In Iron Man 2, when Whiplash strikes the cars on the racetrack with his plasma weapon, why doesn't it cut into the racetrack asphalt, given it cuts through the cars so easily?
They do cut through the asphalt.
Throughout the scene, if you pay close attention, you can see that the whips have an effect on the asphalt.
Here, when Whiplash first reveals his whips, they burn the ground.
The arrows point to the darkened streaks where the whips burned the ground.
In this next still, we can see that there is some destruction to the asphalt between Tony's legs.
Notice the rocky appearance outside the whip mark? That's the crumbling asphalt.
Here's another shot of Whiplash doing nothing but damaging the track as he approaches Tony in a flurry of whipping.
You can see the scorch and cut marks easily, but I highlighted them with more arrows.
So why aren't there marks after he attacks the cars?
I think there are three very plausible reasons we don't see the whips make marks beneath the cars:
- Force needs to be applied to the asphalt in order to do more than scorch it. The crumbling effect seen between Tony's legs only happens when Whiplash forcefully strikes towards the ground. When using a whip, the force is applied only until the whip hits its "target". If that target is above something, such as the ground, then the whip will merely "fall" the rest of the way to the ground. Whiplash is aiming only low enough to ensure he cuts through the cars, but still high enough to not forcefully strike the ground.
- Although the whips cut through the cars like a hot knife through butter, it may be possible for Whiplash to feel some resistance as the whips slice through. Looking at the scene, it also appears that Whiplash pulls/curls the whips away immediately after they've cut through the cars (both the first car and Tony's). Whiplash is highly skilled at using his whips, and when he detects that they stop meeting resistance he retracts them (before they cut into the ground).
- The scene cuts very quickly from shot to shot, and the shots usually don't give visual coverage of the same angle or area twice. Since we know that the cuts do happen sometimes, we can assume that they happen every time. The cuts and camera angles change too quickly and inconsistently for us to get a good visual confirmation.