8 ms·

Nitpick: The bus is more dangerous because it has more momentum (m • v), not necessarily more force.

by qwyxzy 11y ago

waterhouse 11y ago
Double nitpick: I think kinetic energy is actually the more relevant quantity. Citation after a bit of googling: http://physics.stackexchange.com/questions/141779/what-causes-damage-kinetic-energy-or-momentum http://physics.stackexchange.com/questions/141779/what-cause...
- deleted 11y ago
  [deleted]
- pdkl95 11y ago
  This is correct. My grandfather was a physicist who reconstructed some of the nastier accidents in CA. He was always looking for two values for most situations: delta-v, and (mv^2)/2. The 'v' being the delta-v of the two vehicles when they collided. The damage done will generally be a function of the kinetic energy that hit it. Note that this implies that unless there are other dangerous road conditions (fog, ice, etc), the safest speed is "the same speed as everybody else" so the delta-v is minimized. Fun note for HN: he had software built for DOS that I helped him get running in dosbox so he could run it on a modern computer. It would reconstruct the motion of the vehicles from the final resting positions and the depth of the dents. working backwards from the implied kinetic energy. Apparently the DOS version was a port of his original FORTRAN source... on punch cards.
  - spectre256 11y ago
    Sounds like some software that belongs in the internet archive, if it's possible!
    - pdkl95 11y ago
      That's a good idea! I'll have to talk to my grandfather about it.
    - eru 11y ago
      And on something like github, for people to experiment with.
colanderman 11y ago
Actually neither of those matter much anyway. The energy transferred is primarily a function of the velocity difference (squared) and the mass of the lighter of the two objects. That is, a pedestrian will experience a collision with a car and with a bus in much the same way. If the two objects are close to the same mass, i.e. car vs. car, the energy transfer will be reduced by up to 75%, but otherwise the mass of the larger object is immaterial. That is, a car-car collision at 60 MPH does equal damage as a bus-car collision at 30 MPH.
- lultimouomo 11y ago
  This. Extreme example: a 1000 ton freight train "hitting" you at 0.1km/h it's not doing the same damage as a 1000kg car hitting you at 100km/h.
cowardlydragon 11y ago
Nitpick: isn't it the kinetic energy M x V x V that is dangerous as the square of velocity?

Nitpick: The bus is more dangerous because it has more momentum (m • v), not necessarily more force.

by qwyxzy 11y ago

Nitpick: The bus is more dangerous because it has more momentum (m • v), not necessarily more force.

Double nitpick: I think kinetic energy is actually the more relevant quantity. Citation after a bit of googling: http://physics.stackexchange.com/questions/141779/what-causes-damage-kinetic-energy-or-momentum http://physics.stackexchange.com/questions/141779/what-cause...

deleted 11y ago
[deleted]
pdkl95 11y ago
This is correct. My grandfather was a physicist who reconstructed some of the nastier accidents in CA. He was always looking for two values for most situations: delta-v, and (mv^2)/2. The 'v' being the delta-v of the two vehicles when they collided. The damage done will generally be a function of the kinetic energy that hit it. Note that this implies that unless there are other dangerous road conditions (fog, ice, etc), the safest speed is "the same speed as everybody else" so the delta-v is minimized. Fun note for HN: he had software built for DOS that I helped him get running in dosbox so he could run it on a modern computer. It would reconstruct the motion of the vehicles from the final resting positions and the depth of the dents. working backwards from the implied kinetic energy. Apparently the DOS version was a port of his original FORTRAN source... on punch cards.
- spectre256 11y ago
  Sounds like some software that belongs in the internet archive, if it's possible!
  - pdkl95 11y ago
    That's a good idea! I'll have to talk to my grandfather about it.
  - eru 11y ago
    And on something like github, for people to experiment with.

colanderman 11y ago

Actually neither of those matter much anyway. The energy transferred is primarily a function of the velocity difference (squared) and the mass of the lighter of the two objects. That is, a pedestrian will experience a collision with a car and with a bus in much the same way. If the two objects are close to the same mass, i.e. car vs. car, the energy transfer will be reduced by up to 75%, but otherwise the mass of the larger object is immaterial. That is, a car-car collision at 60 MPH does equal damage as a bus-car collision at 30 MPH.

lultimouomo 11y ago
This. Extreme example: a 1000 ton freight train "hitting" you at 0.1km/h it's not doing the same damage as a 1000kg car hitting you at 100km/h.

cowardlydragon 11y ago

Nitpick: isn't it the kinetic energy M x V x V that is dangerous as the square of velocity?