[nyc640]

If we're talking cars hitting you at 80mph, sure you're probably dead no matter what. But a 9,000 lb car hitting you at 35mph is traveling with the same momentum as a 3,000 lb car going 105mph. You might survive a 3,000 lb car hitting you at 35 mph, you're unlikely to survive the other.

[mwint]

This feels... wrong, somehow - we wouldn't say that a 100,000lb shuttle transporter hitting you at 1mph will do the same damage as your 3,000lb car at 35mph. There's way more dynamics at play here.

[Rury]

It is kind of wrong. What matters is the force impacted upon the person (their acceleration), not momentum. Basically, change in velocity over time is what really matters, but because momentum is conserved, it influences what the total change in velocity is.

[bentcorner]

I agree. If I jump up and down and impact the earth with my feet it's quite harmless. I suspect above some lower bound it doesn't matter how much something weighs, it's all about how fast something hits you (because it's mostly you that's being accelerated).

Although the weight (and correspondingly, the size) of a car does play into sight lines, maneuverability and how fast the car can decelerate.