So, when Gina hit Dave with her car, Dave flew through the air. Yet when Nr. 2 wants Dave to take a taxi but can't get one, he simply grabs hold of one and stops it.
I know that Dave is a spaceship, but if "he" flies through the air when getting hit by a car, how is he powerfull and heavy enough to stop a car by grabbing it?
It is explained by the Engineer that the collision with the car was unexpected, so their "impact shields" weren't activated. Opposite to that when No. 12 stops the cab he had given orders to the crew to be prepared. Of course it can be argued that while their shield were on, the energy levels were very low - or, as is being said in the film, "their power is nearly depleted." Also, you have to consider that the car's momentum was probably higher than that of the cab. ("I've never been caught speeding. In New York. This year.")
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