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Conservation of momentum in combination of angular and linear momentumby sr241
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#1
Feb1611, 02:32 AM

P: 83

angular momentum and linear momentum is conserved, but what happen when combination angular momentum and linear momentum occurs?
for example a ball hits a horizontal paddle wheel on a base(which is free to move in any directions). then what happen to linear momentum of ball and paddle wheel? will it be equal to linear momentum before collision? or does the linear momentum after collision will be reduced by angular momentum gained by of paddle wheel (paddle wheel was initially at rest and due to tangential collision of ball it starts rotating after collision) 


#2
Feb1611, 05:24 AM

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P: 41,464

Both linear and angular momentum will be separately conserved during the collision.



#3
Feb1611, 06:28 AM

P: 112

Yes, momentum before = momentum after; likewise angular momentum.
It is not that linear momentum gets "converted to" angular momentum. It is that your system of ball and paddle wheel already has angular momentum before the collision occurs. The ball's trajectory before collision is a straight line with tangential separation [itex]r[/itex] from the centre of mass of the paddle wheel. [itex]\vec L = \vec r \times \vec p[/itex]. Since [itex]r[/itex] and [itex]p[/itex] are nonzero, the ballpaddle system possesses angular momentum. 


#4
Feb1611, 08:25 AM

P: 83

Conservation of momentum in combination of angular and linear momentum
But what about Kinetic Energy(KE) of ball and paddle system before and after collision, will it be split into rotational KE and Linear KE and their sum equals initial KE of Ball 


#5
Feb1611, 09:38 AM

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P: 41,464

If the ball travels in a straight line towards the paddle wheel's COG, why do you think it will start rotating? Originally you said the collision was 'tangential', which I interpreted to mean not along the COG. 


#6
Feb1611, 10:47 PM

P: 83

no ball travels parallel to center of mass of paddle wheel and hit paddle wheel tangentially.
assume collisions are perfectly elastic. then what about rotational kinetic energy and linear kinetic energy, before and after collision. Will it conserved independently just as in case of linear and angular momentum. Or will it be sums of rotational and linear kinetic energy 


#7
Feb1711, 04:46 AM

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P: 41,464




#8
Feb1711, 06:44 AM

P: 83




#10
Feb1711, 07:57 AM

P: 83

what happens when ball travels through center of mass of paddle wheel and base and hits paddle wheel tangentially . now how is going to be the conservation of angular momentum and linear momentum. Also tell me about the conservation of Rotational and Linear Kinetic Energy 


#11
Feb1711, 03:41 PM

P: 112

sr241, in response to your diagram: my best guess of the situation after collision is as follows. (Let the point CM be the centre of mass of the paddlebase system. The ball has initial momentum [itex]{p_{{\rm{b,i}}}}[/itex] and final momentum [itex]{p_{{\rm{b,f}}}}[/itex].)
Momentum before = momentum after = [itex]{p_{{\rm{b,i}}}}[/itex]. Angular momentum before = angular momentum after = 0 (all w.r.t. CM). (This is ignoring any complicating factors, such as the possibility of the ball's trajectory being deflected offaxis upon collision with the paddle. I'm assuming that the ball continues rightwards along its original linear trajectory, or else bounces normally off the paddle.) This is just an intuitive guess, so please take my answer with a pinch of salt until somebody either confirms or refutes it. 


#12
Feb1711, 09:29 PM

P: 83

If ball possess angular momentum before collision then that would be P_{2} and corresponding linear momentum would be P_{3}. you mean to say P_{2} and P_{3} will separately conserved, then in a way it is the sum of P_{2} + P_{3} = P_{1} (actual linear momentum before collision) that is conserved. or in other words sum of angular momentum and linear momentum will be conserved. or say some part of initial linear momentumP_{1} will go into angular momentum 


#13
Feb1811, 04:07 AM

P: 83

http://www.youtube.com/watch?v=ul9_0F0T9ZA in the video linear momentum is conserved. and angular momentum is conserved by ball starts rotating in opposite direction of bar. 


#14
Feb1811, 07:46 AM

Mentor
P: 41,464

To calculate the angular momentum of the ball with respect to the original position of the paddle's center of mass, you would evaluate r X P_{1}, where r is the position vector of the ball measured from the paddle's center of mass. This a vector cross product. The magnitude of that angular momentum will equal rP_{2}. (Note: P_{2} is the tangential component of P_{1}, not the angular momentum.) In this collision, two things are conserved: The angular momentum and the linear momentum. The angular momentum I discussed above; the linear momentum is just P_{1} = P_{2} + P_{3}. 


#15
Feb1811, 08:32 AM

P: 83

if two balls of same mass (a,b  a moving initially and b at rest) collides along center of mass; after collision a will be at rest and b will be moving with same velocity as "a". but if a's path is parallel to b's center of mass, a will not be at rest after collision; a will be moving along same path with velocity = a's initial velocity  b's final velocity, ( thus linear momentum is conserved) . but a and b starts rotating with respect to their center of mass in opposite direction. thus angular momentum is conserved, right ?
How to calculate a's and b's final velocity and angular velocity from the path and initial velocity of "a" ? does moment of inertia has anything to do with this? 


#16
Feb1811, 05:49 PM

P: 112

I might have confused things when I wrote, in my first post, "...the ballpaddle system possesses angular momentum." I ought to have added, "about its (the ballpaddle system's) own centre of mass". Please note that the above applies to your diagram in post #12, i.e. in the case where the ballpaddle (or ballpaddlebase) system possesses nonzero angular momentum about its own centre of mass. For your final question, regarding the two balls of equal mass, we need only consider the conservation laws which Doc Al has already stated: conservation of linear momentum, and conservation of angular momentum. (For simplicity, let all trajectories lie in the xy plane.) Let's say ball A travels with velocity [itex]\vec u[/itex] along the positive [itex]x[/itex] axis. Ball A strikes ball B offcentre. Let the final velocities of A and B be [itex]\vec v[/itex] and [itex]\vec w[/itex] respectively. Then, due to conservation of linear momentum: [tex]\left {\vec u} \right = {u_x} = {v_x} + {w_x}[/tex] [tex]{v_y} + {w_y} = {u_y} = 0[/tex] [tex] \Rightarrow {v_y} =  {w_y}[/tex] For a perfectly elastic collision, balls A and B each rebound with zero spin, and their velocities are at 90º to each other. This conserves linear momentum and linear kinetic energy. If the balls impart some rotation/spin to one another upon collision (due to friction, deformation etc.) then a portion of the initial kinetic energy of A is transferred to rotational/spin kinetic energy of A and B. The remainder of A's initial kinetic energy is transferred to linear kinetic energy of A and B. The balls' rebound velocities will then necessarily make an angle 0º < [itex]\theta [/itex] < 90º with each other. (Say if you need help proving this.) My feeling is that if the balls do impart some spin angular momentum to one another, then the angular momentum of A will be equal and opposite to that of B. (I haven't proved this. Perhaps someone could comment...?) The angular momentum of the system (w.r.t. some arbitrary point P) is constant. The linear momentum of the system is constant. The linear kinetic energy of the system can change. The rotational kinetic energy of the system can change. 


#17
Feb1911, 07:09 AM

P: 83

http://www.youtube.com/watch?v=BVgHV1LgzvM
the video shows two equal mass bars collide their linear momentum is separately conserved. but they are spinning in same direction. could it be because of direction of n in the cross product for angular momentum = p*r*sin theta * n in the first bar n is towards us and in second bar n away from viewer. thus angular momentum can be conserved separately even if bars are rotating in same direction. then what is the significance of n in angular momentum? 


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