# How velocity builds up in space

1. Oct 14, 2008

### lpbug

According to Newton's first law of motion: "An object at rest will remain at rest and an object at motion will remain at a constant velocity unless acted upon by an external force."
I was just wondering if this law applies to space; since there are no friction in air and little to no gravity, I was just wondering if velocity could build up in space. For example, I shot a rocket into outer space and the more velocity I add to it the faster it goes and even if I take out the source of propulsion, it'll still travel as fast as I set it without ever slowing down.

2. Oct 15, 2008

### rcgldr

If you include the spent fuel expelled by the rocket's engine as part of the rocket's mass, there are no external forces, and momentum is conservered. However chemical energy is converted into kinetic energy, most of this into the fuel which ends up with a very high velocity with respect to the rocket engine.

3. Oct 15, 2008

### Naty1

In general, Newton's laws apply in space as here on earth. gravity, friction, and other forces may vary, but the LAWS remain intact.

"I was just wondering if this law applies to space; since there are no friction in air and little to no gravity, I was just wondering if velocity could build up in space..."

Velocity can build in space due to F = MA and V=AT as a body approaches a planet for example...where gravity exerts a force on the body and the body accelerates. This effect can be ustilized to good effect in space probes where the "slingshot effect" can be used to speed up vehicles.

"For example, I shot a rocket into outer space and the more velocity I add to it the faster it goes and even if I take out the source of propulsion, it'll still travel as fast as I set it without ever slowing down."

If you assume no friction in space then one view is that there are no forces to slow it down. In fact space is curved by gravity in different places,more so near large masses like stars, so over time the rocket will turn and follow gravitational curves a bit,,,if you ignore these, it will retain a constant fixed "straiht line" velocity.

Once the rocket engine is turned off in free space, no friction and no other forces, the rocket will continue at the velocity achieved...it won't slow down nor speed up because there are no additional forces acting....restart the rocket engine...apply a force... acceleration ensures and velocity increases...

4. Oct 15, 2008

### LURCH

As a matter of fact, in the extremely low friction environmentthe two parts of this law become one. If you put yourself in the Rockets place, with no nearby objects against which to compare yourself, you will find that you're able to tell the difference between "accelerating" and "not accelerating," but the difference between "moving very fast at a constant velocity," "moving very slowly at a constant velocity," and "sitting still" will require you to ask, "compared to what?"

So, moving at a constant velocity literally is remaining at rest.

5. Oct 15, 2008

### lpbug

Ok... well, so if i keep on exerting force upon the "rocket" I can achieve a build-up in the rocket's velocity right? So it'll keep on going faster and faster.... is that correct?

6. Oct 16, 2008

### LURCH

Yes, that is correct. Without wind resistance, there is no "terminal velocity." As long as you keep pushing, the rocket (or any object) will keep accelerating.

7. Oct 16, 2008

### Raap

A semi-related question: Is it possible to get something to stand completely still out in space? I'm not talking about on the lowest level, I know there'll always be some movement there, but larger objects. Would there be anyway to tell if an object didn't move at all, considering everything else in space is moving? Some way to measure an object's kinetic energy perhaps?

8. Oct 16, 2008

### Staff: Mentor

Since velocity can only be measured with respect to other objects, so too, kinetic energy.

9. Oct 16, 2008

### lpbug

wait but it builds up right? I mean it will buld up like:
i start out 20Mph and slowly i will accelerate to 100mph and keep adding up right?

10. Oct 16, 2008

### Hootenanny

Staff Emeritus
Hmm, yes.

I think I can see where your going with this...

11. Oct 16, 2008

### Staff: Mentor

...the rules that work at low speed start breaking down at high speed. Special Relativity takes over for Newtonian mechanics.