# What happens if you jump on a "spin ship"?

If you're on a spin ship, or rotating wheel space station, and you jump, then you're no longer being accelerated by the rotation. What would happen?

First, I'll establish some terms to make discussing this easier. I've also popped in an image courtesy of 2001: A Space Odyssey to help with the visualization. Not the most realistic movie, but the visuals are helpful. Moving at or near the spinning surface in the direction of spin is spinward, moving up away from the surface toward the center of rotation is up, and moving down toward the surface away from the center of rotation is down.

If you jumped up, intuitively it seems you would keep your horizontal velocity and keep moving spinward until your feet reconnected with the surface. But if the radius of rotation is large, the surface would be very nearly flat and barely curved, so considering you wouldn't be "falling" back down toward the surface it seems you could coast floating just above the surface for quite a while before finally landing when the curvature catches up with you. In other words, "gravity" for objects in mid-jump would be lower than that for objects on the surface. Is that accurate? What would jumping on a spin ship actually be like?

• "Not the most realistic movie..." Really, in what way? Can you cite an example, I'm curious! Did Kubrick take care to get the strength of artificial gravity correct in “2001: A Space Odyssey”?
– uhoh
Jan 11 '21 at 3:19
• Great question by the way! In addition to remembering that "gravity" decreases as one moves "up" towards the center of the hub though, we have to remember that our tangential or forward velocity doesn't decrease. My hunch is that our astronaut would land "forward" of were they jumped.
– uhoh
Jan 11 '21 at 3:30
• Keep on mind that drum with larger diameter would need higher surface speed for the same amount of artificial gravity (angular speed will be lower, but not "translational" speed of the surface). So lower curvature is intuitively compensated by higher forward speed you are approaching it after jump. For the rest of physics, check Coriolis force. Jan 11 '21 at 8:00
• Does this answer your question? Creating your own artificial gravity by running. (Part 1 - the basic idea) Jan 11 '21 at 13:33
• @CarlWitthoft It's helpful for general information, but my question specifically concerns what happens when you jump, which is not discussed at all in that other question, though they do mention jumping as a form of exercise. So it doesn't answer the question, but I really appreciate the information! Jan 11 '21 at 14:54

## 3 Answers

We must remember that there is no spoon gravity here and that as soon as our astronaut is no longer in contact with the floor they must have an essentially straight line trajectory.

Below is a simulation in an inertial frame moving towards Jupiter along with Discovery 1. Let's assume that the astronaut is initiall standing and has a tangential velocity equal to the $$\omega r_0$$ where $$\omega$$ is $$2 \pi / T$$ where the period of rotation $$T$$ is 20 seconds and $$r_0$$ is 5.3 meters from @OrganicMarble's SciFi SE answer to Did Kubrick take care to get the strength of artificial gravity correct in “2001: A Space Odyssey”?

If they jump "up" i.e. towards the center of rotation with a radial velocity of -1.2 m/s (I estimate 2.6 m/s is maximum possible) they will "land" about 3 seconds later, at about 1.7 meters in front of where they started, and almost flat on their face!

They should either have their arms extended ahead of time to break their "fall", or jump with what on earth would feel like a partial backflip in order to land "upright".

update: I wondered what happens if one jumps harder. Here are the trajectories for several jump velocities in integer steps of 1.2 m/s (the 1.2 comes from my first attempt at a 3 second jump).

Counterintuitively at first, in this regime it seems the harder and "higher" you jump the faster you "come down" again!

import numpy as np
import matplotlib.pyplot as plt
from scipy.integrate import solve_ivp

def deriv(t, X):
g0 = 9.80665 # m/s^2 exactly https://en.wikipedia.org/wiki/Standard_gravity
x, v = X.reshape(2, -1)
acc = -g0 * np.array([0, 1]) * 0 # times zero because there is no gravity!
return np.hstack((v, acc))

halfpi, pi, twopi = [f * np.pi for f in (0.5, 1, 2)]

r0 = 5.3 # meters = 35 feet/2  https://scifi.stackexchange.com/a/239904/51174
T = 20. # seconds https://scifi.stackexchange.com/a/239904/51174
omega = twopi / T # s^-1

v_tan = omega * r0

v_jumps = 1.2 * np.arange(1, 6)
X0s = [np.array([0, -r0, v_tan, v_jump]) for v_jump in v_jumps] # 2.6 m/s max from https://space.stackexchange.com/a/31729/12102

times = np.linspace(0, 3, 301)
t_span = times[[0, -1]] # first and last

answers = []
for X0 in X0s:
answer = solve_ivp(deriv, t_span, X0, t_eval=times)
answers.append(answer)

fig, (ax1, ax2) = plt.subplots(1, 2)

for answer in answers:
x, y, vx, vy = answer.y

cos, sin = [f(-omega * times) for f in (np.cos, np.sin)] # rotate backwards
xr, yr = x * cos - y * sin, y * cos + x * sin # rotating frame

xc, yc = [r0 * f(np.linspace(0, twopi, 201)) for f in (np.cos, np.sin)]

ax1.plot(xc, yc, '-k')
ax1.plot(x, y)
ax1.plot(x[::100], y[::100], 'o')
ax1.text(x[-1]+0.4, y[-1], 't=3.0 sec', fontsize=12)
ax1.text(x[-1]+0.4, y[-1], 't=3.0 sec', fontsize=12)
xo, yo = [r0 * f(omega*times[::100] - halfpi) for f in (np.cos, np.sin)]
ax1.plot(xo, yo, 'ok')
ax1.text(xo[-1]+0.4, yo[-1], 't=3.0 sec', fontsize=12)

ax1.plot([0], [0], 'ok')
ax1.set_aspect('equal')
ax1.set_title('r0 = 5.3 m, T_rot = 20 sec, v_jump = n times 1.2 m/s')

ax2.plot(xc, yc, '-k')
ax2.plot(xr, yr)
ax2.plot(xr[::100], yr[::100], 'o')
ax2.text(xr[-1]+0.4, yr[-1], 't=3.0 sec', fontsize=12)

ax2.plot([0], [0], 'ok')
ax2.set_aspect('equal')
ax2.set_title('rotating frame')

plt.show()

• Fantastic! And what happens to the distance travelled and air time as you increase the ship radius? Jan 11 '21 at 17:55
• @TheEnvironmentalist What's not show here is for very small velocity jumps, air time also decreases, for 0.24 m/s jump velocity (1/5 of 1.2 m/s) is only 0.89 seconds. So in (this estimation of) Discovery-1's artificial gravity 3 seconds at 1.2 m/s is the maximum air time. A larger ship at the same level of simulated gravity would probably require a higher radial jump velocity to maximize air time. I'll bet there is an analytical solution out there somewhere that solves for $t_{max}$ as a function of $g_{sim}$ and $r_0$ and that might be the basis of a new reduced-gravity-sports question.
– uhoh
Jan 11 '21 at 23:12
• For an ancient (well, 60 year old) illustration, see "Frames of Reference" at the 17:00 time hack This is a clear version : aeon.co/videos/… Jan 12 '21 at 4:48
• @DJohnM I love it, thank you for that bit of retro :-)
– uhoh
Jan 12 '21 at 7:39

On a sufficiently large spin ship the result would be indistinguishable from normal gravity, at least to human perception.

The idea behind a spin ship is simple: instead of gravity constantly pulling your body downward towards the ground as would happen on a planet, the ship rotates such that the floor is being constantly pulled "upward" towards you by the structure of the ship. This constant pull on the floor towards the ship's center is called centripetal force, and it's what prevents the floor from simply falling apart and flying off into space in a straight line, as would happen if the floor was not connected to the rest of the ship.

Like any other force, centripetal force causes the objects it pulls on (in this case, the floor beneath your feet) to accelerate. In the case of centripetal force this is known as centripetal acceleration, and on a ship designed for Earth-like gravity the ship will need to spin fast enough that the floor is being pulled towards the center of the ship at a rate of ~9.8m/s^2, the same rate at which objects accelerate towards the ground on Earth.

If you jump on a spin ship, you'll no longer be touching the floor and so will continue moving in a straight line "spinward". The floor however, will continue to accelerate towards the center of the ship as the station spins, making it feel as though you are "falling" back to the ground at a rate of 9.8m/s2, just as you would on Earth. This illusion of gravitational force is commonly known as centrifugal force1.

On a smaller ship (such as the one in 2001: A Space Odyssey) the illusion isn't perfect. As other answers have pointed out, there are additional factors at play in a spin ship which would lead to very noticeable, counterintuitive behavior for the occupants of such a vessel. Notably, varying "gravity" (centrifugal force) depending on altitude and your speed of travel in the direction of spin, plus Coriolis force2. However, the larger the ship's diameter the smaller and less noticeable those unwanted effects become on a human scale. Centrifugal force is proportional to radius (F = m*ω^2*r), so a large diameter spin ship doesn't need to spin as fast (in terms of angular velocity) to maintain Earth-like gravity, and small differences in altitude and speed don't matter as much when the ship is comparatively large. On a sufficiently large spin ship (a couple kilometers in diameter or so), these effects would be nearly imperceptible on a human scale, and jumping would feel identical to how it does on Earth.

1Not to be confused with the centripetal force discussed earlier. Centrifugal force is a fictitious but handy force that is used when we are looking at things from a rotating perspective (as a passenger on a spin ship would) but want to think of it as if we were stationary. Centripetal force is a real force that keeps the floor of our spin ship connected to the ship's center as it spins.

2Coriolis force is another fictitious force which would be felt when you move "up" or "down" in the spin ship. On larger ships it would be less noticeable at human scales, as the distances involved would be smaller compared to the radius of the ship.

• "floor is accelerated towards the center of the ship" ? Does it ever get there? Jan 11 '21 at 22:02
• I've made a small edit to add a link or two. "floor is accelerated towards the center of the ship" is suboptimal. For myself, I was never able to figure out how to choose the right words to talk about what's happening in a rotating frame, so I always "hide" in an as-inertial-as-possible frame and language thereof. Great answer, +1!
– uhoh
Jan 11 '21 at 23:07
• @uhoh When I talked about the floor accelerating I actually wasn't taking about centrifugal acceleration. The floor has to be accelerating in an inertial reference frame, otherwise it would just continue off in a straight line and the spin ship would fall apart. Trying to think of a better way to phrase that... Jan 12 '21 at 0:12
• Yes, the radial spokes are constantly pulling each floor segment towards the center, otherwise they would go in a straight line. If we put a spring scale in each spoke we'd certainly measure a force; that's real, not fictitious. Hmm...
– uhoh
Jan 12 '21 at 0:40
• Great edit, +1! Jan 16 '21 at 16:14

Look at this image, the sparks move away tangentially from the grinding wheel.

This movement is valid for the spin ship too, as long as it is possible inside the spinning ship. The jumping astronaut will move in tangential direction until he hits the floor again. I assume the circumference speed of the ship is much faster than the running speed of the astronaut.

• What would that mean for the actual movement of the astronaut? What would the body of the person inside the ship actually do? Jan 11 '21 at 2:46
• Have a look to the excellent simulation by uhoh. The left sides of image 1 and image 2 shows the tangential movement.
– Uwe
Jan 11 '21 at 10:42
• @TheEnvironmentalist that question can't be answered until you tell us what frame of reference you wan the answer in. In one perfectly valid frame the astronaut remains stationary (or at least their centre of mass does) from the moment they lose contact with the floor until the moment they hit the floor again. The ship is moving and spinning around them, Jan 16 '21 at 15:56