5 ms·

I agree that momentum should be the prime consideration. Especially as it relates to heavy trucks. Those guys shouldn’t be allowed to go over 55.

tzs 4y ago
Is momentum what actually matters? A Ford F-150 at 45 MPH has less momentum than a typical locomotive has at 1 MPH, but I'd sure rather be hit by the latter.
- nemo44x 4y ago
  Velocity is squared so yes that part matters more. Your extreme example is an edge case, sure, but not realistic. But overall, momentum is the force applied.
  - yibg 4y ago
    That doesn’t sound right. A 1mph train hitting you won’t kill you, it’ll just push you along. The sudden acceleration of all or parts of your body is important.
    - nemo44x 4y ago
      Stand there for a few seconds or minutes. It’s what glaciers did. They travelled far slower than 1mph and took out mountains. I don’t make the physics, I just live with them.
      - paulryanrogers 4y ago
        Right but you assume the person would be stupid enough to plant their feet and get slowly dragged under the train. Reaction time drops sharply as MPH increases.
      - yibg 4y ago
        Unlike mountains, people are not nailed to the ground waiting to be carved out, they move when they get pushed by a slow but massive object. Only a tiny amount of the train's momentum will get transferred to the person, roughly 1mph x mass of the person worth.
  - ironmagma 4y ago
    p = mv, so in momentum velocity is simply linear. It's in KE = 1/2 * m * v^2 that has the square (kinetic energy).
dreamcompiler 4y ago
Part of the reason I like driving on the Autostrada in Italy is that trucks are speed-restricted. Their speed limit is posted on the back of the truck itself so it's easy for anybody to see if the truck is speeding, and I'm led to believe that the penalty for speeding trucks is pretty severe. This may also be true in other countries; I don't know. It's not the case in the US and I wish it were.
FabHK 4y ago
Why momentum and not energy? I'd say the latter. It is energy that deforms your body. So, reduce the speed by 15%, reduce energy by 28% (2*15% to first order, or 1-(1-15%)^2 exactly). Or, to put it differently, if the vehicle has twice the mass, speed should be reduced by 30%.

tzs 4y ago

Is momentum what actually matters? A Ford F-150 at 45 MPH has less momentum than a typical locomotive has at 1 MPH, but I'd sure rather be hit by the latter.

nemo44x 4y ago
Velocity is squared so yes that part matters more. Your extreme example is an edge case, sure, but not realistic. But overall, momentum is the force applied.
- yibg 4y ago
  That doesn’t sound right. A 1mph train hitting you won’t kill you, it’ll just push you along. The sudden acceleration of all or parts of your body is important.
  - nemo44x 4y ago
    Stand there for a few seconds or minutes. It’s what glaciers did. They travelled far slower than 1mph and took out mountains. I don’t make the physics, I just live with them.
    - paulryanrogers 4y ago
      Right but you assume the person would be stupid enough to plant their feet and get slowly dragged under the train. Reaction time drops sharply as MPH increases.
    - yibg 4y ago
      Unlike mountains, people are not nailed to the ground waiting to be carved out, they move when they get pushed by a slow but massive object. Only a tiny amount of the train's momentum will get transferred to the person, roughly 1mph x mass of the person worth.
- ironmagma 4y ago
  p = mv, so in momentum velocity is simply linear. It's in KE = 1/2 * m * v^2 that has the square (kinetic energy).

dreamcompiler 4y ago

Part of the reason I like driving on the Autostrada in Italy is that trucks are speed-restricted. Their speed limit is posted on the back of the truck itself so it's easy for anybody to see if the truck is speeding, and I'm led to believe that the penalty for speeding trucks is pretty severe. This may also be true in other countries; I don't know. It's not the case in the US and I wish it were.

FabHK 4y ago

Why momentum and not energy? I'd say the latter. It is energy that deforms your body. So, reduce the speed by 15%, reduce energy by 28% (2*15% to first order, or 1-(1-15%)^2 exactly). Or, to put it differently, if the vehicle has twice the mass, speed should be reduced by 30%.