# Thread: propulsion system and 3rd Newton's law

1. Is it possible to push the space ship forward not using the propulsion system based on the 3rd Newton's law? Not using magnetic fields and other complicated devices. I am thinking about something simple. Are you able to convert the power of your muscles to push it forward like you do riding your bicycle? When people talk about the space voyages they always mention propulsion systems which need matter to be thrown in one direction to push the ship into oposite direction. What do you think?

Tomasz

2.

3. I think that any propulsion system that ignores Newton's laws is doomed to failure.

4. Originally Posted by ztomasz
Is it possible to push the space ship forward not using the propulsion system based on the 3rd Newton's law? Not using magnetic fields and other complicated devices. I am thinking about something simple. Are you able to convert the power of your muscles to push it forward like you do riding your bicycle? When people talk about the space voyages they always mention propulsion systems which need matter to be thrown in one direction to push the ship into oposite direction. What do you think?

Tomasz
It does not matter how the prooulsion system is designed, it will conform to conservation of momentum. Bicycles conform. So do rockets. In fact, if they did not they would not work.

You can also make a propulsion system with light, rather than something with positive rest mass. But it will still conform to conservation of momentum. So would a system based on magnetic fields, or anything else.

5. Originally Posted by ztomasz
Is it possible to push the space ship forward not using the propulsion system based on the 3rd Newton's law? Not using magnetic fields and other complicated devices. I am thinking about something simple. Are you able to convert the power of your muscles to push it forward like you do riding your bicycle?

Tomasz
But even riding your bike relies on Newtons 3rd law. As you start to pedal, your wheels push backward on the Earth, throwing it backward. Of course, since the Earth is some 10^23 times more massive than you are, its movement backwards is really, really really, small.

6. So slowly moving a piston from one part of the ship and stopping it quickly at the other will not induce a net motion. (?)

7. Originally Posted by KALSTER
So slowly moving a piston from one part of the ship and stopping it quickly at the other will not induce a net motion. (?)
If you trow a baseball from the front of the ship to your friend at the back of the ship, the ship will start to drift forward while the ball is moving, but it will stop when the guy at the back of the ship catches it. So there will be motion, but ultimately no net velocity change. If you want to keep moving, your friend at the back of the ship will need to open a window or something so that the ball can fly out of the ship.

8. Originally Posted by KALSTER
So slowly moving a piston from one part of the ship and stopping it quickly at the other will not induce a net motion. (?)
Nope. You might be thinking about how this works if you are on some wheeled device. You shift your weight one way slowly and then jerk quickly in the other direction, and thus creep along the floor.

This effect is caused by the difference between static and rolling friction. When you move slowly, the force isn't enough to overcome the static friction, but when you jerk in the other, it is enough to get you past the static friction, and once moving you are dealing with the smaller rolling friction, so the net friction is smaller in one direction than the other. You are still using the Earth to push yourself forward.

9. Originally Posted by KALSTER
So slowly moving a piston from one part of the ship and stopping it quickly at the other will not induce a net motion. (?)
No. More generally the center of mass of a closed system never accelerates in any inertial reference frame. That is because the forces in a closed system always sum to zero.

Even with a rocket, say in a vacuum to simplify the discussion, the center of mass of the rocket body together with the exhaust gasses does not accelerate. In most naturally chosen inertial reference frames it is stationary.

10. Three answers, all of them good; thanks. Yeah that is what I was thinking too. I just remember always trying to get around the fact (while I was in early high school) by inventing all sorts of intricate slowdown arrangements. One involved firing a rocket within an enclosure spanning the whole length of the ship, which then slams into the other side and imparts a net motion. None of these work because of the reasons you cited. Moving the piston from one side to the other imparts the same net momentum, since it is a smaller force applied over a longer time period. The impulse is the same, but opposite.

The reference frame problem DrRocket talks about is interesting to me. Let's say you float in space with a bucket full of water, start spinning it around and then let go. The bucket will take off on a perpendicular trajectory relative to the line formed by your arms, but then will you take off in an opposite trajectory to the bucket forming a right angled Z? I'd think so.

11. Originally Posted by KALSTER
Three answers, all of them good; thanks. Yeah that is what I was thinking too. I just remember always trying to get around the fact (while I was in early high school) by inventing all sorts of intricate slowdown arrangements. One involved firing a rocket within an enclosure spanning the whole length of the ship, which then slams into the other side and imparts a net motion. None of these work because of the reasons you cited. Moving the piston from one side to the other imparts the same net momentum, since it is a smaller force applied over a longer time period. The impulse is the same, but opposite.

The reference frame problem DrRocket talks about is interesting to me. Let's say you float in space with a bucket full of water, start spinning it around and then let go. The bucket will take off on a perpendicular trajectory relative to the line formed by your arms, but then will you take off in an opposite trajectory to the bucket forming a right angled Z? I'd think so.
Yes that is correct. You will. It is all handled through conservation of momentum (a vector so it takes care of directional questions as well as magnitudes).

12. Do you have any idea how to convert momentum of the rotating mass into linear movement of the body. Do you think its possible?

13. Originally Posted by ztomasz
Do you have any idea how to convert momentum of the rotating mass into linear movement of the body. Do you think its possible?
A rotating mass does not have any net momentum. The momentum of the particles on one side of the axis is cancelled by the opposite momentum of the particles on the other side. Therefore, no, it's not possible unless you can get some traction on another object and propel it in a direction opposite to your direction of travel.

There are two propulsion methods that I know of that do not require ejecting matter from the space ship. Momentum is still conserved. They are the solar sail,
http://en.wikipedia.org/wiki/Solar_sail
and the slingshot effect.
http://en.wikipedia.org/wiki/Gravity_assist

14. Thank you for all the answers. I expected most of them. If they were different most probably we would have bases on Mars. Anyways some time ago I had this brilliant idea about the propulsion system which could be powered by any form of mechanical energy, doesnt need the atmosphere and tons of fuel - just electricity which we know how to produce even in deep space. I am not a scientist, not even an engineer but I have basic knowledge of physics and math and some basic imagination. For the time being I have no access to any garage and tools and was thinking to use some kind of software which could simulate the laws of physics and would allow me to build the virtual prototype. Have you heard anything about such a software? What do you recommend?

15. Theoretically, giving out photons in a single direction could provide a velocity in the opposite direction. This could be accomplished by use of mirrors.

The momentum of the photons is incredibly tiny, though, so this is not feasible.

16. Originally Posted by drowsy turtle
Theoretically, giving out photons in a single direction could provide a velocity in the opposite direction. This could be accomplished by use of mirrors.
The momentum of the photons is incredibly tiny, though, so this is not feasible.
Proponents of solar sails, as per Harold's link, would disagree. You could either use the sun, or direct a powerful laser at the sail. (Refer to The Mote in God's Eye by Niven and Pournelle for an example.)

17. OP Obviously needs a basic physics text

18. Originally Posted by ztomasz
Do you have any idea how to convert momentum of the rotating mass into linear movement of the body. Do you think its possible?
You can convert the energy to linear movement, but it is not a direct conversion of momentum.

All that is needed to convert the energy to linear movement is something to react the forces. That is the basic principle behind a flywheel.

More simply, dig a hula hoop (remember those) out of the attic. Throw it out with a backwards spin and it will return to you -- and what happens is that the energy stored in the rotation of the hoop is converted to linar motion due to the reaction force available due to friction.

Analysis of the conversion process involves conservation of energy rather than conservation of momentum, because as noted the net linear momemtum of a spinning body is zero, so you must have application of some outside force in order for the end state to have non-zero momentum.

In an ideal case in which there is no slippage of the hula hoop, the original energy stored in the hula hoop will be converted to kinetic energy of translation less the residual energy of rotation of the hula hoop at whatever speed corresponds to the final rolling motion.

 Bookmarks
##### Bookmarks
 Posting Permissions
 You may not post new threads You may not post replies You may not post attachments You may not edit your posts   BB code is On Smilies are On [IMG] code is On [VIDEO] code is On HTML code is Off Trackbacks are Off Pingbacks are Off Refbacks are On Terms of Use Agreement