Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum 
May 11th 2018, 08:32 AM

#1  Junior Member
Join Date: May 2018
Posts: 4
 Bullet in 2D coordinate system
Hello everyone, i have this kind of problem, but i dont think i have enought knowledge of physics to solve it.
So here it is: The BULLET is in 2D coordinate system moving in time (T), with starting speed(V0) , that is decressing over time with acceleration(A).Coordinates of a bullet (X,Y) are always known.
BULLET movement is defined by his spike! [[[Comment: Just like in real life, if you fire a bullet from a x,y,z coordinate in real world in time t, he cannot move backwards, it moves where the force leads him, but that "vector" force is defined by his spike.]]] Important thing: BULLET can be rotated in 2D space in time (T). Rotation is (0<=angel<=360)degrees. So at any time (T), an invisible force can rotate a BULLET for any angel. Center of rotation of a BULLET is back of his tail.. [[[Comment:Imagine a bullet was moving from a X=0,Y=100 with a starting speed (V0) in 2D coordinate system.That means that X coordinate of a bullet is always changing in time(T) and Y stays the same,Y=100.If invisble force rotated him for an angel, lets say PI/2 anticlockwise(but dosnt matter wether is clockwise or anticlockwise as long it is the angel of total rotation(that means that 1 coordiante stay same and other 1 is changing)). We can say that a total rotation has been done,becose now X coordinate is the same,and Y coordinate is now changing. But if its not the angel of total rotation, we can for example use PI/3, now both X and Y coordiantes of a bullet are chaning.]]] Mathematicly define all proccess that are happening in time(T).
Dont consider BULLET's mass, gravitational pull, or any kind of loss of BULLET's speed from other sorts of forces, except acceleration, that is always decressing.If BULLET stopped moving becose he lost his speed, he can be fired again at the same place where he stopped, but must be fired in direction that is facing.Remember: Only invisble force can change BULLET's angel after its fired in time (T). [[[Comment:If you need any more information, il give.]]] [[[Comment:I forgot to say, but it is very IMORTANT.If angel of Xaxis and BULLET is 0 degrees, and BULLET is rotated in time (T) for lets say PI/2, rotation CANNOT HAPPEN INSTANTLY, ROTATION IS ALSO HAPPENING IN TIME (T), with slowly incresing to PI/2.
So for any angel ANGEL, rotation of a BULLET to angel ANGEL is also happening in time (T) whether clockwise or anitclockwise.
If BULLET's rotation is in motion, and he got rotated for PI/6, the last rotation of a BULLET stops(last rotation in this case is Pi/2) and new on to PI/6 strats to increse/decrese.
Last edited by Subject; May 11th 2018 at 08:57 AM.

 
May 11th 2018, 08:54 AM

#2  Senior Member
Join Date: Oct 2017 Location: Glasgow
Posts: 426

Question: "Mathematically define all processes that are happening in time(T)."
There's all sorts of physics phenomena: mechanics, electromagnetism, solid state physics, relativity... however, you only want to consider the main ones that are going to affect the bullet.
I think it's fairly obvious that this is a mechanics problem. Mechanics deals with the motion of objects as forces are applied to them.
Whenever you have a mechanics problem, draw a freebody diagram. A freebody diagram is a simplified picture of the object along with all of the possible forces that are affecting the object. The forces that you need to add to the diagram are the result of the kinds of processes that are happening... is there air resistance? Is there gravity? The question gives you hints to identify the kind of things you need to include.
Once you have the freebody diagram, you can start considering Newtons laws of motion. What's the resultant force on the bullet? Does the resultant force help define an equation of motion?
With regards to rotation, consider that there are rotational and translational equations of motion that can arise from Newton's laws. Try looking it up
EDIT: also... why is there an "invisible force" that rotates the bullet? Is this not a real force? Are you trying to model a rotating bullet in Unity, or something, and you want the input required to get a rotating bullet in your scene? If that's the case, you might want to consider a fictitious torque instead. In reality, the torque that rotates an object as it flies is a real torque imposed by air resistance because the force applied to the bullet across its surface results in a torquecouple.
Last edited by benit13; May 11th 2018 at 09:02 AM.
Reason: whooops.....

 
May 11th 2018, 09:06 AM

#3  Junior Member
Join Date: May 2018
Posts: 4

Mathematicly define all proccesses that are happening in time(T).
My answer:Yes thats the question.
is there air resistance? Is there gravity? The question gives you hints to identify the kind of things you need to include.
My answer:
I defined everything there is need to know. Read the post.
My answer to rest of the post:
I realy dont know physics, i know some basic things, but this problem is way to complex for me.
So i just need mathematical explanation, or any kind of formulas that i can use, and if i plug in for example V0=const0,T=const1,A=const2 etc... To know how will BULLET be positioned in 2D space...

 
May 11th 2018, 10:57 AM

#4  Forum Admin
Join Date: Apr 2008 Location: On the dance floor, baby!
Posts: 2,778

Originally Posted by Subject Mathematicly define all proccesses that are happening in time(T).
My answer:Yes thats the question.
is there air resistance? Is there gravity? The question gives you hints to identify the kind of things you need to include.
My answer:
I defined everything there is need to know. Read the post.
My answer to rest of the post:
I realy dont know physics, i know some basic things, but this problem is way to complex for me.
So i just need mathematical explanation, or any kind of formulas that i can use, and if i plug in for example V0=const0,T=const1,A=const2 etc... To know how will BULLET be positioned in 2D space... 
I'm confused about this one as well. I see that you are trying to model something that might be called an "ideal bullet." Most of this is essentially done.
Question 1) Why are you considering a rotation of the bullet after it's in the air? Unless there is something to torque it after it leaves the barrel any rotation at the start will continue. If it doesn't rotate at the start then it won't rotate later.
Comment: This vaguely looks like a problem to count the number of degrees of freedom the bullet has. (Two spatial and one rotational.)
Question 2) The OP says that you already know all (x, y). But nothing in the problem says that the acceleration is constant so unless you want to get into Calculus and estimate the acceleration from the curve there is nothing more that can be done.
Question 3) What's this business of "If bullet stopped moving becose he lost his speed, he can be fired again at the same place where he stopped, but must be fired in direction that is facing.." I have no idea what this means unless this question is a model for a game of some kind?
Dan
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May 11th 2018, 11:27 AM

#5  Junior Member
Join Date: May 2018
Posts: 4

Question 1, answer:
This is not ideal bullet. Its a bullet that obeys only some laws of physics.Rotation of a bullet is not a bullet spining around, i cosindered a speed vector in direction where is bullet facing(at start), and change of direction of that vector where bullet is facing.And that change happens when invisible force affects the bullet's movment for some angel.
Comment answer:
I dont know what is deegres of freedom of a bullet.
Question 2, answer:
If you give a object some starting speed V0, in time T he will pass road S and his speed will decrese over time untill it stops. So i think you have to consinder acceleration. And in what ever X,Y coordinate bullet is found, and is being fired again in time T=0+(lets say like that) his acceleration is always decresing. More like, acceleration in T=2s is less than acceleration in T=1s. I hope you understand me.
Question 3, answer:
Its not a model for a game.The business of that is i that you can repeat what you did at start of the time moving T=0, diffrence is that bullet is now not in coordinate Y=100 and X=0, but it has changed for something like Y=160,X=50 with a change of some angel, and i dont know what that is that angel.We notice that angel is changed becose Y=160 and its not constant like Y=100, and if only X coordinate has change(that would happen if there was no change in angle, so bullet would go like just straight.)

 
May 11th 2018, 11:29 AM

#6  Junior Member
Join Date: May 2018
Posts: 4

I think i just wrote my answer, but it seems it does not appear...?
Edit: And this comment does, what is this? haha
It said that it waits "Moderator approval" as i remember, so if anyone can please check, i cant type it all again...

 
May 11th 2018, 01:28 PM

#7  Forum Admin
Join Date: Apr 2008 Location: On the dance floor, baby!
Posts: 2,778

Originally Posted by Subject I think i just wrote my answer, but it seems it does not appear...?
Edit: And this comment does, what is this? haha
It said that it waits "Moderator approval" as i remember, so if anyone can please check, i cant type it all again... 
You should be able to see it now. The spam filter acts up every now and again.
Dan
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