When a mass rotates in a plane about a fixed point, its angular momentum is directed along:

When a mass rotates in a plane about a fixed point, its angular momentum is directed along:

• A. The radius
• B. A line at an angle of 45 deg to the plane of rotation
• C. Tangent to orbit
• D. Axis of rotation

Answer: (D)

The main idea is that angular momentum is given by the cross product r × p. For planar motion about a fixed point, r lies in the plane and p is tangent to the path within that same plane, so their cross product points perpendicular to the plane. That perpendicular direction is exactly the axis about which the mass rotates. So the angular momentum vectors align with the rotation axis. The sense (toward or away from you) is set by the right-hand rule: curling from r to p gives the direction along the axis. This is why the angular momentum is directed along the axis of rotation, not along the radius, tangent, or some in-plane direction.

The main idea is that angular momentum is given by the cross product r × p. For planar motion about a fixed point, r lies in the plane and p is tangent to the path within that same plane, so their cross product points perpendicular to the plane. That perpendicular direction is exactly the axis about which the mass rotates. So the angular momentum vectors align with the rotation axis. The sense (toward or away from you) is set by the right-hand rule: curling from r to p gives the direction along the axis. This is why the angular momentum is directed along the axis of rotation, not along the radius, tangent, or some in-plane direction.