# Question about momentum in vacuum of space

1. Jun 3, 2012

### kris_yo

If you were, say a person moving forward in space and threw something, would you still go backwards? I know that fuel would push you forward (like a rocket's propulsion system) and a fire extinguisher would also propel you in the opposite direction, but do you need a source of fuel for that to happen? In other words, if you had a bag of stuff and you (just your body, not in a vehicle of any sort) are already in motion in space, (going forward) could you slow yourself down by throwing the things in that bag in the opposite direction that you are moving?
Thanks!!

2. Jun 4, 2012

### Staff: Mentor

A person throwing something or pushing on something would experience an force that would accelerate them in the opposite direction. With enough force this could indeed cause them to go backwards. If you threw a ball in space it would be the "fuel" for your movement. (Of course in this specific case you would also start to rotate end over end as well) A rocket engine or thruster's purpose is to vent mass in one direction in order to accelerate the engine, and thereby the vehicle, forward, just like you are doing with the ball. Depending on your speed and the amount of stuff you had in your bag, you could indeed slow yourself down by throwing things.

3. Jun 5, 2012

### e^(i Pi)+1=0

If a 90kg man were to throw a 4.5 kg object at 3m/s he would be propelled backwards at .15m/s

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