[pgustavson]

Originally Posted by lgKido

Sorry, but that's not the correct, and it's my duty to unravel 30 years of your inaccuracy (no offense intended).

If two objects are moving in parallel their relative velocity can be calculated:
For example, two cars are moving in the same direction along a road. Car A is traveling at +30 m/s and Car B is travelling at +20 m/s. Their relative velocity is 30 – 20 = +10 m/s.

If the two cars are moving in opposite directions, the velocity of Car A is +30 m/s and the velocity of Car B is –20 m/s. Their relative velocity is 30 –(–20) = 30 + 20 = +50 m/s.

So in the video, you have Object A (the car) moving at 120 kmh in one direction and Objedt B (the wall) moving at 0 kmh. Simple relative velocity of the pair is 120 - (-0) = 120 + 0 = 120 kmh........

Ok, I'm a civil engineer and studied relative velocities in Engineering Dynamics........

That’s all well and good, but the relative velocities w/r to one another is, well, irrelevant. The system has all the energy that it has regardless of how it’s distributed. In the case discussed above, making a number simplifying assumptions, a car hitting a wall at 120km/hr will experience the same kinetic energy transfer as if it hit another identical car also travelling at 120km/hr directly towards it. In the car/wall scenario 100% of the kinetic energy dissipation is carried by the car. In the car/car scenario, there is twice the energy in the system, but it is dissipated equally between the two cars. Again, this only holds if a lot of simplifying assumptions are made (e.g. no energy goes into heating the wall, both cars absolutely identical, no friction losses, etc….) This problem is a classic one that all physics students are introduced to; and many think that the 2-car scenario is somehow worse than the car vs wall because the relative velocity is twice as high.