Center of Mass

Definition of Center of Mass

Every single physical system has associated itself with a certain point whose motion characterizes the motion of the whole system. When the system moves under an external force, then this point moves as if the entire mass of the system were concentrated at it, and also the external force was applied at it. This point is therefore called the ‘center of mass’ of the system. The motion of a system can be described in terms of the motion of its center of mass.   


While studying the dynamics of the movement of the system of particles as a whole, do not be bothered about the dynamics of an individual particle of the system. Rather focus only on the dynamics of a unique point corresponding to that system. The movement of this unique point is analogous to the movement of a single particle whose mass is equivalent to the sum of all independent particles of the system and the outcome of all the forces exerted on all the particles of the system by surrounding bodies (or) action of a field of force is exerted directly to that particle. This point is known as the center of mass of the system of particles. The idea of the center of mass (COM) is functional in analyzing the complicated movement of the system of objects, especially when two and more objects collide or an object explodes into fragments.


What is the Center of Mass?

A position that is defined relative to an object or a system of objects is known as the Centre of Mass of that object. In a system, the average position of all parts that are weighted according to their Masses is defined as the Centre of Mass. The Centre of Mass for the simple rigid objects having uniform density is located at the Centroid. It is possible that sometimes the Centre of Mass of an object does not lie over it. In the case of a uniform disc, the Centre of Mass would lie on its Centre, while the Centre of Mass in the case of a ring lies on the Centre but there is not any material. 


The Centre of Mass for complicated shapes can be defined as the unique position where the sum of the weighted position vectors of all the parts equals zero. 



Importance of Centre of Mass

The Centre of Mass of a system is that one point where any uniform force is acted upon the object. It is important to find the Centre of Mass of objects as it makes it easy to solve the Mechanics’ problems in order to describe the Motion of complicated and oddly shaped objects. While doing calculations, we assume that all the Mass of an oddly-shaped object is concentrated in a tiny object that is located at the Centre of Mass, and this tiny object that we have assumed is known as the point Mass.


The formula for the Centre of Mass

The Vector Addition of the weighted position vectors that point towards the Centre of Mass of each object, gives the Centre of Mass of that system. The Centre of Mass is calculated separately for the components along each axis. 


For x-axis:

\[x_{com}=\frac{m_1x_1+m_2x_2+m_3x_3+...}{m_1+m_2+m_3+...}\]


For y-axis:

\[y_{com}=\frac{m_1y_1+m_2y_2+m_3y_3+...}{m_1+m_2+m_3+...}\]


\[x_{com}\] and \[y_{com}\] together will give the coordinates for the Centre of Mass of a system.

Center of mass formula for point objects: 


\[z_{com}=\frac{\sum_{i=0}^{n}m_iz_i}{M}\]


Center of mass formula for any geometric shape: 


\[x_{com}=\frac{l}{2}\]


Center of Mass of a Two-Particle System

Let us consider a system of two particles of masses m1 and m2 located at points A and B respectively.             


Let r1 and r2 be the position vectors of the particles relative to a fixed origin O. Then, the position vector rcm of the center of mass C of the system is defined by 

(m1+m2) rcm =m1 r1+m2 r2.


The product of the total mass of the system and the position vector of the center of mass is equal to the sum of the products of the masses of the two particles and their respective position vectors.  


Center of Gravity

The imaginary point through which on an object or a system, the force of Gravity is acted upon is known as the Centre of Gravity of that system. Usually, it is assumed while doing mechanical problems that the gravitational field is uniform which means that the Centre of Gravity and the Centre of Mass is at the same position. 


Solved Example

Example 1: m₁=2kg and m₂=5kg are two-point Masses located at y₁=10m and y₂=-5m respectively. Calculate the Centre of Mass.


We know that


 \[y_{com}=\frac{m_1y_1+m_2y_2+...}{m_1+m_2+...}\]


\[y_{com}=\frac{2\times 10+5\times (-5)}{2+5}\]


\[y_{com}=\frac{20-25}{7}\]=\[-\frac{5}{7}\]


Therefore, \[y_{com}=-\frac{5}{7}\]


Example 2: In the HCL molecule, the separation between the nuclei of the two atoms is 1.27 A. Find the location of the center of mass of the molecule. A chlorine atom is 35.5 times heavier than a hydrogen atom.   


Solution: The center of mass of the HCL molecule will be on the line joining H and CL atoms. Let the HCL molecule be along the X-axis, the H atom being at the origin (x=0). The center of mass relative to the H atom is given by 


\[x_{cm}=\frac{m_1x_1+m_2x_2}{m_1+m_2}\]


Where m1 and m2are the respective masses of H and CL atoms, and x1and x2 their distances relative to the H atom. 


Here, m2=35.5 m1, x1=0 and x2=1.27 A


\[x_{cm}=\frac{(m_1\times0)+(35.5m_1\times1.27A)}{m_1+35.5m_1}\]=\[\frac{35.5\times1.27A}{36.5}\]=1.235A


Example 3: Two bodies of masses 0.5 kg amnd 1 kg are lying in the X-Y plane at points (-1, 2) and (3,4) respectively. Locate the center of mass of the system.  


Solution : Here, we have 


m1=0.5 kg, m2=1 kg, x1= -1, y1=2, x2=3, y2=4.


By definition, the coordinates of the center of mass of the two bodies are given by: 


\[x_{cm}=\frac{m_1x_1+m_2x_2}{m_1+m_2}\]=\[\frac{(0.5)(-1)+(1)(4)}{(0.5)+(1)}\]=\[\frac{5}{3}\]


\[y_{cm}=\frac{m_1y_1+m_2y_2}{m_1+m_2}\]=\[\frac{(0.5)(2)+(1)(4)}{(0.5)+(1)}\]=\[\frac{10}{3}\]


The coordinates of C>M> are (5/3, 10/3).

Follow and practice more center of mass examples to get a better understanding.

FAQs on Center of Mass

What are the important differences between the Centre of Mass and the Centre of Gravity?

In the case of the Centre of Mass, the weights of the objects are done and divided according to their Masses while the Centre of Gravity is that point where the object acts in response to Gravity. The Centre of Mass is focused on the Mass of the body and is dependent upon the Mass distribution while the Centre of Gravity is focused on the weight of the body and is dependent upon the acceleration that is caused due to the presence of Gravity in an object.

What are some of the characteristics of the Centre of Mass?

The Centre of Mass is considered to be an essential part of Astrophysics and Astronomy and the characteristics of the same are-

  • The Center of Mass is dependent on the position of the object.

  • Mass of an object equals zero in space.

  • The Center of Mass is changed if any external force is exerted on the object.

  • The Center of Mass is not dependent on the location or position of the object.

  • In the case of symmetrical objects, the Centre of Mass and Centre of Gravity lies in one place.

What are some of the characteristics of the Centre of Gravity?

Some of the important characteristics that the Centre of Gravity possess are-

  • Asymmetrical objects have a Centre of Gravity.

  • The Center of Gravity is dependent upon the size and shape of the object.

  • The stability of an object is affected when it is not able to balance itself.

  • In order to maintain the stability of an object, the Centre of Gravity is useful.

  • Physical processes can determine the Centre of Gravity of an object.

How is the motion in a rigid body described?

The general motion of a rigid body with Mass m can be described as follows-

  • The translational motion can be described for a rigid body in such a way that it is a point particle with a Mass m that is located at the Centre of Mass.

  • With respect to the Centre of Mass of the body, the rotation of the particle is independently described.

In order to learn more about rigid bodies, students can check out Vedantu’s study material on rigid bodies, for a better understanding of the concept.

What is meant by the term Linear Momentum?

When the product of the total Mass and the velocity of the Centre of Mass are equal, the system is said to be in Linear Momentum. In Mathematical terms, it can be written as p=mv. This also states that the momentum and the Mass of the object and its velocity are directly proportional to each other. This implies that if the Mass of an object or its velocity is greater, the Momentum will also be greater.

Give a Real-Life Center of Mass Example?

We know that the trajectory of a projectile under gravity is a parabola. Therefore, when a firecracker is projected from the earth, it goes up along a parabolic path and explodes in mid-air at some point. Let us say P explodes into many fiery fragments which fly off along their own parabolic paths, depending on the position of P. Since the explosion is caused by internal forces alone, the ‘center of mass’ of all the fragments continues to move along the same initial parabolic path that the unexploded cracker would have followed. (In fact, after the explosion, there is nothing at the location of the center of mass which is just a mathematical point.)

What is a Rigid Body?

 In practice, we come across extended bodies that may either be deformable or non-deformable, that is, rigid. An extended body is also a system of an infinitely large number of particles that have infinite small separations between them. When a body deforms, the disconnection between the distance of its particles and their relative locations also changes. Whereas, a rigid body is an extended object in which the separations and relative location of all of its constituent particles will always remain the same under all circumstances.


It is the average position of all the parts of the system, weighted based on their masses. While dealing with a simple rigid object that has a uniform density, the center of mass will be located at the centroid.

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