# The Naked Scientists Forum

### Author Topic: Would a bullet do as much damage if shot on a small planet?  (Read 4069 times)

#### Im a Geek on the Edge

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##### Would a bullet do as much damage if shot on a small planet?
« on: 09/02/2009 01:09:21 »
If I made a bullet out of expanded polystyrene (Styrofoam) and shot someone with it it probably would not do much damage. It simply would not have the weight and therefore not the inertia to create much of a wound. But what about if I was standing on a tiny planet with gravity so small that a conventional lead bullet weighed the same as the polystyrene one did on earth. Now the metal bullet is lightweight. Will it therefore do no great damage? I feel the answer has to be that the bullet on a small planet would do as much damage as it would do on Earth. It's obvious but I can't explain why as it seems counterintuative.

#### John Chapman

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##### Would a bullet do as much damage if shot on a small planet?
« Reply #1 on: 09/02/2009 01:12:08 »
Hi Geek

The reason you're struggling to explain the obvious is that you are mixing up 'weight' with 'mass'. Mass doesn't change under normal conditions and as long as we stay on this planet and travel at a relatively constant speed and direction neither does weight. Therefore our experience is that a certain mass will have a particular weight and so we learn that one is equivalent to the other. However, they are not the same thing. Mass is all about the number and size of the particles that make up the object whereas weight is the force of attraction between that object and, in this case, each of the two planets in either scenario. Inertia, ie the fact that an object will continue to move at the same speed & direction unless acted upon by another force, is proportional to mass not weight, although on this planet it amounts to much the same thing. However, on the smaller planet although the bullet would have less weight it would still have the same mass and consequently the same inertia. It would take the same amount of resistance to bring it to a halt and so would travel the same distance into the body before it stopped.

If inertia was linked to weight rather than mass then in space, where weight is more or less zero, the bullet wouldn't travel at all once it had moved far enough away from the casing to let the exhaust gas escape. It would probably travel out the end of the gun barrel and stop just in front of the gun. I think!
« Last Edit: 09/02/2009 01:29:35 by John Chapman »

#### lightarrow

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##### Would a bullet do as much damage if shot on a small planet?
« Reply #2 on: 09/02/2009 07:37:45 »
If I made a bullet out of expanded polystyrene (Styrofoam) and shot someone with it it probably would not do much damage. It simply would not have the weight and therefore not the inertia to create much of a wound. But what about if I was standing on a tiny planet with gravity so small that a conventional lead bullet weighed the same as the polystyrene one did on earth. Now the metal bullet is lightweight. Will it therefore do no great damage? I feel the answer has to be that the bullet on a small planet would do as much damage as it would do on Earth. It's obvious but I can't explain why as it seems counterintuative.

You and your enemy are inside a lift; suddenly, the lift's cables breaks and you fall down withought weight; you shoot at him and you don't make him anything?

#### Chemistry4me

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##### Would a bullet do as much damage if shot on a small planet?
« Reply #3 on: 09/02/2009 07:48:02 »
Sorry? I do not understand.

#### daveshorts

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##### Would a bullet do as much damage if shot on a small planet?
« Reply #4 on: 09/02/2009 10:14:58 »
Yes basically gravity doesn't affect the amount of energy or momentum your bullet has, and therefore how much damage it will do. The only effect it will have is on the trajectory of the bullet. So if you are aiming at something a long way away you would have to aim in a different place to hit it because the bullet would fall more slowly.

#### lightarrow

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##### Would a bullet do as much damage if shot on a small planet?
« Reply #5 on: 09/02/2009 14:52:19 »
Sorry? I do not understand.
In a falling lift there is no weight (Einstein).

Edit:
At the benefit of someone, the fact Einstein said it, would mean nothing if it wasn't at the same time universally recognized by physics.
« Last Edit: 09/02/2009 15:29:26 by lightarrow »

#### ScientificBoysClub

• Guest
##### Would a bullet do as much damage if shot on a small planet?
« Reply #6 on: 09/02/2009 15:07:33 »
If I made a bullet out of expanded polystyrene (Styrofoam) and shot someone with it it probably would not do much damage. It simply would not have the weight and therefore not the inertia to create much of a wound. But what about if I was standing on a tiny planet with gravity so small that a conventional lead bullet weighed the same as the polystyrene one did on earth. Now the metal bullet is lightweight. Will it therefore do no great damage? I feel the answer has to be that the bullet on a small planet would do as much damage as it would do on Earth. It's obvious but I can't explain why as it seems counterintuative.

well you need more velocity to attain momentum .... and you need more mass as much as density of Comet or Astroid .....
when bullet enters into atmosphere of a planet it gets heated up and evaporated due to air resistance !!

#### Im a Geek on the Edge

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##### Would a bullet do as much damage if shot on a small planet?
« Reply #7 on: 09/02/2009 22:13:58 »
Thank you John Chapman for clearly explaining the difference between mass and weight.

But ScientificBoysClub, what is the difference between speed and velocity?

#### RD

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##### Would a bullet do as much damage if shot on a small planet?
« Reply #8 on: 09/02/2009 23:06:03 »
Just to complicate matters ...

a planet smaller than Earth would have a less dense atmosphere than Earth: thinner air.
So the air resistance on the smaller planet would be less,
so the velocity of the bullet would not be slowed to the same extent by air as on Earth.

So the same type of bullet would do more damage on the smaller planet than on Earth as it would be travelling faster when it hit the target,  (if all other variables are constant).
« Last Edit: 09/02/2009 23:13:36 by RD »

#### The Naked Scientists Forum

##### Would a bullet do as much damage if shot on a small planet?
« Reply #8 on: 09/02/2009 23:06:03 »