# Inertia in space?

1. Sep 29, 2014

### Prof Null

Imagine that you are in a spacecraft and you then open a hatch, of a type like the door of a Gemini capsule (it hinges outward). Does the action of the hatch opening outward then hitting a stop impart force to the ship?
Note that this is in deep space, that is, ignoring outside gravity fields and such.

2. Sep 29, 2014

### Matterwave

If the capsule is under no external forces and is not producing any thrust, then it's center of mass will move in a straight line at a constant speed. Opening the hatch changes the center of mass of the system a little bit, so the system will adjust accordingly; however, the center of mass velocity will remain constant.

3. Sep 29, 2014

### TumblingDice

Two of Newton's laws can help examine and explain why nothing more than what matterwave explained will occur. According to Newton's first law, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force. In this case there is no external force being applied, so there can be no change in velocity - no acceleration - when viewed from yours or any other inertial frame of reference.

According to Newton's third law, when one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body. So anything that you do while you remain onboard, whether you open a door, jump up and down, or throw heavy weights around, all forces you apply will be met with equal and opposite forces. The net result will be zero for the spaceship and its contents. The ship will not spin, or wobble, or anything as all forces sum to zero for the system.

The only way to have an effect on the space ship would be to throw something out the hatch after you open it. :w In this case, the force you apply to throw something out of the ship would result in an equal force throwing you (and the ship) in the opposite direcion. The mass of the object you throw doesn't matter as much as the force - how "hard" you throw it. The mass will have some contribution to the net result,. A greater mass will contribute more to the net result because the ship has that much less mass to accelerate when the object leaves.

4. Sep 29, 2014

### Bandersnatch

It should get some angular displacement though, shouldn't it? That is, it should spin while the hatch is moving from the closed to open positions so that the angular momentum can be conserved.

5. Sep 29, 2014

### TumblingDice

Rutt-rho! I don't know that answer for sure. I do understand what you're speaking towards, and thinking this would be true if the ship was spinning to begin with. (EDIT: The rate of rotation would decrease to conserve angular momentum.) And spinning can be detected locally. But if it has no angular momentum to begin with, why would that change?

Last edited: Sep 29, 2014
6. Sep 29, 2014

### Bandersnatch

It doesn't matter what the angular momentum was. The point is that at any time during the opening of the hatch it must be the same - whether it was zero or non-zero to begin with.
The motion of the hatch has got both radial and tangential components that vary in magnitude as it swings on its hinges. The hatch moving tangentially w/r to the centre of mass has got some angular momentum that must be compensated by the rest of the capsule rotating the other way.

7. Sep 29, 2014

### TumblingDice

Ah! Thanks, Bandersnatch! Please check me here if I'm following correctly. So the capsule will go through a rotational motion while the hatch is being opened, and will stop when hatch open is complete. Since this is only a reorientation of the system, it doesn't matter how much force is applied to open the hatch or the time duration the force is applied. The net rotational motion would be the same. Also, when the hatch is closed, the ship will return to its original orientation.

I'm going to have to re-wrap some things in my head. I couldn't imagine movement occurring in my first reply without some type of acceleration, and no acceleration w/o external force.

8. Sep 29, 2014

### Bandersnatch

Yeah, that's about right. I wouldn't say "net rotation motion would be the same" but "angular displacement would be the same", since the angular velocity of the rotation would differ depending on how much torque you apply. Kicking the hatch open would rotate the capsule faster than gently pushing it open.

9. Sep 29, 2014

### TumblingDice

<sigh> I've been posting during that period when I can't sleep at 1am and login to PF on my tablet. :w Just took a break on the patio and maybe it was the cool air that helped. I recalled how gyroscopes were used to adust or stabilize pitch, yaw and attitude of a craft. Not a "light bulb" moment, it felt more like a "kick myself in the butt" moment.

I think the OP has been answered. Can I push the thread further and posit, if the astronaut "spun up" a gyro, s/he could use it to reorient the craft about its CoM by applying force (torque?) to manipulate the gyro?

10. Sep 29, 2014

### Bandersnatch

Sure. He could even reorient the craft by walking along the perimeter. Or spinning his arm while holding to the craft.
Reorientation using conservation of angular momentum is the same idea used by cats when falling, or by astronauts as shown here at about 25:00
http://techtv.mit.edu/collections/l...cle-smarts-stability-translation-and-rotation.

11. Sep 29, 2014

### TumblingDice

You've helped me to understand the larger picture well enough to view the OP of opening the hatch as a trivial example. I should have remembered orientation can be changed w/o external force, but this exercise will definitely help keep the neurons stickier. Thanks!

12. Oct 9, 2014

### Chronos

A good way of looking at this is the Machian principle. Inertia is relative to the gravitational field of distant bodies in the universe.

13. Oct 10, 2014

### BobG

Here:

In other words, yes - except if it were capable of doing what you suggest to a spacecraft, it would be called a reaction wheel or a momentum wheel (momentum wheels have a bias speed to supply gyroscopic stability and reaction wheels don't - the wheels themselves are identical if you wanted to buy one on the internet).

"Gyroscopes" are obviously the same thing, but very small (intentionally so). They sense a change in orientation instead of cause one. Obviously same principle, though.

14. Oct 10, 2014

### TumblingDice

Thanks, Bob! So the significant difference between reaction/momentum wheels and gyroscopes is purposeful size. Large to influence, and small to measure. Is that right? Reminds me of a Navy friend who once told me how to tell a ship from a boat. "You can put a boat on a ship, but you can't put a ship on a boat." ;)