3 ms·

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 a

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.

s1artibartfast 2y ago
you cant change reference frames mid problem to follow an object when calculating kinetic energy. The frame is the original velocity of car A. >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. If that were true, I could set a reference plan as a bullet in a gun, and when I fired it everyone in the world dies from rapid acceleration as their kinetic energy goes from 0 to 3000mph.
- teamonkey 2y ago
  You can compare frames of reference before and after collision and check the relative speeds of the entities involved.
  - s1artibartfast 2y ago
    Yes, and if you do that, The change in speed is always the same 50mph different. you change 50 if you hit a unmoving wall. IF you start at 0 or 100mph, you also change 50.
    - teamonkey 2y ago
      I don’t get the 50mph change; the change in velocity for each car is going to be 100mph, regardless of the frame of reference, no? Each car approaches the impact point at 50mph, each car recedes at 50 in the opposite direction after a perfect collision, so each has a dv of 100. And if my initial reference frame is car A, travelling at 50, then before impact car A appears to be travelling at 0 while car A is approaching at 100. After collision both cars experience a dv of 100, so car A appears to be receding at 100mph while B appears stationary. For the wall, the car approaches at 100 and recedes at 100 for a delta of 200mph. But in any case the velocity delta doesn’t affect the energy, it just has to be conserved. As I’ve said elsewhere we are trying to find the collision energy in the moment of impact from the perspective of car A (the question is whether you driving into a wall the same as you driving into another opposite car at half the speed). For that you have to consider a frame of reference relevant to car A, and not a stationary reference that is not following either car.

s1artibartfast 2y ago

you cant change reference frames mid problem to follow an object when calculating kinetic energy. The frame is the original velocity of car A. >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. If that were true, I could set a reference plan as a bullet in a gun, and when I fired it everyone in the world dies from rapid acceleration as their kinetic energy goes from 0 to 3000mph.

teamonkey 2y ago
You can compare frames of reference before and after collision and check the relative speeds of the entities involved.
- s1artibartfast 2y ago
  Yes, and if you do that, The change in speed is always the same 50mph different. you change 50 if you hit a unmoving wall. IF you start at 0 or 100mph, you also change 50.
  - teamonkey 2y ago
    I don’t get the 50mph change; the change in velocity for each car is going to be 100mph, regardless of the frame of reference, no? Each car approaches the impact point at 50mph, each car recedes at 50 in the opposite direction after a perfect collision, so each has a dv of 100. And if my initial reference frame is car A, travelling at 50, then before impact car A appears to be travelling at 0 while car A is approaching at 100. After collision both cars experience a dv of 100, so car A appears to be receding at 100mph while B appears stationary. For the wall, the car approaches at 100 and recedes at 100 for a delta of 200mph. But in any case the velocity delta doesn’t affect the energy, it just has to be conserved. As I’ve said elsewhere we are trying to find the collision energy in the moment of impact from the perspective of car A (the question is whether you driving into a wall the same as you driving into another opposite car at half the speed). For that you have to consider a frame of reference relevant to car A, and not a stationary reference that is not following either car.