Angular Momentum


What is Angular Momentum?

Angular momentum, in physics, is a property that characterizes the rotatory inertia of an object in motion about the axis that may or may not pass through the specified object. The Earth’s rotation and revolution are the best real-life examples of angular momentum. For instance, the annual revolution that the Earth carries out about the Sun reflects orbital angular momentum and its everyday rotation about its axis shows spin angular momentum. 

From this example, we can easily conclude that angular momentum is of two types - 

  1. Spin angular momentum. (eg- Rotation)

  2. Orbital angular momentum. (eg- Revolution)

The total angular momentum of a body is the sum of spin and orbital angular momentum. 

In another way, angular momentum is a vector quantity that requires both the magnitude and the direction. For an orbiting object, the magnitude of the angular momentum is equal to its linear momentum. It is given as the product of mass (m) and linear velocity (v) of the object multiplied by the distance (r) perpendicular to the direction of its motion, i.e., mvr. However, in the case of a spinning body, the angular momentum is the summation of mvr for all the particles making the object.           

Some vital things to consider about angular momentum are: 

  • Symbol = As the angular momentum is a vector quantity, it is denoted by symbol L^ 

  • Units = It is measured in SI base units: Kg.m2.s-1

  • Dimensional formula = [M][L]2[T]-1

  • Formula to calculate angular momentum (L) = mvr, where m = mass, v = velocity, and r = radius.

 

Angular Momentum Formula       

The angular momentum of an object having mass (m) and linear velocity (v) with respect to a fixed point can be given as: 

L = mvr sin θ 

Or 

→         →

L   = r x p (in terms of vector product)

Where,

L→= Angular Momentum

v = linear velocity of the object

m = mass of the object

p→ = linear momentum

r = radius, i.e., distance amid the object and the fixed point around which it revolves.

Moreover, angular momentum can also be formulated as the product of the moment of inertia (I) and the angular velocity (ω) of a rotating body. In this case, the angular momentum is derivable from the below expression:

→        →

L = I x ω

Where, 

  • L→is the angular momentum.

  • I is the rotational inertia.

  • ω is the angular velocity.

The direction of the angular momentum vector, in this case, is the same as the axis of rotation of the given object and is designated by the right-hand thumb rule.  

 

Right-Hand Thumb Rule 

The right-hand thumb rule gives the direction of angular momentum and states that if someone positions his/her hand in a way that the fingers come in the direction of r, then the fingers on that hand curl towards the direction of rotation, and thumb points towards the direction of angular momentum (L), angular velocity, and torque.        

     

Angular Momentum and Torque 

For a continuous rigid object, the total angular momentum is equal to the volume integral of angular momentum density over the entire object. Here, torque is defined as the rate of change of angular momentum. Torque is related to angular momentum in a way similar to how force is related to linear momentum. Now, when we know what the angular momentum and torque are, let's see how these two are related. To see this, we need to find out how objects in rotational motion get moving or spinning in the first position. Let's take the example of a wind turbine. We all know that it's the wind that makes the turbine spins. But how is it doing so? Well, the wind is pushing the turbine's blade by applying force to blades at some angles and radius from the axis of rotation of the turbine. In simple words, the wind is applying torque to the turbine. Hence, it is torque what gets rotatable objects spinning when they are standing still.  Moreover, if the torque is applied to an object which is already spinning in the same direction in which it is spinning, it upsurges its angular velocity. Hence, we can say that torque is directly proportional to the angular velocity of a rotating body. Since torque can change the angular velocity, it can also change the amount of angular momentum as the angular momentum depends on the product of the moment of inertia and angular velocity. This is how torque is related to angular momentum. 

Consider a string is tied to a point mass. Now, if we apply torque on the same point mass, it would start rotating around the centre. Here, the particle of mass m would move with a perpendicular velocity V┴ to the radius r of the circle. Now, the magnitude of L→ will be:

L = rmv sin ϕ

   = r p⊥

= rmv⊥

= r⊥p

= r⊥mv

Where,  

  • Φ is the angle formed between r→ and p→

  • p⊥ and v⊥ are the segments of p→ and v→ perpendicular to r→ .

  • r⊥ is the perpendicular distance between the extension of p→ and the fixed point.

Note: The equation or formula L = r⊥mv representing the angular momentum of an object changes only when you apply a net torque. Hence, if no torque is applied, then the perpendicular velocity of the object will alter according to the radius (the distance between the centre of the circle, and the centre of the mass of the body). It means velocity will be high for a shorter radius and low for a longer one.