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Suppose I'm in a rocket, accelerating away from the Earth. While I'm accelerating away from the Earth in the rocket, I can feel the force of acceleration. Now, let's suppose we take away the Earth, and therefore, it's gravity, along with all other high-mass objects that exert a visible attraction on objects. Will I still feel the force of acceleration with any other forces acting upon the rocket?

Here are some pictures that convey what I'm trying to say:

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If we take away the Earth's gravity, and the gravity of all celestial bodies, will the force of acceleration in the rocket still be felt?

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  • $\begingroup$ One of the things I've discovered since I started teaching physics is that language matters. When you say "the force of acceleration", what do you mean? I imagine that you have a mental picture that you're trying to put into words, but the phrase you've chosen is not part of the standard lexicon, so I'm left guessing at the meaning you intend. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Nov 4, 2015 at 1:50
  • $\begingroup$ @dmckee I've added a picture. I hope it helps a little. $\endgroup$
    – Ethan Bierlein
    Commented Nov 4, 2015 at 1:59

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Let's discuss a whole set of cases.

  • You're standing on the planet. I'm guessing that you would describe the sensation as 'feeling' gravity pulling you down.

  • You're on a skydiving trip, standing in the plane. You still 'feel' gravity.

  • You're on the skydiving trip and you've just step out of the plane, but not had time to pick up speed. Here you don't 'feel' any gravity.

  • Later on, you deploy you chute and come to a rather modest terminal velocity. Now you 'feel' gravity again.

  • You've trained very hard, been selected as an astronaut and you've hanging out in the International Space Station. No 'feeling' of gravity.

  • Get in your Mars bound ship and turn on the rockets. 'Feel' gravity.

  • Still in that Mars bound ship, you've completed transfer orbit insertion and the rockets have turned off. No 'feeling' of gravity.

  • Arrived at Mars' orbit, with the planet coming up fast from behind. The rockets come back on to achieve match with the planet. The rockets come on and the 'feeling' of gravity returns.

So which three cases have no 'feeling' of gravity? Just stepped out of a plane, in free orbit around the Earth and in free orbit between the planets.

Two things that can't explain the full set of cases.

  1. It's not whether or not you are subject to a gravitational force from the planet. That happens in both the sky diving and ISS cases.

  2. It can't be an acceleration relative the planet. You have that when stepping out of the plane and in the ISS (no 'feeling'), but also in the rocket on transfer insertion and matching with the destination ('feeling')

But what all the case where you 'feel' gravity have in common is that a material substance is pushing on you. On earth it is the ground (or floor). In the plane it is the floor of the plane. In the accelerating rocket it is the deck or more likely your crash couch. In the chute it is the harness.

And the cases when you don't 'feel' acceleration is when you are in free fall.

So the thing that causes the 'feeling' of gravity is the normal force between you and a material object.

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Do I need to have a force acting in an opposite direction to feel the force of acceleration?

If the rocket accelerates and you are inside you will always feel a force, same as the force of gravitation, acting on you even if the rocket if far from the earth and any celestial body.

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  • $\begingroup$ Thank you for your answer. I've edited my question though. I'm wondering what happens if the gravity of all celestial bodies are removed. $\endgroup$
    – Ethan Bierlein
    Commented Nov 4, 2015 at 2:16
  • $\begingroup$ @EthanBierlein: the answer remains the same. Even in the absence of any gravitational bodies you'll feel that force. $\endgroup$
    – Gert
    Commented Nov 4, 2015 at 2:24
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If we take away the Earth's gravity, and the gravity of all celestial bodies, will the force of acceleration in the rocket still be felt?

Yes, you'll still feel the acceleration and you can demonstrate this even on Earth and in your car.

Car accelerating.

As your car accelerates at a rate $a$, you experience a force $F=ma$ with $m$ your mass. Of course you also experience gravity ($mg$) but that force is perpendicular to the first one and so $F$ is not affected by gravity. As long as your car accelerates the back of your seat will always push you in the same sense of the acceleration.

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