Mass Impact on Hinged Rod: Single Plane Explained

[captainjack2000]

1. A mass is hitting the end of a hinged rod. After impact they begin to rotate.
The motion is then confined to a single plane. why? what plane?

3. I am a bit confused by what they mean by a single plane. Are they saying that there are only x,y coordinates? Is this just a condition for the conservation of angular momentum or a reason why linear momentum cannot apply?

 

[Gokul43201]

The motion of any point on a hinged object is always confined to a single plane in the lab-frame. This is the plane that is normal to the axis provided by the hinge.

Consider, for example, the motion of the keyhole on a door, as the door rotates.

 

[captainjack2000]

I'm sorry but I still don't understand why it has to be considered in the single plane?

 

[captain.joco]

The motion of the mass and the rod is a circle. Circle lies in a place, perpendicular to the angular momentum vector of the objects moving round the hinge... I think