Center of Mass of a Rigid Body

# Center of Mass of a Rigid Body

## Introduction:

The center of mass of an object or set of objects is a place specified relative to it. It’s the average position of all the system’s components, weighted by their mass. The centroid is the location of the center of mass for basic rigid objects with homogeneous density. The object’s center of mass does not always fall in the same place.

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The center of mass of a ring, for example, is at its center, where no material exists. Humans need a broader mathematical definition of the center of mass for more sophisticated shapes: it is the unique place where the weighted position vectors of all parts of a system total to zero.

## A brief outline:

### Define center of mass

• The center of mass of the object or system is interesting because it is the place at which any uniform force acting on the object acts.
• This really is useful since this simplifies the solution of mechanical problems involving the motion of unusually shaped objects and complex systems.
• We can treat an irregularly shaped object as though all of its mass is concentrated in a tiny object near the center of mass for calculation purposes. This fictitious item is sometimes referred to as a point mass.
• When we press down on a rigid item at its center of mass, it always moves as if it were a point mass. Regardless of its shape, it will not spin around any axis. If an imbalanced force is applied to the item at another place, it will begin to rotate around its center of mass.

Center of gravity: The point through which gravity acts on a body or system is known as the center of gravity. The gravitational field is kept uniform in most mechanics’ situations. The center of gravity and the center of mass is then in the same place. Because they are often in the same place, the words center of gravity and center of mass are frequently interchanged.

### System of Particles:

A very well assemblage of a collection of entities that may or may not engage with one another or are related to each other is referred to as a system of particles. They could be genuine particles of hard bodies moving in translation. The particles that interact with one another exert force on one another.

The interaction forces F i j and F j i between the i t h and j t h particles. The internal force of the system refers to the forces of mutual contact between the particles of the system.

Internal forces constantly exist in opposite directions and in pairs of equal magnitude. External forces may act on all or parts of the particles in addition to internal forces. A force operating on any one particle is included in the system by another body outside the system is referred to as an external force.

## Important concepts:

• Researchers cope with stretched bodies in practice, which can be deformable, non-deformable, or rigid. A system comprising an endlessly large number of particles with an infinitely small space between them is also known as an extended body.
• The difference between the distance between a body’s particles and their relative locations changes as it deforms. A rigid body is a long entity in which the divisions and relative positions of all of its constituent particles are constant under all conditions.
• It’s the average position of all the system’s components, weighted by their mass. The centroid is the location of the center of mass for a simple rigid object with a homogeneous density.
• We can use gravity forces on the object to calculate the center of mass of an object in an experimental setting. This is possible due to the fact that the center of mass in the parallel gravity field at the earth’s surface is the same as the center of gravity.
• Furthermore, a body with a symmetry axis and constant density will have its center of mass on this axis.
• Similarly, the center of mass of a circular cylinder with constant density will be on the cylinder’s axis. If we’re talking about a spherically symmetric body with constant density, the COM will be in the sphere’s center.

If we consider it in a broad sense, the center of mass of any body will almost always be a constant point of that symmetry.

You can write the equation to a particle system. The equation is applied to each axis independently.

X com = ∑n i=0mixi​​/ M

Y com​ = ∑n i=0mi y i/ M

Z com = ∑n i=0miz i​​/ M

When we have point objects, we can utilize the calculation above. If we need to identify the centre of mass of a stretched item, such as a rod, we must employ a different approach. A differential mass and its position must be considered, and then integrated across the full length.

X com​ = ∫ x d m​/M

Y com​ = ∫ y d m​/M

Z com​ = ∫ z d m​/M

## A body’s center of mass has a continuous mass distribution.

If the given item is not discrete and the distances between them are not particular, the center of mass can be determined by considering an infinitesimal element of mass (d m) at a distance of x, y, and z from the chosen coordinate system’s origin.

x c m=x d m/d m

Y c m= ∫y d m/d m

Z cm= ∫z d m/d m

## In the form of vector

r cm​​=r d m/d m

In the case of a single rigid body, the center of mass is fixed in reference to the body, and it will be positioned at the centroid if the body has a uniform density. The center of mass of empty or loose objects, including a horseshoe, can occasionally be found outside the physical body. The center of mass may not match the position of any one member of the system in the event of a distribution of independent bodies, such as the planets of the Solar System.

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Question: What are the key distinctions between the Center of Mass and the Center of Gravity?

Answer: The weights of the objects are done and split according to their Masses in the case of the Centre of Mass, whereas the Center of Gravity is the point where the object responds in reaction to Gravity. The Centre of Mass is concerned with the body’s mass and is affected by its distribution, whereas the Centre of Gravity is concerned with the body’s weight and is affected by the acceleration generated by the existence of Gravity in an item.

Question: What is the best way to describe motion in a rigid body?

• The following is a description of the general motion of a rigid body with mass m:
• A rigid body’s translational motion can be characterized as a point particle with a mass m that is positioned at the Center of Mass.
• The spin of the particle is described separately with respect to the body’s Center of Mass.

Question: Give an example of a real-life centre of mass.

Answer: The trajectory of a projectile beneath gravity is known to be a parabola. As a result, when a firecracker is launched from the ground, it travels up a parabolic path before exploding in mid-air. Assume P explodes into many hot fragments, each of which flies off on its own parabolic course, depending on P’s position. Because the cracker’s explosion is produced only by internal forces, the ‘center of mass of all the fragments continues to proceed along such an initial parabolic path as the unexploded cracker.

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