/sci/ - Science & Math » Thread #7611405

[48 / 1 / ?]

36KiB, 1024x683, cannon.jpg

Anonymous Fri 23 Oct 2015 19:18:32 No.7611405 View Reply Report Delete

Quoted By: >>7611434 >>7611515 >>7611520 >>7611578 >>7611663 >>7611716

Hey guys I need some help with a debate me (an idiot) and my buddy (an idiot) are having.

So imagine you have a small cannon barrel and you are shooting a ball out of it. Now the first time you shoot it, it is just sat on a box and when fired it sends the ball forward and the barrel backwards. The second time you fire it with it solidly backed, and when fired the barrel doesn't move.

Wîll the second shot go further or will it not matter?

Anonymous

Anonymous Fri 23 Oct 2015 19:20:41 No.7611410 Report Delete

Quoted By: >>7611444

I think it will go farther the second time but I have no qualifications, or any form of reasoning.

Anonymous

Anonymous Fri 23 Oct 2015 19:29:37 No.7611434 Report Delete

Quoted By: >>7611444

>>7611405
I think it'll go further. We're assuming that this "solidly backed" thing is an object with infinite inertia, yeah?

Anonymous

Anonymous Fri 23 Oct 2015 19:32:56 No.7611443 Report Delete

Quoted By: >>7611523 >>7611559

The second shot will definitely go farther. Conservation of energy.

Anonymous

Anonymous Fri 23 Oct 2015 19:33:04 No.7611444 Report Delete

Quoted By: >>7611449 >>7611457

>>7611410
>>7611434
Really?
I was thinking it would go the same distance, maybe I'm a dumbass
I don't understand why the cannons lack of backward movement should make the ball travel further

Anonymous

Anonymous Fri 23 Oct 2015 19:36:07 No.7611449 Report Delete

Quoted By: >>7611456 >>7611490 >>7611526 >>7611928

>>7611444
There is a constant amount of energy released from detonating the gunpowder. In the first case, that energy is split between moving the cannon and moving the ball. In the second case, the cannon can not move, so all the energy goes into moving the ball.

It might help to imagine what would happen if the cannon had almost 0 mass. In the first case, the ball would remain perfectly still (until gravity dragged it down), and the cannon itself would shoot backward. In the second case, the ball would still be fired forward.

Anonymous

Anonymous Fri 23 Oct 2015 19:39:25 No.7611456 Report Delete

Quoted By: >>7611526

>>7611449
ok
I was thinking the backward energy just gets absorbed by the wall or something

Anonymous

Anonymous Fri 23 Oct 2015 19:39:40 No.7611457 Report Delete

Quoted By: >>7611490 >>7611526

>>7611444
Because for the backward movement to be totally suppressed, whatever is holding it back must be part of a perfectly elastic collision.

Anonymous

Anonymous Fri 23 Oct 2015 19:52:33 No.7611490 Report Delete

Quoted By: >>7611502

>>7611449
>>7611457
off topic but this got me thinking about the physics class I dropped out of
we learned that even if you press your finger to the wall, on an atomic level you're not really touching it
in this instance what exactly is between your finger and the wall?

Anonymous

Anonymous Fri 23 Oct 2015 19:58:50 No.7611502 Report Delete

Quoted By: >>7611510

>>7611490
Nothing but electromagnetic forces keep you from actually touching the wall.

Anonymous

Anonymous Fri 23 Oct 2015 20:01:44 No.7611508 Report Delete

Quoted By:

I hope >>7609560 didn't take those replies seriously.

Anonymous

Anonymous Fri 23 Oct 2015 20:02:00 No.7611510 Report Delete

Quoted By: >>7611530

>>7611502
I don't understand
If I'm not touching the wall there must be something inbetween the wall and my finger right?
Is there air?

Anonymous

Anonymous Fri 23 Oct 2015 20:02:29 No.7611511 Report Delete

Quoted By:

Nope

you just moved the earth a fractional amount rather than the cannon

Anonymous

Anonymous Fri 23 Oct 2015 20:04:06 No.7611515 Report Delete

Quoted By:

>>7611405
newton's third law obviously

Anonymous

Anonymous Fri 23 Oct 2015 20:06:39 No.7611520 Report Delete

Quoted By:

>>7611405
You basically want to know if recoil reduces the power of a gun. Ask /k/

Anonymous

Anonymous Fri 23 Oct 2015 20:09:19 No.7611523 Report Delete

Quoted By:

>>7611443
>Conservation of energy.
... actually conservation of momentum, but you're on the right track. If the cannon is free to move, then it gets half the total momentum, and the ball gets the other half. If the cannon is constrained from moving, then the ball gets all the momentum.

Anonymous

Anonymous Fri 23 Oct 2015 20:10:50 No.7611526 Report Delete

Quoted By: >>7611563

>>7611449
>>7611457
Just realized y'all are retards
>>7611456
is actually exactly what happens

Anonymous

Anonymous Fri 23 Oct 2015 20:12:45 No.7611530 Report Delete

Quoted By: >>7611539

>>7611510
Think of it this way, everything has a field around it. Its a very weak field only detectable near the surface of the object but its there and it repels other fields.

When you touch things you are actually touching the field and feeling it repelling your field.

There are situations where this repulsion can be overcome but its not easy and believe me you wouldn't want the nuclei of the atoms in your hand touching the nuclei of the atoms in the wall.

Anonymous

Anonymous Fri 23 Oct 2015 20:15:46 No.7611539 Report Delete

Quoted By:

>>7611530
yeah I'm aware of the explosion bit
I don't understand this whole field business, but what I'm taking away is between the finger and the wall there is only field
I guess I'll look into it if I'm bored

Anonymous

Anonymous Fri 23 Oct 2015 20:26:36 No.7611559 Report Delete

Quoted By: >>7611582

>>7611443
>conservation of energy
>non conservative force doing work
go back to physics 101 m8

Anonymous

Anonymous Fri 23 Oct 2015 20:31:33 No.7611563 Report Delete

Quoted By:

>>7611526
>y'all are retards
thanks fer sharin' yer opinion, Tex

Anonymous

Anonymous Fri 23 Oct 2015 20:39:39 No.7611578 Report Delete

Quoted By: >>7611600

>>7611405
it wont matter because the recoil in both scenarios is the same. In the first scenario, the cannon gains momentum in the opposite direction of the cannonball's momentum, which means the cannon has a velocity in the opposite direction of the cannonball. Now, because the box has friction with respect to the cannon and with respect to the earth, the velocity of the cannon with respect to the earth will decrease to nothing because the friction will send momentum from the cannon to the earth until their motions sync up. The same goes for the well backed cannon, but instead of using friction to facilitate the transfer of momentum, a normal force is used to facilitate the transfer of momentum.

Anonymous

Anonymous Fri 23 Oct 2015 20:40:40 No.7611582 Report Delete

Quoted By:

>>7611559
What force here would be path dependent though? (ignoring friction from barrel)

Anonymous

Anonymous Fri 23 Oct 2015 20:44:23 No.7611585 Report Delete

Quoted By:

It will go further in the second shot because no energy is being lost to the cannons rearward movement.

Anonymous

Anonymous Fri 23 Oct 2015 20:56:29 No.7611600 Report Delete

Quoted By: >>7611604 >>7611641

>>7611578
also, if you take your frame of reference to be the earth, before the cannon is fired in either example, the kinetic energy and momentum of the cannonball is 0 and the kinetic energy and momentum of the cannon is 0 because each of their velocities is 0 compared to the earth. However, two different laws apply to the moment the cannonball is fired. The conservation of momentum and the conservation of energy. The conservation of momentum forces the cannonball to have the same magnitude of momentum as the earth and cannon system (of course, the direction is opposite), but it does not limit how high the magnitude of momentum can be. That is where conservation of energy comes in, with that and the earth frame of reference, only one possible pair of velocities is acceptable for the cannon and earth system and cannonball, so, yes, the recoil is the same no matter how the cannon is slowed down.

Anonymous

Anonymous Fri 23 Oct 2015 20:59:15 No.7611604 Report Delete

Quoted By:

>>7611600
I meant that the frame of reference is the earth ONLY before the cannonball is fired. The frame of reference should not travel as the earth itself gains momentum.

Anonymous

Anonymous Fri 23 Oct 2015 21:09:14 No.7611615 Report Delete

Quoted By: >>7611624

You have a mass-less spring with potential energy e when fully compressed and a block of mass m.
What is the velocity of the blocks in each scenario?

(1) The spring is fully compressed between two blocks, each of mass m and then released.
(2) One end of the spring is attached to a solid wall, fully compressed between the wall and a block and released.

(1) $\sqrt{\frac{e}{m}}$
(2) $\sqrt{\frac{2e}{m}}$

Anonymous

Anonymous Fri 23 Oct 2015 21:13:31 No.7611624 Report Delete

Quoted By:

>>7611615
I should say speed, rather.

Anonymous

Anonymous Fri 23 Oct 2015 21:21:31 No.7611639 Report Delete

Quoted By: >>7611641 >>7611651 >>7611654

The block and the cannon would then participate in a theoretically perfectly inelastic collision right? In such a collision energy from the eart cannon system would be lost as heat into the block?

Anonymous

Anonymous Fri 23 Oct 2015 21:23:05 No.7611641 Report Delete

Quoted By:

>>7611639
that was meant for >>7611600

Anonymous

Anonymous Fri 23 Oct 2015 21:26:10 No.7611651 Report Delete

Quoted By: >>7611657

>>7611639
Where is the collision? The cannon is sliding during the first shot.

Anonymous

Anonymous Fri 23 Oct 2015 21:27:26 No.7611654 Report Delete

Quoted By: >>7611658

>>7611639
true, but the point is that the momentum and energy in the cannon and earth system has only a single solution no matter if the cannon is on a box, or if it is stopped by a wall.

Anonymous

Anonymous Fri 23 Oct 2015 21:28:47 No.7611657 Report Delete

Quoted By: >>7611665

>>7611651
I meant in the second situation

Anonymous

Anonymous Fri 23 Oct 2015 21:28:53 No.7611658 Report Delete

Quoted By:

>>7611654
(I meant that the single solution is the same solution for the box scenario and the wall scenario)

Anonymous

Anonymous Fri 23 Oct 2015 21:31:22 No.7611663 Report Delete

Quoted By:

>>7611405
force wave travels back into the wall, bounces off and back into canon. The canon shakes or gets hot or deforms/cracks. If the shaking happens before the ball leaves the barrel, it will affect the trajectory.

Anonymous

Anonymous Fri 23 Oct 2015 21:32:42 No.7611665 Report Delete

Quoted By:

>>7611657
No collision. Assume the cannon is solidly affixed to the earth and there is no strain in any of the components. Actually, it doesn't fucking matter.

Anonymous

Anonymous Fri 23 Oct 2015 21:40:57 No.7611692 Report Delete

Quoted By: >>7611708

Assuming the cannon isn't touched, cannonball A will be farther from the cannon than cannonball B because the cannon will roll, but will not be farther from the point where the ball was shot.

Anonymous

Anonymous Fri 23 Oct 2015 21:48:37 No.7611708 Report Delete

Quoted By:

>>7611692
Prove it.

Anonymous

Anonymous Fri 23 Oct 2015 21:51:38 No.7611716 Report Delete

Quoted By: >>7611836

>>7611405
The second shot will go just as far (to some small uncertainty).

The momentum that the cannon received without the support is now passed to the support (so more or less it's transferred to the earth unless you're on a ship or something). The explosion just puts an equal force on the ball and cannon, and will not be changed by the support.

Anonymous

Anonymous Fri 23 Oct 2015 22:59:22 No.7611836 Report Delete

Quoted By: >>7611843

>>7611716
Let T be the time it takes for the cannonball to exit the barrel of the cannon.
Let P and Vol be the pressure and volume, respectively, in the barrel at time t.
At some time t<T, assume that the cannonballs in both shots have accelerated to the same velocity.
The relative velocity of the cannon and ball in shot 1 is $v_{ball}+v_{cannon}$ . In shot 2 the velocity of the cannon is negligibly small, so it's just $v_{ball}$
That means the distance traveled down the bore is greater in shot 1 because
$t \cdot (v_{ball}+v_{cannon}) > t \cdot (v_{ball})$
So
$Vol_{shot 1} \propto \frac{v_{ball}+v_{cannon}}{v_{ball}} \cdot Vol_{shot 2}$
And thus
$P_{shot 1} \propto \frac{v_{ball}}{v_{ball}+v_{cannon}} \cdot P_{shot 2}$
So the acceleration on the cannonball in shot 1 is less than in shot 2, how can this be?

Anonymous

Anonymous Fri 23 Oct 2015 23:02:29 No.7611843 Report Delete

Quoted By: >>7612539

>>7611836
Why does that latex display properly in preview, but not when I post it?

Anonymous

Anonymous Fri 23 Oct 2015 23:27:38 No.7611883 Report Delete

Quoted By: >>7611887

It doesn't matter, even if the cannon is braced, the reaction of the cannon ball being fire is just transferred to the Earth.

Anonymous

Anonymous Fri 23 Oct 2015 23:32:07 No.7611887 Report Delete

Quoted By:

>>7611883
wrong

Anonymous

Anonymous Fri 23 Oct 2015 23:42:34 No.7611910 Report Delete

Quoted By:

The pressure within the barrel would be slightly higher on the second shot and the cannonball would go further.

In the un-braced cannon the expanding gasses are able to move backwards

Anonymous

Anonymous Fri 23 Oct 2015 23:52:54 No.7611928 Report Delete

Quoted By: >>7611943 >>7611996

>>7611449
I can't believe these are the people on sci.

The energy that normally moves the cannon is transferred into the "backing" behind it dumbasses.

Anonymous

Anonymous Sat 24 Oct 2015 00:00:41 No.7611943 Report Delete

Quoted By:

>>7611928
For the most part, actual smart people don't have time for bullshit like /sci/
And the ones who do don't bother with threads like this

Anonymous

Anonymous Sat 24 Oct 2015 00:33:47 No.7611996 Report Delete

Quoted By:

>>7611928
Are you saying the cannon and cannonball have equal energies in both cases? Read the first problem (1. Two blocks and a spring) here:

https://web2.ph.utexas.edu/~itiq/303K/chiu/physiquiz/OMPhQuiz_10_p84-92.pdf

OMG! How is it possible that the heavier block has less energy! Back to Physics 101 you go!

Anonymous

Anonymous Sat 24 Oct 2015 12:33:09 No.7612539 Report Delete

Quoted By:

>>7611843
Put spaces in your equations whenever you can. 4chan doesn't like very long strings without whitespace in them.

Subject

Name

E-mail

Password

Capcode	All Only User Posts Only Verified Posts Only Moderator Posts Only Manager Posts Only Admin Posts Only Developer Posts Only Founder Posts
Show Posts	All Only With Images Only Without Images Only Spoiler Images Only Non-Spoiler Images
Deleted Posts	All Only Deleted Posts Only Non-Deleted Posts
Ghost Posts	All Only Ghost Posts Only Non-Ghost Posts
Post Type	All Only Sticky Threads Only Opening Posts Only Reply Posts
Results	All Grouped By Threads
Order	Latest Posts First Oldest Posts First

On these boards

Your latest searches