Question

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

• A. a line perpendicular to the plane of rotation
• B. the line making an angle of $45^{\circ}$ to the plane of rotation
• C. the radius
• D. the tangent to the orbit

Answer 1

The Correct Option is A

The question focuses on the concept of angular momentum in the context of rotational motion. Angular momentum is a key physical quantity in rotational dynamics, and it has a specific direction dictated by the right-hand rule.

**Explanation:**

When a mass rotates about a fixed point, it possesses angular momentum. Angular momentum (\mathbf{L}) is given by the cross product of the position vector (\mathbf{r}) and the linear momentum (\mathbf{p}) of the object:

\mathbf{L} = \mathbf{r} \times \mathbf{p}

According to the right-hand rule, if you point the fingers of your right hand in the direction of the position vector (\mathbf{r}) and curl them in the direction of the linear momentum (\mathbf{p}), your thumb points in the direction of the angular momentum (\mathbf{L}).

In this scenario, since the mass is rotating in a plane, the direction of the angular momentum is perpendicular to the plane of rotation. This is because the right-hand rule dictates that the cross product of vectors perpendicular to each other results in a vector that points perpendicularly away from their plane.

Therefore, the angular momentum of a rotating mass in a plane is directed along a line perpendicular to the plane of rotation.

**Conclusion:** The correct answer is a line perpendicular to the plane of rotation. This aligns with the standard physical principle governing angular momentum.