Bullets and Gravity

[ysnnim]

On the BBC home page, which I glance at before starting work, this caught my eye. it was in the QI section...

'A bullet fired horizontally from a gun, and a second bullet dropped by hand from the same height as the gun, will both hit the ground at the same time. The forward motion of an object has no effect on the pull of gravity.'

Ran the idea through my head... and whilst I am no genius or scientist (and do not have any conveyor belts handy), it just doesn't seem 'right'. Surely the fired bullet has two forces being applied, both the force from the 'firing' of the bullet, in the horizontal direction and the gravitational one. Just struck me as a mildly interesting fact...

(Can't really demo it in the kitchen, as the wife keeps getting in the way... if she buggers off, will report back findings)

Back to work now...

[TonyHetherington]

ysnnim said:

Surely the fired bullet has two forces being applied, both the force from the 'firing' of the bullet, in the horizontal direction and the gravitational one.

Consider only the vertical forces; i.e. gravity. Both bullets have exactly the same acting upon them.

[Mattt]

Read up on vectors

[GravelBen]

Off the top of my head I think it works - just the fired bullet also travels horizontally as it falls. Of course it has to be fired exactly horizontally (in the perpendicular to gravity sense) for this to apply, any up or down in the firing angle will result in an additional vertical force being applied.

[Justayellowbadge]

Curvature of the Earth may have a tiny impact as 'horizontal' is a tangent...

[Russ35]

This is another subject that has been tested on the Mythbusters TV show. Both hit the ground at the same time.

[WorAl]

Don't forget this must be performed with bullets of the same weight. So if you are going to experiment you have to take the tip out of the cartridge if using a real bullet. Air rifle probably better to use.

Also took me a while to get my head around it but 'tis true.

[sinizter]

WorAl said:

Don't forget this must be performed with bullets of the same weight. So if you are going to experiment you have to take the tip out of the cartridge if using a real bullet. Air rifle probably better to use.

Also took me a while to get my head around it but 'tis true.

Does not need to be the same weight.

[toasty]

Just to be the devil's advocate, apparently not.

http://scienceblogs.com/dotphysics/2009/10/mythbus...

[FasterFreddy]

Justayellowbadge said:

Curvature of the Earth may have a tiny impact as 'horizontal' is a tangent...

I don't think so, because the gravitational force is equal along the trajectory of the bullet, regardless of how far it travels.

[GravelBen]

FasterFreddy said:

Justayellowbadge said:

Curvature of the Earth may have a tiny impact as 'horizontal' is a tangent...

I don't think so, because the gravitational force is equal along the trajectory of the bullet, regardless of how far it travels.

Unless we start taking into account gravitational variations and deviation of the vertical.

[JD123d]

Would rifling (as in the rotation of the bullet to improve accuracy) have an effect. Sure that the above conclusion is correct.

[king arthur]

FasterFreddy said:

Justayellowbadge said:

Curvature of the Earth may have a tiny impact as 'horizontal' is a tangent...

I don't think so, because the gravitational force is equal along the trajectory of the bullet, regardless of how far it travels.

Actually if you fired the bullet from far enough above the ground, it would go into orbit.

[Strangely Brown]

Already proved to be correct by those Mythbuster chappies.

http://www.youtube.com/watch?v=D9wQVIEdKh8

[WorAl]

sinizter said:

WorAl said:

Don't forget this must be performed with bullets of the same weight. So if you are going to experiment you have to take the tip out of the cartridge if using a real bullet. Air rifle probably better to use.

Also took me a while to get my head around it but 'tis true.

Does not need to be the same weight.

Yes it does as a heavier object reached terminal velocity sooner.

Unless you're being pedantic and were actually meaning mass.

[AJS-]

That's a nice thing to learn. Surprising, but I can see the sense behind it.

[WorAl]

Strangely Brown said:

Already proved to be correct by those Mythbuster chappies.

http://www.youtube.com/watch?v=D9wQVIEdKh8

How accurate was the gun though? i.e. was it actually perfectly level with the ground? the time between impacts was so small a tiny variation in the trajectory could have made all the difference.

Edited by WorAl on Tuesday 1st June 10:56

[Buzz word]

In terms of pure vectors I would tend to agree. However I think it depends if any of the constraints change on firing. Whilst in a vaccum or on the moon for example dropping hammer and feather the statement may be true. I think with the atmosphere included other factors could come into play. If the bullet changes shape on firing it will have an effect. If the bullet by spinning generates lift somehow it will affect the time in flight. All these things will change how long the object will take to fall on earth. In a vaccum not at all.

[mattdaniels]

Did you never do the experiment in science lessons at school using two tennis balls? One rolled off the side of the desk the other dropped? Both hit the floor at the same time.

[SystemParanoia]

http://galileoandeinstein.physics.virginia.edu/mor...

Newtonian mountain

edit :

EVERYTHNIG falls at 9.81 m/s, the only variable is how much velocity you give it in the horizontal direction.

at hypersonic velocities and beyond, you may want that bullet to be made from tungston if you dont want it to melt into nothing lol

Edited by SystemParanoia on Tuesday 1st June 11:18