The epic fight over the tropical planet of Scarif in Rogue One remains one of my favorite battles in all of Star Wars. Since there is a small chance you’ve not yet seen the movie, I will give you a spoiler alert.
Still here? Excellent. Let me explain the scene. Two star destroyers wage a pitched battle against a whole bunch of rebel ships. Several Y-wing starfighters disable one of the star destroyers, and a hammerhead corvette shoves it into the other one. It’s a great move by the rebels, but I’m not here to discuss military tactics. I’m here to talk about physics, and this battle offers a great opportunity for that.
I will break it down into three parts: the collision, the debris trail, and the motion of the star destroyer.
The rebels in the hammerhead clearly braced for impact. The Imperial officers did not, and they careen across the bridge. It looks cool, but what would really happen?
First, let’s assume the star destroyer has a significantly greater mass than the hammerhead. I’d guess it is on the order of 100 times more massive. And mass matters. When the hammerhead strikes the star destroyer, it exerts a force on the destroyer. Because force changes the momentum of an object (where momentum is the product of mass and velocity), the star destroyer moves in the same direction as the hammerhead.
Forces are an interaction between two objects. This means that when the hammerhead pushes on the star destroyer, the star destroyer pushes back with the same magnitude force and thus the same change in momentum. But the same change in momentum does not mean the same change in velocity. Since the hammerhead (probably) has a smaller mass, it will experience a much greater change in velocity than the star destroyer. This means the impact should toss the hammerhead’s crew around even more than it tosses the Imperial guys around.
Maybe something else is going on. Perhaps the ships use an inertia field of some kind to prevent…