Can we accelerate in space ?

1. Sep 21, 2010

johncena

Imagine we are in a space craft which is moving with a constant velocity in empty space. If we are not influenced by any gravitational forces, and if we restart the engine, will it be accelerated ?

2. Sep 21, 2010

zgozvrm

I would hope so. Without acceleration, how would you be able to control your speed and direction? Without acceleration, it would be difficult to control our satellites. Also, docking with the space station would be a real pain.

3. Sep 21, 2010

johncena

How it can be accelerated without an external force ?

4. Sep 21, 2010

zgozvrm

I don't know. Ask someone at NASA. I just know that you can't change direction or speed without acceleration.

5. Sep 21, 2010

D H

Staff Emeritus
The rocket itself provides the external force by ejecting mass.

6. Sep 21, 2010

DaveC426913

The rocket-and-exhaust as a closed system will maintain its centre of mass. Exhaust goes left, rocket goes right. CoM remains in place.

7. Sep 22, 2010

Sakha

By conservation of momentum. If gas is ejected from the back with a momentum, the ship will feel an increase of the same mometum but to the other direction. And by definition, change in momentum over time is force. Suppose you are in space with some rocks in your pocket, if you throw those rocks backwards, you will start moving forward.

8. Sep 23, 2010

Drakkith

Staff Emeritus
It's the same principle as firing a gun. The propellant ignites, propelling the projectile down the barrel and sending force back along the weapon and eventually into the person holding it. Hence the "jerk" when you fire a gun.

A rocket engine works on the same principle, but instead of a projectile, it sends the spent fuel backwards to propel itself forwards.

9. Sep 24, 2010

Dcat

It s the momentum conservation (or motion quantity p=mv), and the same explanation why gun slide recoil.

The mas of gas that the jet engine blow out, multiplied for its velocity, must be the same of the mass of the space ship, multiplied for the space ship velocity.
mv=MV

You have two kind of jet reactor, esoreactor (typical planes), where you have that there is also an incoming mass of air, and endoreactor (rocket kind), where all the propellent is inside the rocket and there is no incoming mass. The first can't work in space, as there is no air.

You can also measure the speed force as the mass flow of ejected combusted gas and the gas velocity F=m'*v.