy density. Mixed in dynamite has 7.5 MJ/Kg and alone has 6.4 MJ/Kg (Is this correct? I expected a bigger value). I found a list of these values for a wide range of materials. In particular it says that green wood has 10 MJ/Kg, air dry wood has 15 MJ/Kg and oven dry wood has 20 MJ/Kg. http://physics.info/energy-chemical/ http://physics.info/energy-chemical/ Other values: Gasoline ~45 MJ/Kg, Methane 55.5 MJ/Kg, Hydrogen 142 MJ/Kg
- dnautics 13y ago
  Think about it, do you think you could move a car to 140 mph by burning a kit kat bar (in space, with no friction)? Consider ion propulsion drives, which use a teeny tiny bit of force (less than the weight of paper) but are highly efficient. What percent of a kit kat bar do you think it takes to get an astronaut on the ISS (say, 1/30 the mass of a car) to bound across the ISS at 10 mph? But yes, the comparison is supposed to be to put in natural contexts, and so this is flawed example since we don't really live in space.
- maaku 13y ago
  1) Realize that you are a highly articulated animatronic robot, built with inefficient meat actuators; 2) think about all the things you could do in the hour or so that a king sized Kit Kat bar could keep you energized; 3) visualize the trillions of cell-sized actuations required to perform each step of each task previously listed, and the neural computation required to organize these actions; 4) compare that with the relative simplicity of turning wheel axels to propel a car down the road; 5) marvel at the power of a Kit Kat bar powering the magnificent machine that is you.
shabble 13y ago
I'm getting the KE of a car to be roughly double your estimate: assuming ~4000lb[1] car, 140mph ~= 62.6m/s, 0.5*m*v*v = 0.5 * (4000*0.454) * 62.6^2 = 3.5E6Joules[2] ~= 830kcal Did you miss the 0.5, or use a different Spherical Co^Wstandard automobile mass? Which is still only 2 kitkats :) The other potentially confusing thing (judging by the sibling comments here) is that this isn't the energy required to accelerate an actual car from 0 to 140mph. It's the kinetic energy embodied in a car traveling at that speed. Wind & tyre resistance losses, engine/transmission, etc, etc. It's really the (upper bound of the) amount of energy you could expect it to impart if it hit something, or the amount of heat the brakes would need to dissipate to stop it. Finally, for all those commenting on how much energy there appears to be in a kitkat, consider the famous E=MC^2, and just how staggeringly huge that C^2 term really is (9E16J/kg, if you're wondering) Getting all that out in a usable form, however, is left as an exercise for the reader. [1] http://www.nytimes.com/2004/05/05/business/05weight.html http://www.nytimes.com/2004/05/05/business/05weight.html http://hypertextbook.com/facts/2000/YanaZorina.shtml http://hypertextbook.com/facts/2000/YanaZorina.shtml [2] http://www.wolframalpha.com/input/?i=0.5+*+%284000+*+0.454%29+*+%2862.6^2%29 http://www.wolframalpha.com/input/?i=0.5+*+%284000+*+0.454%2...
- nonce42 13y ago
  I didn't expect so much controversy here, I just wanted to get a feel for the order of magnitude. I went with 2000 pounds for the car weight, which is the weight of a 1976 Honda Accord - 1976 because of a search error on my part. 4000 pounds is the average new car or light truck, so that number is a bit high because of the trucks. In any case, the energy dumped into this experiment is like a big car crash concentrated into a tiny capsule. Anyone who has used a treadmill at the gym will recognize that it's a lot of work to burn off 400 calories. Not enough to get a car up to 140 MPH because the body isn't 100% efficient.
stox 13y ago
Of course, you're talking about the chemical energy in a Kit Kat bar. Were it completely converted to energy, it would produce around 5.8 Terra-watt years of power assuming a lossless conversion.

by nonce42 13y ago

PhasmaFelis 13y ago

"...about 400 kilocalories, or a king size Kit Kat bar. This is the energy of a car going 140 miles per hour." Wow. How can those two things be equivalent? I know there's a lot I don't know about physics...

Florin_Andrei 13y ago
Kinetic energy is the speed, squared, times the mass, halved. 140 mph is not a small number; to the power of two it's rather large. Now multiply the result by the mass of a car, which is still pretty huge even halved. A racing car barreling down the speedway packs a hell of a punch.
- StavrosK 13y ago
  I think the GP's question was more like "how the hell can a single Kit Kat bar get a whole car to 140 mph?".
  - Florin_Andrei 13y ago
    I see. Well, a kitkat bar actually has a decent amount of energy in it. Very roughly, the same energy as the same mass of wood, or gasoline. Chemical fuels don't vary too much in terms of energy density. They stay within the same order of magnitude, at most. And yes, food is chemical fuel.
    - maaku 13y ago
      No, a Kit Kat bar has way more energy per gram than wood, or even TNT. A king size Kit Kat bar packs basically the same energy as 0.5kg of TNT. That's a lot of power. Probably only a straight-up chemical fuel like gasoline or methane packs more punch per gram.
      - gus_massa 13y ago
        Strange. I did some research and found some numbers. The energy density of Kit Kat is 22 MJ/Kg (5.2 Kcal/g) http://www.wolframalpha.com/input/?i=%28Kit+Kat+calories+to+megajoules%29+%2F+%28Kit+Kat+weight+to+kilograms%29 http://www.wolframalpha.com/input/?i=%28Kit+Kat+calories+to+... TNT has 2.8 MJ/Kg in an explosion and 4.2 MJ/Kg in a combustion. It has too much Carbons. In an explosion, it doesn’t react with the oxygen in the air, because it’s too fast. There is an interesting discussion in http://en.wikipedia.org/wiki/Trinitrotoluene#Energy_content http://en.wikipedia.org/wiki/Trinitrotoluene#Energy_content In general, the explosives have to explode without using the Oxygen in the air, so they have a lot of Oxygen and Nitrogen atoms inside. The fuels for slow combustions can have almost only Carbon and Hydrogen atoms to store the energy more efficiently and get the Oxygen from the air. Nitroglycerin has less Carbons, so has more energy density. Mixed in dynamite has 7.5 MJ/Kg and alone has 6.4 MJ/Kg (Is this correct? I expected a bigger value). I found a list of these values for a wide range of materials. In particular it says that green wood has 10 MJ/Kg, air dry wood has 15 MJ/Kg and oven dry wood has 20 MJ/Kg. http://physics.info/energy-chemical/ http://physics.info/energy-chemical/ Other values: Gasoline ~45 MJ/Kg, Methane 55.5 MJ/Kg, Hydrogen 142 MJ/Kg
dnautics 13y ago
Think about it, do you think you could move a car to 140 mph by burning a kit kat bar (in space, with no friction)? Consider ion propulsion drives, which use a teeny tiny bit of force (less than the weight of paper) but are highly efficient. What percent of a kit kat bar do you think it takes to get an astronaut on the ISS (say, 1/30 the mass of a car) to bound across the ISS at 10 mph? But yes, the comparison is supposed to be to put in natural contexts, and so this is flawed example since we don't really live in space.
maaku 13y ago
1) Realize that you are a highly articulated animatronic robot, built with inefficient meat actuators; 2) think about all the things you could do in the hour or so that a king sized Kit Kat bar could keep you energized; 3) visualize the trillions of cell-sized actuations required to perform each step of each task previously listed, and the neural computation required to organize these actions; 4) compare that with the relative simplicity of turning wheel axels to propel a car down the road; 5) marvel at the power of a Kit Kat bar powering the magnificent machine that is you.

shabble 13y ago

I'm getting the KE of a car to be roughly double your estimate: assuming ~4000lb[1] car, 140mph ~= 62.6m/s, 0.5*m*v*v = 0.5 * (4000*0.454) * 62.6^2 = 3.5E6Joules[2] ~= 830kcal Did you miss the 0.5, or use a different Spherical Co^Wstandard automobile mass? Which is still only 2 kitkats :) The other potentially confusing thing (judging by the sibling comments here) is that this isn't the energy required to accelerate an actual car from 0 to 140mph. It's the kinetic energy embodied in a car traveling at that speed. Wind & tyre resistance losses, engine/transmission, etc, etc. It's really the (upper bound of the) amount of energy you could expect it to impart if it hit something, or the amount of heat the brakes would need to dissipate to stop it. Finally, for all those commenting on how much energy there appears to be in a kitkat, consider the famous E=MC^2, and just how staggeringly huge that C^2 term really is (9E16J/kg, if you're wondering) Getting all that out in a usable form, however, is left as an exercise for the reader. [1] http://www.nytimes.com/2004/05/05/business/05weight.html http://www.nytimes.com/2004/05/05/business/05weight.html http://hypertextbook.com/facts/2000/YanaZorina.shtml http://hypertextbook.com/facts/2000/YanaZorina.shtml [2] http://www.wolframalpha.com/input/?i=0.5+*+%284000+*+0.454%29+*+%2862.6^2%29 http://www.wolframalpha.com/input/?i=0.5+*+%284000+*+0.454%2...

nonce42 13y ago
I didn't expect so much controversy here, I just wanted to get a feel for the order of magnitude. I went with 2000 pounds for the car weight, which is the weight of a 1976 Honda Accord - 1976 because of a search error on my part. 4000 pounds is the average new car or light truck, so that number is a bit high because of the trucks. In any case, the energy dumped into this experiment is like a big car crash concentrated into a tiny capsule. Anyone who has used a treadmill at the gym will recognize that it's a lot of work to burn off 400 calories. Not enough to get a car up to 140 MPH because the body isn't 100% efficient.

stox 13y ago

Of course, you're talking about the chemical energy in a Kit Kat bar. Were it completely converted to energy, it would produce around 5.8 Terra-watt years of power assuming a lossless conversion.