4 ms·

The total kinetic energy is not the same, it scales as v^2?

• WalterBright 2y ago
  The kinetic energy is relative to the reference frame being used.
• s1artibartfast 2y ago
  expanding on Walters response, each car has the same kinetic energy as if it hit a wall at 50mph. Together, they have the energy of two cars at 50 each hitting a wall, and this energy is split between the two cars. If somehow ALL of the kinetic energy was absorbed only by one car , it still wouldnt be equal to 100mph, do the the square. Together, two cars traveling at 50mph have the same energy as one car traveling at 70mph. The only case when two objects colliding at 50mph is like 100mph is if the final reference frame is still traveling at 50mph. a Car going 50pmh hitting a train going 50mph would be like the car hitting a wall at 100mph (net velocity change of 100mph.
  • LegionMammal978 2y ago
    If a car hit an actual brick wall, it would likely be closer to your 50 mph final reference frame scenario, with the wrecked car and wall remnants both moving near that speed. It would take some good anchoring for the ground to exert enough force on the wall to keep it immovable.
    • s1artibartfast 2y ago
      yes, but now we are just torturing the example. If I was in a car, I would brake or turn.
  • teamonkey 2y ago
    From the reference frame of car A, it is as if it is stationary and car B - or the wall - is travelling towards it at 100mph. The mass of the wall is required to determine which system has more energy. Or, take it from the perspective of car B or the wall. In both cases, car A is approaching with a relative speed of 100mph.
    • tbrownaw 2y ago
      The wall is indestructible and anchored to the earth, so it has effectively infinite mass.
      • teamonkey 2y ago
        An infinite mass is impossible. But in any case it just limits the usefulness of that frame. I believe the correct approach is to look at it from the perspective of the object that you in car A are striking. Taken from the perspective of the wall, car A strikes it at 100mph. From the perspective of car B, car A also strikes it with the same speed (and kinetic energy). IF the two cars have equal mass, this will also be the impact energy on car A. And that’s what we’re trying to find for both cases.
        • tbrownaw 2y ago
          > I believe the correct approach is to look at it from the perspective of the object that you in car A are striking. The most convenient reference frame for colliding objects is the combined center of mass of those objects. It isn't affected by the collision, and for perfectly inelastic collisions the objects will come to rest with respect to that frame. This makes the math a lot simpler that working in some other frame.
    • s1artibartfast 2y ago
      In the reference frame you propose, car A is traveling 50mph after the collision. It goes from sitting still to going backwards at 50pmh, for a net change of 50mph. The difference of velocity between cars 100mph in every reference frame you can pick. Each of the cars always experience a change of 50mph from the collision, no matter what reference frame you pick. People are confused because the pick a reference frame for car A, but don't hold it constant for their calculations, as required by physics.
      • teamonkey 2y ago
        No, if you’re taking car A as a reference point it behaves as if stationary before and after collision - it’s everything else that moves. The wall approaches at 100mph and, in a totally elastic collision, it recedes at 100mph, as does car B.