Centre of Mass

The centre of mass of an object is the point where all of its mass is evenly distributed. It is also known as the centroid or the geometric centre.

Calculating the Centre of Mass

The centre of mass of an object can be calculated by finding the average of the positions of all of its particles. For a continuous object, this can be done by integrating the mass density over the entire volume of the object.

The centre of mass of a system of particles is given by the following formula:

$$ \overrightarrow{R} = \frac{\sum_i m_i \overrightarrow{r}_i}{M} $$

where:

• $\overrightarrow{R}$ is the centre of mass
• $m_i$ is the mass of the $i$th particle
• $\overrightarrow{r}_i$ is the position of the $i$th particle
• $M$ is the total mass of the system

Properties of the Centre of Mass

The centre of mass has a number of important properties, including:

• The centre of mass is always located within the object.
• The centre of mass is the point at which the object would balance if it were suspended from a string.
• The centre of mass is the point through which all of the forces acting on the object must pass in order for the object to be in equilibrium.

Applications of the Centre of Mass

The centre of mass is used in a variety of applications, including:

• Engineering: The centre of mass is used to calculate the stability of structures and machines.
• Physics: The centre of mass is used to study the motion of objects.
• Astronomy: The centre of mass is used to calculate the orbits of planets and stars.

The centre of mass is an important concept in physics and engineering. It is used to calculate the stability of structures, the motion of objects, and the orbits of planets and stars.

Motion of Centre of Mass

The center of mass of a system of particles is the point at which the total mass of the system can be considered to be concentrated. The motion of the center of mass is determined by the total external force acting on the system.

Equations of Motion for the Center of Mass

The equations of motion for the center of mass of a system of particles are:

$$\overrightarrow F_{ext}=m\overrightarrow a_{CM}$$

where:

• $\overrightarrow F_{ext}$ is the total external force acting on the system
• $m$ is the total mass of the system
• $\overrightarrow a_{CM}$ is the acceleration of the center of mass

The motion of the center of mass is a fundamental concept in mechanics. It is used to describe the motion of a system of particles as a whole, and it is independent of the internal forces acting on the system.

Centre of Gravity

The centre of gravity (CG) of an object is the point at which all of its weight is evenly distributed. It is also known as the centre of mass.

Calculating the Centre of Gravity

The centre of gravity of an object can be calculated by finding the average of the positions of all of its particles. This can be done using the following formula:

$$ CG = (1/M) * ∑(mᵢ * rᵢ) $$

where:

• CG is the centre of gravity
• M is the total mass of the object
• mᵢ is the mass of each particle
• rᵢ is the position of each particle

Properties of the Centre of Gravity

The centre of gravity of an object has a number of important properties, including:

• It is the point at which the object’s weight is evenly distributed.
• It is the point at which the object will balance if it is suspended from a string.
• It is the point at which the object will rotate if it is subjected to a force.

Applications of the Centre of Gravity

The centre of gravity is an important concept in a number of fields, including:

• Engineering: The centre of gravity is used to design structures that are stable and resistant to tipping over.
• Physics: The centre of gravity is used to study the motion of objects.
• Sports: The centre of gravity is used to improve performance in sports such as golf, baseball, and tennis.

The centre of gravity is a fundamental concept in physics and engineering. It is the point at which all of an object’s weight is evenly distributed. The centre of gravity has a number of important properties and applications.

Conditions of Equilibrium of a Rigid Body

A rigid body is an idealization of a solid object in which deformation is neglected. In other words, a rigid body is assumed to be perfectly stiff. This assumption is often made in engineering mechanics when the deformations of the object are small compared to its overall dimensions.

The conditions of equilibrium for a rigid body are:

1. The net force acting on the body must be zero. This means that the vector sum of all the forces acting on the body must be zero.
2. The net torque acting on the body must be zero. This means that the vector sum of all the torques acting on the body must be zero.

These two conditions are necessary and sufficient for a rigid body to be in equilibrium.

1. Net Force = 0

The first condition of equilibrium states that the net force acting on the body must be zero. This means that the vector sum of all the forces acting on the body must be zero.

$$\sum F = 0$$

where:

• $\sum F$ is the net force acting on the body
• $F$ is a force acting on the body

This condition can be expressed in terms of the components of the forces acting on the body. In three dimensions, the net force is given by:

$$\sum F_x = 0$$

$$\sum F_y = 0$$

$$\sum F_z = 0$$

where:

• $\sum F_x$ is the net force in the $x$-direction
• $\sum F_y$ is the net force in the $y$-direction
• $\sum F_z$ is the net force in the $z$-direction

2. Net Torque = 0

The second condition of equilibrium states that the net torque acting on the body must be zero. This means that the vector sum of all the torques acting on the body must be zero.

$$\sum \tau = 0$$

where:

• $\sum \tau$ is the net torque acting on the body
• $\tau$ is a torque acting on the body

This condition can be expressed in terms of the components of the torques acting on the body. In three dimensions, the net torque is given by:

$$\sum \tau_x = 0$$

$$\sum \tau_y = 0$$

$$\sum \tau_z = 0$$

where:

• $\sum \tau_x$ is the net torque in the $x$-direction
• $\sum \tau_y$ is the net torque in the $y$-direction
• $\sum \tau_z$ is the net torque in the $z$-direction

Applications of the Conditions of Equilibrium

The conditions of equilibrium are used to analyze the forces and torques acting on a rigid body and to determine whether the body is in equilibrium. This information is essential for designing and analyzing structures and machines.

Some examples of applications of the conditions of equilibrium include:

• Analyzing the forces and torques acting on a bridge to determine whether it is safe
• Designing a machine to ensure that it is stable
• Determining the forces acting on a person’s body when they are standing, walking, or running

The conditions of equilibrium are a fundamental principle of engineering mechanics and are used in a wide variety of applications.

Centre of Mass and Centre of Gravity FAQs

1. What is the difference between the centre of mass and the centre of gravity?

• The centre of mass of an object is the point at which all of its mass is evenly distributed. It is also known as the centroid.
• The centre of gravity of an object is the point at which the force of gravity acts on the object. It is also known as the centre of weight.

2. How do you find the centre of mass of an object?

• For a symmetrical object, the centre of mass is located at the geometric centre of the object.
• For an irregularly shaped object, the centre of mass can be found by using the following formula:

$$ Centre\ of\ mass = (Σmx/Σm, Σmy/Σm, Σmz/Σm) $$

where:

• $Σmx$ is the sum of the products of the masses of the particles and their x-coordinates
• $Σmy$ is the sum of the products of the masses of the particles and their y-coordinates
• $Σmz$ is the sum of the products of the masses of the particles and their z-coordinates
• $Σm$ is the total mass of the object

3. How do you find the centre of gravity of an object?

• For a symmetrical object, the centre of gravity is located at the same point as the centre of mass.
• For an irregularly shaped object, the centre of gravity can be found by using the following formula:

$$ Centre\ of\ gravity = (Σmgx/Σm, Σmgy/Σm, Σmgz/Σm) $$

where:

• $Σmgx$ is the sum of the products of the masses of the particles, their x-coordinates, and the acceleration due to gravity
• $Σmgy$ is the sum of the products of the masses of the particles, their y-coordinates, and the acceleration due to gravity
• $Σmgz$ is the sum of the products of the masses of the particles, their z-coordinates, and the acceleration due to gravity
• $Σm$ is the total mass of the object

4. What are some examples of the centre of mass and centre of gravity?

• The centre of mass of a human body is located at about the navel.
• The centre of gravity of a human body is located at about the hip joint.
• The centre of mass of a baseball is located at the centre of the ball.
• The centre of gravity of a baseball is located slightly below the centre of the ball.

5. Why is the centre of mass important?

• The centre of mass is important because it is the point at which all of the forces acting on an object are balanced. This means that the object will not rotate around its centre of mass.
• The centre of mass is also important for understanding the motion of objects. For example, the centre of mass of a projectile will follow a parabolic trajectory.

6. Why is the centre of gravity important?

• The centre of gravity is important because it is the point at which the force of gravity acts on an object. This means that the object will fall towards its centre of gravity.
• The centre of gravity is also important for understanding the stability of objects. For example, an object with a high centre of gravity is more likely to tip over than an object with a low centre of gravity.