Canonball!

[thecoach]

A train is travelling at exactly 20mph in a perfectly straight line, with a canon pointing directly upwards, all natural factors are obsolite.

The canon shoots a canonball 20 meter's in the air.

Where would the ball land?

[sebandbay]

Straight back into the cannon!

[Loosehead]

sebandbay is correct.

I'll field the predictable pedants.

He is correct if the atmosphere is removed, otherwise some wind resistance will cause the ball to miss the cannon narrowly. As far as the thought experiment is intended he is correct.

[Clanad]

Finally, Loosehead and I may agree on something... since the moving train is a closed inertial system for this discussion, a simple proof for the question is the example of a person jumping straight up in the proverbial moving train. The person jumping comes back down on the same spot, no? Discounting any air resistance, the same thing applies to the shot fired from the vertical cannon...

[ACB312]

Hang on, I'm a bit lost here. Once the cannon has shot the cannonball there is no further force propelling the cannonball in the same direction as the train. The train is still being pushed along at 20mph. So it seems to me that the cannonball would land somewhat behind the cannon. Is this being a bit simplistic? Please feel free to enlighten me.

[fatgaz]

i agree with acb312, the person jumping in the train actually is jumping forward to land back down in the same spot, much the same as throwing a ball and catching it in a moving car. So the cannon ball should come down somewhere behind the train

[mibn2cweus]

fatgaz, If this were the case when jumping on the equator we would have to jump forward at a speed of over 1000 mph, (the speed of the Earth's rotation at the equator), in order to land back at the same spot. As far as the cannon and ball are concerned they are not in motion relative to the constant velocity of the train. Once the train reaches a constant velocity the only energy required to continue at that velocity is to compensate for friction. For the same reason it takes a great distance to bring a fully loaded moving train to a stop.

[Hamish]

If all natural factors are obdolite than the ball never lands as not gravitational force is acting on it.

[JudgeJ]

Perhaps there is a simpler way to understand this. If the train were stationary, it must be agreed that the ball will fall back into the cannon. If the train is moving at 20mph, then the ball leaves the cannon with a forward velocity of 20mph. If it is accepted, for this argument, that air resistance is not a factor, then there is nothing to reduce this forward motion. The ball continues to move forward at 20mph, as does the train, so the ball returns to the cannon.

ABC312�s contention that there is no further force propelling the ball forward is quite correct, but similarly there is no force acting to reduce its forward speed either, so it continues forward at 20mph.

[Loosehead]

One word fatgaz, ABC, Relativity!

Viewed from the train the ball goes up then down, viewed from the ground the ball traces an arc through the sky from cannon back to cannon.

That Einstein geezer new his onions!

Nothing is actually stationary so all movement,speed etc is relative. How fast am I going typing this? Well 1000mph on the Earth, 60000mph orbiting the sun, relative to the desk 0!

[fatgaz]

thank-you for explaining that for me loosehead, I shall be sleeping well tonite lol