# In zero gravity!

If spacecraft moving in a stable speed or acceleration suddenly stopped would an astronaut continue moving forward (such as a man in a train) ?

• One of the most important ideas in physics is that the laws describing how things work in one place should be the same laws that describe how things work in every place. The explanation of what happens to passengers in a train crash uses the word "momentum," not "gravity." The explanation for what happens to passengers in a spaceship crash should be no different. (except for the speed and energy, which may be several orders of magnitude greater in the spaceship case.) – Solomon Slow Jun 26 at 18:15
• @safesphere, It's not an answer if it doesn't explain "momentum." I'm too lazy for that. – Solomon Slow Jun 27 at 15:24

“Zero gravity” is something that doesn't exist. Maybe you mean the situation of freely orbiting spacecraft, in which cosmonauts don't feel any attraction as they are orbiting together with the spacecraft.

Yes, the astronaut will continue to move forward, with all tragic consequences for him.

Note:

In the case of stable or unstable acceleration the astronaut will feel the attraction (in the opposite direction of the acceleration), even if spacecraft don't stop.

• A different way of saying the same thing is: You can't feel a uniform gravitational field. What you feel here on Earth that you don't feel on board the space station is the contact force that the Earth exerts on the soles of your feet as it stops you from following the path that gravity and momentum want you to follow. On board the ISS, nothing prevents you from following that path. – Solomon Slow Jun 26 at 18:20
• Why do you say the astronaut will feel the attraction. If he's strapped into a seat he will feel the straps pulling against him, and if he is floating inside the spacecraft he will feel the splat when he hits the wall, but he doesn't feel any "attraction". – Hot Licks Jun 27 at 2:01

Yes, the astronaut will move forward. The Law of Inertia of Newton holds good both in presence or absence of Gravity.