Question: What would happen if you shot a bullet with a gun, in space (not on the earth)?

Keith Siew answered on 17 Jun 2013:

Well Kelan I think you could work out what would happened by applying Newton’s laws of motion and thinking about the forces present/absent in space versus earth.

So we’ll start with the 3rd law “If body A exerts a force on body B, body B exerts an equal and opposite force on A”.

Modern bullets contain all the components in the shell to trigger an explosion (so the lack of oxygen in space won’t stop it firing)… so when the hammer hits the bullet it triggers a chemical reaction which generates a hot fast expanding cloud of gases in the barrel of the gun. This cloud is going to push against the bullet and the gun together forcing them away from each other… but on earth you’re being held on the spot by gravity and the friction of your shoes on the ground, so while the bullet flies out of the gun, you remain fairly stationary because you’re pushing back against the recoil of the gun. But in space you don’t have the gravity of Earth to anchor you or friction.. so the gas leaving the gun pushes against you and you against it forcing you apart… so you’d end up flying off in the opposite direction of the bullet you shot… probably doing somersaults!

Now if we consider Newton’s 2nd law “The acceleration (a) of a body is directly proportional to, and in the same direction as, the net force (F) acting on the body, and inversely proportional to its mass (m). Therefore F=ma” what does that mean…

Well it means that the because the bullet has a small mass the force of the explosion in the gun is going to cause a massive acceleration of the bullet and it will speed along… however because the mass of you and the gun is much greater, the acceleration will be much slower… so you’ll just sort of drift off slowly in the opposite direction you fired your bullet, while the bullet speeds along… well at the speed of a speeding bullet… haha

Finally if we think about Newton’s 1st law ” An object at rest will remain at rest unless acted upon by an external force. An object in motion remains in motion, and at a constant speed, unless acted upon by a force” what can we conclude.

Well a bullet shot on earth would be acted on by gravity which would slowly pull it towards the ground giving the bullet’s trajectory (direction) an arc. We also have to consider that the bullet has to pass through the air which will cause friction and slow the bullet down until it eventually crashes into the ground after its velocity (speed) no longer can overcome the force of gravity. So these forces are acting on the object in motion to reduce its velocity. But space is a vacuum so there is no air to provide friction and no gravity (not entirely true but if we’re assuming there are no nearby stars or planets then the microgravity would be minimal) to act on the bullet so in theory it would travel on at the same speed in a straight line pretty much forever until something stopped it!

(In reality this is not entirely correct because there are miniscule particles in space that fly off from nebulas or exploding stars that could cause tiny amounts of friction that would add up over time. Also light from stars would actually hit the bullet and give it a tiny nudge in another direction… its the whole idea behind solar sails being used to propel satellites across space… and finally as I mentioned before there is always some gravity… although it may be tiny it would eventually over time have an effect on the direction of the bullet or slow it down.)

Well there we go… so in a nut shell if you fire a gun in space… the bullet would shoot off in a straight line going on without any signs of stopping at the same speed. While you would be pushed backwards and sent floating in space… oh and let’s not forget… space is a vacuum… no one can hear you scream and they wouldn’t hear your gun shot either… no air for sound waves to travel in!