Physics Question 2

lurrrvs2sp00ge

its me again with a similar question. if i am standing at the back of the train facing the back of the train and all the previous variables apply, would the bullet come flying out of the gun or would it simply drop to the ground with the speeds cancled out. since it already has movement one way and that exact speed is applied in the opposite direction then shouldnt it just stay where it left the gun

Gurgle

Simple vector maths - it falls to the ground.

lurrrvs2sp00ge

hmh not quite the teaster like the last one, pretty much just put up and shot down lol

If you assume that the muzzle velocity is 100km/h, and that the train is moving at 100km/h, then I don't know. My instincts tell me that it would just fall to the ground, and all that I know tell me that it would. But, what would happen to the bullets energy? It can't just "disappear". Relative to the ground it's momentum is zero, therefore relative to the ground it has no kenetic energy. So I suppose it would fall. I think.

Professor_Fink

lurrrvs2sp00ge wrote: »

hmh not quite the teaster like the last one, pretty much just put up and shot down lol

The last one was exactly the same maths, and equally as simple. People just panic when given an easy question. You should see what happens in PhD vivas!

Podge_irl

-JammyDodger- wrote: »

If you assume that the muzzle velocity is 100km/h, and that the train is moving at 100km/h, then I don't know. My instincts tell me that it would just fall to the ground, and all that I know tell me that it would. But, what would happen to the bullets energy? It can't just "disappear". Relative to the ground it's momentum is zero, therefore relative to the ground it has no kenetic energy.

The energy of the system is conserved, i.e. the gun, bullet and train. The bullet originally had kinetic energy due to it moving along with the train, it has no kinetic energy once it has been fired as it will drop to the ground. When the bullet is fired out of the gun it will speed up the train (and the gun) ever so slightly - its pretty much the same principle as rocket engines work on.

JoeB-

But would a rifled bullet still 'rifle'? (i.e, spin for stability) ?

Or does the spinning effect also need a wind rushing over the bullet, usually caused by the bullet moving relative to the air... or is the rifling within the barrel enough?

If we set up a high speed camera at the track side what would the bullet look like before it falls, would it be spinning on the spot?

That's an interesting question.

The rifling on a bullet is to provide air induced stability. This really only occurs at high speeds, i.e. ~500-1000m/s. The train in this scenario is only travelling at ~100m/s, thus, the bullet leaving the train will only be travelling at that speed. So, the rifling wouldn't cause an effect. But, if the train was doing ~1km/s, and you carried out the same experiment, I presume the rifling would stabilize the bullet - as the bullet is travelling at 1km/s relative to the air.

Gurgle

JoeBallantine wrote: »

But would a rifled bullet still 'rifle'? (i.e, spin for stability) ?

Yes, the spin is given by the barrell which is stationary relative to the bullet.