I’ve watched Guardians of the Galaxy Vol. 2 multiple times, but I never noticed this awesome physics scene before. Rocket, the cyborg raccoon, is being chased through a forest at night by Ravagers, and he sets a bunch of traps, including some sort of antigravity mine or repulsor device. When the pursuers approach, he hits the button and they fly up into the air, then tumble back down.
Of course, being Rocket, he can’t just do it once. We get this great view over the treetops, with Ravagers being tossed helplessly up and down, over and over. Boom: It’s a perfect scene for some video analysis. It’s like they made it just for physics classes.
Exoplanetary Motion
As always, we start by figuring out the forces. Once the dudes are beyond the influence of Rocket’s device (whatever it is), there is only one significant force acting on them: the gravitational interaction with the planet. It’s the same kind of downward tug that you feel on Earth as your weight.
OK, we know that on the surface of a planet, this gravitational force has a constant magnitude, equal to the local gravitational field (g) times an object’s mass (m). We also know that a constant force (F) causes objects to accelerate at a fixed rate, and the force equals the product of mass and acceleration (a). Putting those two things together, we get ma = mg:
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Illustration: Rhett Allain
Canceling out m, we find that the acceleration is equivalent to the gravitational field: a = g. (For that reason, it’s often called the “acceleration due to gravity.” I don’t like that term, since it implies the object has to be accelerating.) The point is, mass doesn’t enter into it. Big Ravagers, little Ravagers—they all accelerate downward at the same rate. On Earth that rate would be –9.8 meters/second2. But judging by the four moons in the night sky, this is not Earth!
Are we saying they fall at a constant velocity? No! Objects in free fall, with only gravity acting on them, speed up as they fall. But they speed up at a constant rate.
We can also plot position as a function of time. Starting from a certain height y0 and an initial velocity v0, we can write the relation between vertical position (y) and time (t) using this famous kinematics equation:
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