Does two dimensional motion imply conservation of angular momentum?

[anamariann123]

Hello!

I have a system composed by a particle attached to four springs that lie on the xy plane. The motion of the particle occurs on the xy plane. I wanted to know if the angular momentum is conserved. Thanks for your help :)

 

[K^2]

Momentum is conserved if there is no external torque. If you have multiple springs going to a point, you probably have some torque on that point.

 

[HallsofIvy]

Perhaps you are thinking "the angular momentum vector is perpendicular to the plane of motion. Since this is two dimensional, there cannot be such a vector." That is mistaken. The fact that motion is "two dimensional" means that the motion occurs in two dimensions. It does not mean there is NO third dimension.

 

[anamariann123]

I know the angular momentum is orthogonal to the plane of motion, but I would like to know if it is constant.

 

[K^2]

Not in general, no. If the springs were attached to other objects that are free to move, it would be, because that would be a closed system. But if springs are attached to fixed points, it's the same as supplying external torque, so angular momentum will not be conserved.