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6.2 - CENTRE OF MASS

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Definition of Centre of Mass

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Teacher
Teacher

Welcome everyone! Today, we're discussing the center of mass. Can someone tell me what they understand about this concept?

Student 1
Student 1

I think it's a point where all the mass of an object is concentrated, right?

Teacher
Teacher

Exactly! The center of mass is indeed a point that represents the average position of the mass distribution. It's crucial in analyzing motion because we can treat a complex body as if its entire mass were concentrated at this point.

Student 2
Student 2

How do we actually calculate the center of mass for different systems?

Teacher
Teacher

"Great question! For two particles with masses m1 and m2 at positions x1 and x2, the center of mass can be calculated using the formula:

Application and Importance of Centre of Mass

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Teacher
Teacher

Now let's explore why the center of mass is important. How do you think engineers use it in designing vehicles or buildings?

Student 2
Student 2

They probably need to ensure stability, right?

Teacher
Teacher

"Absolutely! Knowing the center of mass helps in designing stable structures, like ensuring a car has a low center of gravity to prevent tipping.

Examples of Centre of Mass Calculation

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Teacher
Teacher

Let's work through some examples now! Suppose we have three particles located at the vertices of a triangle. How would we proceed to calculate the center of mass?

Student 1
Student 1

We would list their masses and coordinates and then apply the formula for three particles?

Teacher
Teacher

"Exactly! If we have masses m1, m2, and m3 located at (x1, y1), (x2, y2), and (x3, y3), respectively, our formulas for X and Y coordinates would be:

Introduction & Overview

Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.

Quick Overview

The section explores the concept of the center of mass for a system of particles, including its definition and significance in understanding motion.

Standard

This section outlines the definition, calculation, and significance of the center of mass for systems of particles, emphasizing its importance in analyzing motion and providing formulas for calculating the center of mass in various configurations.

Detailed

Detailed Summary

The center of mass (CM) of a system of particles is a crucial concept when studying the motion of extended bodies. It allows us to simplify complex systems into single points where the mass can be considered concentrated. In the case of a system comprising two particles, the center of mass is located at a weighted average of their positions based on their masses. For a system of n particles, the position of the center of mass is found using the formula:

$$ X_{CM} = \frac{\sum m_i x_i}{\sum m_i} $$

where m_i are the particles' masses and x_i are their coordinates. For configurations in two and three dimensions, similar expressions apply for the y and z coordinates, respectively. Significantly, if an object is homogeneous and follows a symmetric distribution, its center of mass coincides with its geometric center. The section further explains that when an extended body is treated as a collection of particles, the laws of motion can be simplified to analyze the motion of the center of mass, providing insight into the dynamics of the entire system.

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Audio Book

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Definition of Centre of Mass for Two Particles

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We shall first see what the centre of mass of a system of particles is and then discuss its significance. For simplicity we shall start with a two particle system. We shall take the line joining the two particles to be the x-axis. Let the distances of the two particles be x1 and x2 respectively from some origin O. Let m1 and m2 be respectively the masses of the two particles. The centre of mass of the system is that point C which is at a distance X from O, where X is given by:

X = (m1x1 + m2x2) / (m1 + m2)

Detailed Explanation

The centre of mass (CM) is a concept that helps us understand how a system of particles behaves as a whole. For a two-particle system, we define the CM as the point where we can consider the entire mass of the system to be concentrated for the purposes of analyzing motion. To find the position of the CM, we calculate a weighted average of the positions of the two particles. The formula given considers each particle's mass and its distance from the origin, emphasizing that heavier particles influence the CM position more than lighter ones.

Examples & Analogies

Imagine balancing a seesaw with two children of different weights. If one child is much heavier, the pivot point needs to be closer to them for the seesaw to balance. Similarly, in the two-particle system, the heavier particle 'pulls' the centre of mass closer to it.

Centre of Mass for Multiple Particles

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If we have n particles of masses m1, m2,...mn respectively, along a straight line taken as the x-axis, then by definition the position of the centre of mass of the system of particles is given by:

X = (Σ(m_i * x_i) / Σ(m_i))

where x1, x2,...xn are the distances of the particles from the origin; X is also measured from the same origin. The symbol Σ denotes summation over n particles. The sum M = Σm_i is the total mass of the system.

Detailed Explanation

For a system of n particles, the CM is calculated similarly but now includes all particles. The formula sums the products of each particle's mass and its position, then divides by the total mass of the system. This approach ensures that all particles contribute to the overall position of the centre of mass, reflecting their relative influences based on their mass and position overall.

Examples & Analogies

Think of a school bus full of students, where some students are heavier than others. To find the average position of the students in the bus, we consider how far each student sits relative to a specific point while also factoring in their weight. Heavier students at the back will 'pull' the centre of mass towards them more than lighter students at the front.

Centre of Mass in Two-Dimensional Space

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Suppose that we have three particles, not lying in a straight line. We may define x- and y-axes in the plane in which the particles lie and represent the positions of the three particles by coordinates (x1,y1), (x2,y2) and (x3,y3) respectively. Let the masses of the three particles be m1, m2 and m3 respectively. The centre of mass C of the system of the three particles is defined and located by the coordinates:

X = (Σ(m_i * x_i) / Σ(m_i))
Y = (Σ(m_i * y_i) / Σ(m_i))

Detailed Explanation

When dealing with particles distributed in a plane (or three-dimensional space), we extend the concept of the centre of mass to include both x and y coordinates (and z if in three dimensions). This means we calculate the x-coordinate and y-coordinate of the centre of mass separately using the same method as before. By doing so, we accurately locate the centre of mass in two dimensions, taking into account how each particle's position in both x and y axes influences the overall location of the CM.

Examples & Analogies

Imagine a triangle drawn on a piece of paper with three coins placed at each vertex. If the coins have different weights, to find the average position of the 'weight' concentrated in the triangle, you must consider their respective positions along both the x and y axes. The centre of mass will be the point that considers both axes to find a balance point for the weights.

Centre of Mass in Three-Dimensional Space

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These results for the particles can easily be generalized to a system of n particles, not necessarily lying in a plane, but distributed in space. The centre of mass of such a system is at (X, Y, Z), where:

X = (Σ(m_i * x_i) / Σ(m_i))
Y = (Σ(m_i * y_i) / Σ(m_i))
Z = (Σ(m_i * z_i) / Σ(m_i))

Here M = Σm_i is the total mass of the system. The index i runs from 1 to n; m_i is the mass of the ith particle and the position of the ith particle is given by (x_i, y_i, z_i).

Detailed Explanation

To find the centre of mass in three-dimensional space, we apply the same principles we used in two dimensions. This means we have three equations, one for each coordinate axis (x, y, and z). Each equation computes the weighted average of positions, factoring in the mass of each particle, allowing us to locate the CM accurately in three-dimensional space.

Examples & Analogies

Picture a large air balloon with different colored weighted balls inside it (like a model of a solar system). Each ball's weight and position impact where the center of gravity of the balloon will be. To find the centre of mass of the balloon, accounting for all dimensions—front, sides, and top—is crucial. Every ball shifts the overall weight, showing that the CM isn't just based on one direction but must consider all three dimensions.

Continuous Mass Distribution

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Eqs. (6.4a), (6.4b) and (6.4c) can be combined into one equation using the notation of position vectors. Let r_i be the position vector of the ith particle and R be the position vector of the centre of mass:

R = (Σ(m_i * r_i) / Σm_i)

The sum on the right hand side is a vector sum.

Detailed Explanation

When dealing with larger bodies made up of many particles, we can treat them as continuous distributions. We therefore replace summation with integration in our calculations. Each small mass element contributes to the overall CM, and we can express the CM as a vector function that simplifies calculations for large or complex systems.

Examples & Analogies

Think of a thick clay sculpture shaped by many tiny bits of clay. Each tiny piece contributes equally to the overall shape, making it challenging to find the CM just by looking at it. Instead, understanding the sculpture as continuous material helps calculate how the weight distributes across its form, revealing where the centre of mass lies.

Homogeneous Bodies and Symmetry

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Often we have to calculate the centre of mass of homogeneous bodies of regular shapes like rings, discs, spheres, rods etc. By using symmetry considerations, we can easily show that the centres of mass of these bodies lie at their geometric centres.

Detailed Explanation

For symmetrical structures like a log or a perfectly round sphere, intuitive reasoning about the shapes can lead us to the conclusion that the centre of mass lies at their geometric center. This is because the mass is distributed evenly on all sides, balancing equally around that point.

Examples & Analogies

Consider a bowling ball: it's perfectly round and weighted evenly throughout. If you balance it on your hand, it centers at the middle. This shows how symmetry directs the physical location of the centre of mass, making it straightforward to predict its balance point without complicated calculations.

Examples of Finding Centre of Mass

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Example 6.1 Find the centre of mass of three particles at the vertices of an equilateral triangle. The masses of the particles are 100g, 150g, and 200g respectively. Each side of the equilateral triangle is 0.5m long.

Answer:
With the x-and y-axes chosen as shown in the diagram, the coordinates of points O, A and B forming the equilateral triangle are respectively (0,0), (0.5,0), (0.25,0.25√3). Let the masses 100 g, 150g and 200g be located at O, A and B respectively.

Detailed Explanation

In this problem, we locate the centre of mass using the defined mass and location of each particle. By applying the equations for CM and substituting the known values, we can accurately compute the location of the centre of mass in a triangular configuration, emphasizing both the individual masses and their respective distances.

Examples & Analogies

Picture placing heavy boxes at different corners of a triangle to balance a seesaw. Heavier boxes closer to the fulcrum shift where the seesaw balances. This exercise helps understand how mass affects the balance point based on their positions, akin to how the centre of mass is calculated in physical systems.

Practical Centre of Mass Applications

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Example 6.2 Find the centre of mass of a triangular lamina. Answer: The lamina (∆LMN) may be subdivided into narrow strips each parallel to the base. By symmetry, each strip has its centre of mass at its midpoint. If we join the midpoint of all the strips we get the median LP. The centre of mass of the triangle as a whole therefore has to lie on the median LP.

Detailed Explanation

By breaking the triangular shape into simpler components and recognizing the symmetry, we can conclude that the overall centre of mass must be at a certain location along drawn medians. This showcases how understanding individual parts of a shape can aid in finding the composite area's centre of mass through patterns and balances.

Examples & Analogies

Imagine a triangular cake: if you wanted to slice it evenly to serve balanced pieces, knowing where to cut through could relate to understanding the cake's centre of mass. Each slice must consider both the mass of cake and where its weight lies, deferring towards that central point for everyone to get equal servings.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Centre of Mass: The average position of all mass in a system.

  • Mass-weighted Average: The average position adjusted for the mass of individual particles.

  • Equations for Centre of Mass: Specific formulas that provide methods for calculating the CM in systems of varying complexity.

Examples & Real-Life Applications

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Examples

  • For two particles with masses m1 and m2, the centre of mass can be calculated using the formula: X_{CM} = (m1x1 + m2x2) / (m1 + m2).

  • Finding the center of mass of a triangle formed by three particles: use the respective coordinates of the particles to apply the center of mass formula.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • Where the mass is spread, you'll find it ahead; the center of mass leads the way, keeps balance all day.

📖 Fascinating Stories

  • Imagine a seesaw; where you place heavier weights shifts the balance point. The center of mass is like that balance point for the whole system.

🧠 Other Memory Gems

  • M.A.R.S - Mass Affects the center of mass's Relative Spot.

🎯 Super Acronyms

C.O.M - Center of Mass represents Concentrated Ongoing Motion.

Flash Cards

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Glossary of Terms

Review the Definitions for terms.

  • Term: Centre of Mass

    Definition:

    The point at which the mass of a system or an object can be considered to be concentrated.

  • Term: Massweighted Average

    Definition:

    An average that takes into account the mass of different particles when determining the center of mass.

  • Term: Equations for Centre of Mass

    Definition:

    Formulas used to calculate the center of mass for different configurations of particles.