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Physics Question: If I'm on a hypothetical train going 100 miles an hour... Answered

Royalesteel recently posted a question in the forums asking if it would be possible to use a K'Nex gun to shoot a GPS tracker onto a moving truck in front of his car. For a moment, I thought that this would be impossible, as you would need a gun that can shoot faster than the speed of the car, but this isn't the case, is it?

Say you're on a hypothetical train that goes 100 miles an hour, and you have a hypothetical gun that shoots a bullet at 100 miles an hour. At first, I thought that this would mean that the bullet shot from the gun would be at a standstill (it would seem that way to you, I mean), but I think that it would really shoot away from you at 100 miles an hour. Total, it would be traveling at 200 mph, I guess.

So, ignoring wind resistance, all you would need is a gun that can shoot for a distance longer than the distance that the cars would travel in the amount of time that the projectile would need to get to the forward car, right?


I don't feel like hand-crafting a unique thanks to all of you individually, so here we go: Thanks.

It all depends on frame of reference.... If this train is directly in front of you (so you are trying to tag it from behind) -- and you are traveling at the same velocity (that is, speed and direction) -- you can "pretend" that you're not moving at all to calculate how fast your projectile must move. We can "pretend" because we're assuming wind resistance is 0. Now - if you're shooting from the side.... How fast is the train moving to the side (left or right relative to it's forward direction)? That should be 0mph (assuming no rocking motion). So you can then calculate how fast a projectile must go to reach it's destination that is not moving away from your :D Does that make any sense? It's all about your reference frame (where you place your coordinate system) :D And when it comes to physics, solid mechanics, dynamics, statics all of that math/engineering - you, as the problem designer/solver, can set your coordinate system wherever you want. You can even make your coordinate system move if you want. An example of a moving coordinate system/reference is a roller coaster. If you want to see how many "G's" passenger is going to experience, you can make your coordinate system move just like the roller coaster car :D But I digress...

Oh - I forgot one case... If you're in front of this train - both you and the train moving at the same velocity.... and you shoot a projectile at the same speed in the backwards direction (at the train).... you're projectile is not moving anywhere with respect to the ground BUT it is moving at the same velocity (but opposite direction) as both you and the train :D

While there's been some interesting discussion of the physics involved, and some interesting tangents, nobody's realy given a quick, easy to understand answer, so I'll give it a try. Basically, if you've got this truck your trying to tag in front of you, and the car your in is travelling at the same velocity, then all you need (ignoring wind resistance) is a gun that has a range greater than the distance between the two vehicles. This is NOT the distance they will travel relative to the ground, simply the distance from the gun to the rear bumper of the truck. So say the truck is 40 ft. in front of you and your traveling at 50MPH. To hit the truck, you would need a gun with a range of 40 ft. or more. Now once you start living in the real world where you can't ignore wind resistance, you'll need significantly more power.

Ah see this gets more interesting when we start talking about light, as einstein did, if your on the front of a lightbeam, traveling at c (speed of light) and you hold a mirror out in front of you, do u ever see a reflection???? Not that a train is a light beam... but still its more interesting this way, in answer than yes its all relative as said above... v simple physics, u should google an A level physics site, that would be a good idea... just to avoid confusion...

W/ref this particular truck-tagging idea, like pissing into the wind, you need a fair bit of power not to be hit in the face... Momentum has to be considered also, i.e the kinetic energy of your projectile.

If you shoot in front of you, it will have to go faster then the car in front of you. If you do it next to you, then it just needs to be fast enough to hit the car, before the car is too far forward. And of course if you shoot it backwards, it just needs to be in the air long enough to hit the car. The point is...once the bullet is outside the car, it would drop straight down. If you shoot it 20 mph, and your going 30mph in the car, it will go 20mph

The point is...once the bullet is outside the car, it would drop straight down. If you shoot it 20 mph, and your going 30mph in the car, it will go 20mph

If you shoot it 20 mph, and your car is going 30 mph, the bullet will go 50 mph.

if its in the car...it wont continue going the same speed it was once it's out of the car. If you drop a penny out the window, it wont keep going the same speed as you, it will drop straight down. The reason it doesn't go to the back of the car when inside, is that the air mass is moving at that speed.

He said ignoring wind resistence. A objecct a rest will stay at rest unless acted upon by another force. Ignoring wind resistence the penny will drop at the same speed as you outside the car. (Physics 101).

I repeat,
If you shoot it 20 mph, and your car is going 30 mph, the bullet will go 50 mph.

oh, didnt notice the ignoring wind resistance part. I was talking as if i were actually going to do that.


11 years ago

Ah. Relativity. CLASSICAL relativity; no Einstein involved. Somewhere around transformation of equations of motion for a rotating frame of reference was when I realized I wasn't cut out to be a physics major. :-(

A problem with all this theory is that "air resistance" is hard to ignore at higher speeds, since drag tends to go up proportional to V2 rather than linearly.