Moment of inertia of a rectangle
Author: o | 2025-04-24
Moment of Inertia of a Rectangle. This calculator provides the calculation of the moment of inertia of a rectangle. Explanation. Calculation Example: The moment of inertia is a
Moment of Inertia of a Rectangle
We speed ω replaces linear speed v, rotationalapply the force. Thus, the quantity analogousto mass includes not only the mass, but also K.E. 1 IZ 2 is analogous to translationaltakes care of the distance wise distribution of 2the mass around the axis of rotation. To know 1 K.E. = 2 mv 2 . Thus, I is defined to be the rotational inertia or moment of inertia (M.I.)the exact relation, let us derive an expression of the object about the given axis of rotation.for the rotational kinetic energy which is the It is clear that the moment of inertia of ansum of the translational kinetic energies of all object depends upon (i) individual masses andthe individual particles. (ii) the distribution of these masses about the given axis of rotation. For a different axis, it will again depend upon the mass distribution around that axis and will be different if there is no symmetry. During this discussion, for simplicity, we assumed the object to be consisting of a finite number of particles. In practice, usually, itFig. 1.12: A body of N particles. is not so. For a homogeneous rigid object ofFigure 1.12 shows a rigid object rotating mathematically integrable mass distribution,with a constant angular speed ω about an the moment of inertia is to be obtained byaxis perpendicular to the plane of paper.For theoretical simplification let us consider integration as I ³r2dm . If integrable mass distribution is not known, it is not possible tothe object to be consisting of N particles obtain the moment of inertia theoretically, butof masses m1, m2, …..mN at respective it can be determined experimentally.perpendicular distances r1, r2, …..rN from theaxis of rotation. As the object rotates, all theseparticles perform UCM with the same angularspeed ω , but with different linear speedsv1 r1Z, v2 r2Z, }} vN rNZ .Translational K.E. of the first particle is 1 1K.E.1 2 m1v12 2 m1r12Z 2 Fig. 1.13: Moment of Inertia of a ring. 1.5.1 Moment of Inertia of a Uniform Ring:Similar will be the case of all the other An object is called a uniform ring ifparticles. Rotational K.E. of the object, is its mass is (practically) situated uniformly on the circumference of a circle (Fig 1.13).the sum of individual translational kinetic Obviously, it is a two dimensional object of negligible thickness. If it is rotating about itsenergies. Thus, rotational K.E. own axis (line perpendicular to its plane and passing through its centre), its entire mass M1 m1r12Z 2 1 m2r22Z 2 }} 1 mN rN2Z 2 is practically at a distance equal to its radius2 2 2 R form the axis. Hence, the expression for the moment of inertia of a uniform ring of mass M? Rotational K.E. and radius R is I = MR2. 1 1 22 m1r12 m2r22 } mN rN2 Z2 IZ 2 N¦Where I m1r12 m2r22 }} mN rN2 mi ri2 i1 131.5.2 Moment of Inertia of a Uniform Disc: of inertias of objects of several integrable geometrical shapes can be derived. Some of Disc
Moment Of Inertia Of Rectangle
Added. W N mN rN2DThus, magnitude of angular momentum of the If the rotation is restricted to a singlebody is given by plane, directions of all these torques are theL m1r12Z m2r22Z } mN rN2Z same, and along the axis. Magnitude of the m1r12 m2r22 } mN rN2 Z IZ resultant torque is then given byWhere, I m1r12 m2r22 } mN rN2 is the W W1 W2 } WN IDmoment of inertia of the body about the given m1r12 m2r22 } mN rN2 D 17where, I m1r12 m2r22 } mN rN2 is the Examples of conservation of angular momentum: During some shows of balletmoment of inertia of the object about the given dance, acrobat in a circus, sports like ice skating, diving in a swimming pool, etc., theaxis of rotation. principle of conservation of angular momentum is realized. In all these applications the product The relation W ID is analogous tof = ma for the translational motion if the L IZ I 2S n is constant (once the playersmoment of inertia I replaces mass, which is acquire a certain speed). Thus, if the moment of inertia I is increased, the angular speed andits physical significance. hence the frequency of revolution n decreases. Also, if the moment of inertia is decreased, the1.10 Conservation of Angular Momentum: frequency increases. (i) Ballet dancers: During ice ballet, theIn the article 4.7 of XIth Std. we have dancers have to undertake rounds of smaller and larger radii. The dancers come togetherseen the conservation of linear momentum while taking the rounds of smaller radius (near the centre). In this case, the moment of inertiawhich says that linear momentum of an of their system becomes minimum and the frequency increases, to make it thrilling. Whileisolated system is conserved in the absence outer rounds, the dancers outstretch their legs and arms. This increases their moment ofof an external unbalanced force. As seen inertia that reduces the angular speed and hence the linear speed. This is essential toearlier, torque and angular momentum are prevent slipping. (ii) Diving in a swimming pool (duringthe respective analogous quantities to force competition): While on the diving board, the divers stretch their body so as to increase theand linear momentum in rotational dynamics. moment of inertia. Immediately after leaving the board, they fold their body. This reducesWith suitable changes this can be transformed the moment inertia considerably. As a result, the frequency increases and they can completeinto the conservation of angular momentum. more rounds in air to make the show attractive. Again, while entering into water they stretchAs seen in the section 1.8, angular their body into a streamline shape. This allows them a smooth entry into the water.romwofhotaaemtrsieeoynnsrttueaimmnsdtoihsreptghpiievosesmtinhtoiebomylnienvnLeetacortformlriunofrempoaemrnmttuhomema.exnitsuomf ?DNddioLtfddwfLte,reddddntrtt riar=utuivFnpg awmnidtrhvudrdueddtpsvptp=e cFdtdrtt.outipme, we get, ?BNuoddtwLtr vuv 0?WThus, r u F ×dLF is the moment of force or torque τ . constant. Example 1.8: A spherical water balloon idf tW 0, dL 0 or L is revolving at 60 rpm. In the course of dt time, 48.8 % of its water leaks out.Moment of Inertia and Product of Inertia for a Rectangle
Is relatively complicated. Plastic section modulus can be exactly calculated in general cross-section editor. In simplified calculation, we can neglect beam bottom flange and assume haunch web and flange thicknesses equal to beam web and flange thicknesses.\[W_{pl,y} = 2 \cdot [(14.6 \cdot 190) \cdot (450+178)/2 +(450+178)/2 \cdot (450+178)/4)] = 1 840 668 \textrm{ mm}^3 \]The stress at the haunch end is:\[ \sigma = M_{Ed}/W_{pl,y} = 642 \cdot 10^6 / 1840668 = 349 \textrm{ MPa}\]Again, we should expect yielding in flanges and not fully utilized web. This agrees well with IDEA StatiCa. Stresses from unit bending moment and cross-sectional properties of the hauch just behind end plate End plateThe shear and normal stresses are transferred into the end plate via welds. Full-penetration butt welds are used for critical welds of flanges. Fillet welds are used at the web where the welds are less loaded. There are several approaches we can use for designing welds at I-section. The most simple is to assume that welds at flanges take bending moments and welds at the web transfer shear force More accurate at the elastic stage is the assumption that weld group transfers the bending moment in ratio of moment of inertias, i.e.:\[M_{flange} = I_{flange}/I_{total}\]\[M_{web} = I_{web}/I_{total}\]where: Mflange – portion of bending moment transferred via flange welds Mweb – portion of bending moment transferred via web welds Note that \(M_{flange}+M_{web} = M_{total}\) Iflange – moment of inertia of flanges Iweb – moment of inertia of web Itotal – total moment of inertia Note that \(I_{flange}+I_{web} = I_{total}\) The shear force is assumed to be taken only by beam web.So we can expect significant shear stresses parallel to the weld axis, \(\tau_\parallel\) and some normal and shear stresses, \(\sigma_\perp\) and \(\tau_\perp\) due to bending.The magnitude of \(\tau_\parallel\) can be calculated by summing up the fillet weld areas at the beam web and the hauch web:\[A_w = 2 \cdot 5 \cdot 421 + 2 \cdot 5 \cdot 118 = 5390\textrm{ mm}^2\]Then we can calculate the expected uniform stress:\[\tau_\parallel = V_{Ed} / A_w = 270 \cdot 10^3 / 5390=50 \textrm{ MPa}\]Comparison to IDEA StatiCa results shows a complicated stress pattern exceeding the calculated value: The load is transferred through the end plate into bolts. Typically, it is assumed that the shear forces are distributed evenly into all bolts. Alternatively, the bolts loaded most in tension are excluded and it is assumed that the bolts in compression zone. Moment of Inertia of a Rectangle. This calculator provides the calculation of the moment of inertia of a rectangle. Explanation. Calculation Example: The moment of inertia is a The Moment of Inertia of a Rectangle. The rectangle is among the simplest shapes for which a moment of inertia can be calculated. Generally, the moment of inertia is calculated by integrating the area of a shape, but the derivation of the moment of inertia ofMoment of Inertia of a Rectangle - Calcresource
Is a two dimensional circular object those are given in the Table 3 at the end of the topic.of negligible thickness. It is said to be uniform 1.6 Radius of Gyration:if its mass per unit area and its composition is As stated earlier, theoretical calculation m mass of moment of inertia is possible only forthe same throughout. The ratio V A area mathematically integrable geometrical shapes.is called the surface density. However, experimentally we can determine the moment of inertia of any object. It depends Consider a uniform disc of mass M and upon mass of that object and how that mass is distributed from or around the given axisradius R rotating about its own axis, which is of rotation. If we are interested in knowing only the mass distribution around the axis ofthe line perpendicular to its plane and passing rotation, we can express moment of inertia of any object as I = MK 2 , where M is massthrough its centre ?V M of that object. It means that the mass of that S R2 . object is effectively at a distance K from the As it is a uniform circular object, it can given axis of rotation. In this case, K is defined as the radius of gyration of the object aboutbe considered to be consisting of a number the given axis of rotation. In other words, if K is radius of gyration for an object, I = MK 2 isof concentric rings of radii increasing from the moment of inertia of that object. Larger the value of K, farther is the mass from the axis.(practically) zero to R. One of such rings ofmass dm is shown by shaded portion in theFig. 1.14. Fig .1.14: Moment of Inertia of a disk. Consider a uniform ring and a uniform Width of this ring is dr, which is so small disc, both of the same mass M and samethat the entire ring can be considered to be radius R. Let Ir and Id be their respective moment of inertias.of average radius r. (In practical sense, dr isless than the least count of the instrument that If Kr and Kd are their respective radiimeasures r, so that r is constant for that ring). of gyration, we can write, dmArea of this ring is A = 2πr.dr ?V 2S r.dr Ir = MR2 = MK 2 ∴Kr = R and 1 r = R 2∴ dm = 2πσr.dr. Id = 2 MR2 = MK 2 ∴ Kd ∴ Kd d As it is a ring, this entire mass is at a It shows mathematically that K isdistance r from the axis of rotation. Thus, the decided by the distribution of mass. In amoment of inertia of this ring is Ir = dm (r2) ring the entire mass is distributed at the Moment of inertia (I) of the disc can now distance R, while for a disc, its mass isbe obtained by integrating Ir from r = 0 to distributed between 0 andMoment of inertia of a rectangle - Civils.ai
SciencePhysicsPhysics questions and answersConceptual Example 14 provides useful background for this problem. A playground carousel is free to rotate about its center on frictionless bearings, and air resistance is negligible. The carousel itself (without riders) has a moment of inertia of 125 kg⋅m2. When one person is standing on the carousel at a distance of 1.50 m from the center, the carousel hasThis problem has been solved!You'll get a detailed solution from a subject matter expert that helps you learn core concepts.See AnswerQuestion: Conceptual Example 14 provides useful background for this problem. A playground carousel is free to rotate about its center on frictionless bearings, and air resistance is negligible. The carousel itself (without riders) has a moment of inertia of 125 kg⋅m2. When one person is standing on the carousel at a distance of 1.50 m from the center, the carousel hasShow transcribed image textTranscribed image text: Conceptual Example 14 provides useful background for this problem. A playground carousel is free to rotate about its center on frictionless bearings, and air resistance is negligible. The carousel itself (without riders) has a moment of inertia of 125 kg⋅m2. When one person is standing on the carousel at a distance of 1.50 m from the center, the carousel has an angular velocity of 0.600rad/s. However, as this person moves inward to a point located 0.997 m from the center, the angular velocity increases to 0.800rad/s. What is the person's mass?Rectangle Moment of Inertia (MoI)
What is a Reinforced Concrete Beam Calculator?Why use a Reinforced Concrete Beam Calculator?Reinforced Concrete Beam CalculatorHow to use a Reinforced Concrete Beam Calculator:Example of Calculator:A reinforced concrete beam calculator is a tool used in structural engineering to estimate parameters related to a reinforced concrete beam. It considers factors such as beam dimensions, concrete strength, steel strength, and applied loads to calculate properties like moment of inertia, modular ratio, depth to neutral axis, and stress in concrete.Why use a Reinforced Concrete Beam Calculator?Structural Design: Assists in designing reinforced concrete beams for specific loads and spans.Material Selection: Helps in selecting concrete and steel materials with appropriate strengths.Load Distribution: Assists in calculating stress and deformation in the beam under applied loads.Code Compliance: Ensures compliance with local building codes and standards. Reinforced Concrete Beam Calculator How to use a Reinforced Concrete Beam Calculator:Enter parameters such as beam width, depth, concrete strength, steel strength, and applied load.Click the "Calculate" button.The calculator estimates properties like moment of inertia, modular ratio, depth to neutral axis, and stress in concrete based on the provided inputs.Example of Calculator: Let's say you have a reinforced concrete beam with the following specifications:Enter beam width: 12 inchesEnter beam depth: 18 inchesEnter concrete strength: 4,000 psiEnter steel strength: 60,000 psiEnter applied load: 50,000 poundsClick the "Calculate" button.Moment of Inertia: 5832.00 in^4Modular Ratio: 0.07Depth to Neutral Axis: 16.88 inchesStress in Concrete: 144.68 psiNoteLike this post? Share it with your friends!Suggested Read –MOMENT OF INERTIA - RECTANGLE - YouTube
References to the object (if defined)disableDyncalc()¶Disable the execution of _dyncalc(). This is used to avoid redundant_precalc() calculations delegated in _dyncalc().isDynamic()¶class schrodinger.application.desmond.packages.analysis.MomentOfInertia(msys_model, cms_model, aids)¶Bases: schrodinger.application.desmond.packages.staf.CompositeAnalyzerMoment of inertia tensorResult is 3x3 numpy.ndarray__init__(msys_model, cms_model, aids)¶__call__()¶Call self as a function.__class__¶alias of builtins.type__delattr__¶Implement delattr(self, name).__dict__ = mappingproxy({'__module__': 'schrodinger.application.desmond.packages.analysis', '__doc__': '\n Moment of inertia tensor\n\n Result is 3x3 `numpy.ndarray`\n ', '__init__': , '_postcalc': })¶__dir__() → list¶default dir() implementation__eq__¶Return self==value.__format__()¶default object formatter__ge__¶Return self>=value.__getattribute__¶Return getattr(self, name).__gt__¶Return self>value.__hash__¶Return hash(self).__init_subclass__()¶This method is called when a class is subclassed.The default implementation does nothing. It may beoverridden to extend subclasses.__le__¶Return self__lt__¶Return self__module__ = 'schrodinger.application.desmond.packages.analysis'¶__ne__¶Return self!=value.static __new__(cls, *args, **kwargs)¶Create and return a new object. See help(type) for accurate signature.__reduce__()¶helper for pickle__reduce_ex__()¶helper for pickle__repr__¶Return repr(self).__setattr__¶Implement setattr(self, name, value).__sizeof__() → int¶size of object in memory, in bytes__str__¶Return str(self).__subclasshook__()¶Abstract classes can override this to customize issubclass().This is invoked early on by abc.ABCMeta.__subclasscheck__().It should return True, False or NotImplemented. If it returnsNotImplemented, the normal algorithm is used. Otherwise, itoverrides the normal algorithm (and the outcome is cached).__weakref__¶list of weak references to the object (if defined)disableDyncalc()¶Disable the execution of _dyncalc(). This is used to avoid redundant_precalc() calculations delegated in _dyncalc().isDynamic()¶class schrodinger.application.desmond.packages.analysis.MomentOfInertiaDirector(msys_model, cms_model, asl)¶Bases: schrodinger.application.desmond.packages.staf.CompositeDynamicAslAnalyzerThis class calculates the principal moment-of-inertia for each of theselected molecules.Result: A list of vectors__call__()¶Call self as a function.__class__¶alias of builtins.type__delattr__¶Implement delattr(self, name).__dict__ = mappingproxy({'__module__': 'schrodinger.application.desmond.packages.analysis', '__doc__': '\n This class calculates the principal moment-of-inertia for each of the\n selected molecules.\n\n Result: A list of vectors\n ', '_dyninit': , '__call__': })¶__dir__() → list¶default dir() implementation__eq__¶Return self==value.__format__()¶default object formatter__ge__¶Return self>=value.__getattribute__¶Return getattr(self, name).__gt__¶Return self>value.__hash__¶Return hash(self).__init__(msys_model, cms_model, asl)¶__init_subclass__()¶This method. Moment of Inertia of a Rectangle. This calculator provides the calculation of the moment of inertia of a rectangle. Explanation. Calculation Example: The moment of inertia is a The Moment of Inertia of a Rectangle. The rectangle is among the simplest shapes for which a moment of inertia can be calculated. Generally, the moment of inertia is calculated by integrating the area of a shape, but the derivation of the moment of inertia of
Moment of Inertia for a Rectangle - YouTube
The concept of conservation of angular momentum.Another example of conservation of angular momentum is a bowling ball hitting a pin 🎳.Angular momentum examplesLet's take a look at some examples of angular momentum where you have to calculate the angular momentum of an object.Example no. 1An object with a moment of inertia of 2 kg ⋅ m² rotates at 1 rad/s. What is the angular momentum of the object?Given:Moment of inertia = 2 kg ⋅ m²Angular velocity = 1 rad/sRequired:Angular momentum=?Solution:The given data suggests we find the solution using the formula:L = I × ωL = 2 × 1L = 2 kg⋅m²/sExample no. 2A 3-kg particle rotates at a constant angular velocity of 2 rad/s. What is the angular momentum if the radius of the circle is 10 cm?Given:Mass = 3 kgRadius = 10 cm = 10/100 = 0.1 mAngular velocity = 2 rad/sRequired:Moment of inertia =?Angular momentum =?Solution:The given data suggests we find the solution using the rotatory body formula for angular momentum:L = I × ωBut first, we need to determine the moment of inertia for the particle using the formula:I = m × r²I = 3 × (0.1)²I = 3 × 0.01I= 0.03 kg⋅m²Now,L = I × ωL = 0.03 × 2L = 0.06 kg⋅m²/sWe hope that these examples help you to understand how to calculate angular momentum with any given data.Speaking of angles and angular concepts, are you aware of angular displacement? Check out our angular displacement calculator if you are interested.Angular momentum in other fields of physicsThere are many other fields where angular momentum plays a significant role:Center force motion;Angular momentum and torque;Orbital mechanics;General relativity;Quantum mechanics;Electrodynamics;Optics;Spin (atomic physics); andConservation of angular momentum in the law of areasUnderstanding the Moment of Inertia of a Rectangle
Favourite LinksStress Transformation Calculator Calculate Principal Stress, Maximum shear stress and the their planesCalculator for Moving Load AnalysisTo determine Absolute Max. B.M. due to moving loads. Bending Moment Calculator Calculate bending moment & shear force for simply supported beam Moment of Inertia Calculator Calculate moment of inertia of plane sections e.g. channel, angle, tee etc. Shear Stress Calculator Calculate Transverse Shear Stress for beam sections e.g. channel, angle, tee etc. Reinforced Concrete Calculator Calculate the Strength of Reinforced Concrete beam Deflection & Slope Calculator Calculate deflection and slope of simply supported beam for many load cases Fixed Beam Calculator Calculation tool for bending moment and shear force for Fixed Beam for many load cases BM & SF Calculator for Cantilever Calculate SF & BM for Cantilever Deflection & Slope Calculator for Cantilever For many load cases of Cantilever Overhanging beam calculator For SF & BM of many load cases of overhanging beam More Links Civil Engineering Quiz Test your knowledge on different topics of Civil Engineering Statically Indeterminate Structures Definition and methods of solving Solved Examples Truss Member Forces calculation using method of joints and method of sections Shear force and bending moment Illustrated solved examples to draw shear force and bending moment diagrams Slope and deflection of beam and Truss Illustrated solved examples to determine slope and deflection of beam and truss Solution of indeterminate structures slope deflection, moment distribution etc. Reinforced concrete beam Solved examples to determine the strength and other parameters Calculation of bending stress in a beam using bending equation and plotting the bending stress diagram Other Useful Links Skyscrapers of the world Containing Tall building worldwide Profile of Civil Engineers Get to know about distinguished Civil Engineers Professional SocietiesWorldwide Civil Engineers Professional Societies Please Tell your Friends about us Search our website for more... Join. Moment of Inertia of a Rectangle. This calculator provides the calculation of the moment of inertia of a rectangle. Explanation. Calculation Example: The moment of inertia is a The Moment of Inertia of a Rectangle. The rectangle is among the simplest shapes for which a moment of inertia can be calculated. Generally, the moment of inertia is calculated by integrating the area of a shape, but the derivation of the moment of inertia ofMoment Of Inertia Of Rectangle - Equation, Derivation
Rotational motionQuantity Symbol/ Quantity Symbol/ Inter-relation, expression Angular expression if possible displacement Linear s u r s θ Tdisplacement Linear velocity = Angular velocity Z dT v Z u r v ds dt dt Linear = Angular D dZ D a u racceleration a dv acceleration dt dt Rotational inertia ³ ¦ I r2dmInertia or mass m or moment of I mi ri2 inertia Linear p = mv Angular L IZ L r u pmomentum momentum W r u f Force f = dp Torque W dL dt Work dt Work W f s W W ------ Power T P dW v Power P dW W Z ------ dt f dt Table 3: Expressions for moment of inertias for some symmetric objects:Object Axis Expression of Figure moment of inertia Thin ring or Central I = MR2hollow cylinder Thin ring Diameter I = 1 MR 2 2Annular ring or thick walled I 1M Central 2 r22 r12hollow cylinder 21Uniform disc or Central I = 1 MR 2 solid cylinder 2Uniform disc Diameter I = 1 MR 2 4 Thin walled Central I = 2 MR 2hollow sphere 3Solid sphere Central I = 2 MR 2 5Uniform symmetric Central I 2 M r25 r15 spherical shell 5 r23 r13Thin uniform rod or Perpendicular to I = 1 ML2 rectangular plate length and passing 12 through centreThin uniform rod or Perpendicular to I = 1 MR 2 rectangular plate length and about 3 one endUniform plate Central I 1 M ( L2 b2 )or rectangular 12parallelepiped 22Uniform solid Central I = 3 MR 2right circular cone 10 Uniform hollow Central I = 1 MR 2right circular cone 2ExercisesUse g = 10 m/s2, unless, otherwise stated. formula (expression) of moment of1. Choose the correct option. inertia (M.I.) in terms of mass M of i) When seen from below, the blades of the object and some of its distance parameter/s, such as R, L, etc. a ceiling fan are seen to be revolving (A) Different objects must have different anticlockwise and their speed is decreasing. Select correct statement expressions for their M.I. about the directions of its angular (B) When rotating about their central velocity and angular acceleration. (A) Angular velocity upwards, angular axis, a hollow right circular cone and acceleration downwards. a disc have the same expression for (B) Angular velocity downwards, the M.I. angular acceleration upwards. (C) Expression for the M.I. for a (C) Both, angular velocity and angular parallelepiped rotating about the acceleration, upwards. transverse axis passing through its (D) Both, angular velocity and centre includes its depth. angular acceleration, downwards. (D) Expression for M.I. of a rod and ii) A particle of mass 1 kg, tied to a 1.2 m that of a plane sheet is the same long string is whirled to perform vertical about a transverse axis. circular motion, under gravity. Minimum iv) In a certain unit, the radius of gyration speed of a particle is 5 m/s. ConsiderComments
We speed ω replaces linear speed v, rotationalapply the force. Thus, the quantity analogousto mass includes not only the mass, but also K.E. 1 IZ 2 is analogous to translationaltakes care of the distance wise distribution of 2the mass around the axis of rotation. To know 1 K.E. = 2 mv 2 . Thus, I is defined to be the rotational inertia or moment of inertia (M.I.)the exact relation, let us derive an expression of the object about the given axis of rotation.for the rotational kinetic energy which is the It is clear that the moment of inertia of ansum of the translational kinetic energies of all object depends upon (i) individual masses andthe individual particles. (ii) the distribution of these masses about the given axis of rotation. For a different axis, it will again depend upon the mass distribution around that axis and will be different if there is no symmetry. During this discussion, for simplicity, we assumed the object to be consisting of a finite number of particles. In practice, usually, itFig. 1.12: A body of N particles. is not so. For a homogeneous rigid object ofFigure 1.12 shows a rigid object rotating mathematically integrable mass distribution,with a constant angular speed ω about an the moment of inertia is to be obtained byaxis perpendicular to the plane of paper.For theoretical simplification let us consider integration as I ³r2dm . If integrable mass distribution is not known, it is not possible tothe object to be consisting of N particles obtain the moment of inertia theoretically, butof masses m1, m2, …..mN at respective it can be determined experimentally.perpendicular distances r1, r2, …..rN from theaxis of rotation. As the object rotates, all theseparticles perform UCM with the same angularspeed ω , but with different linear speedsv1 r1Z, v2 r2Z, }} vN rNZ .Translational K.E. of the first particle is 1 1K.E.1 2 m1v12 2 m1r12Z 2 Fig. 1.13: Moment of Inertia of a ring. 1.5.1 Moment of Inertia of a Uniform Ring:Similar will be the case of all the other An object is called a uniform ring ifparticles. Rotational K.E. of the object, is its mass is (practically) situated uniformly on the circumference of a circle (Fig 1.13).the sum of individual translational kinetic Obviously, it is a two dimensional object of negligible thickness. If it is rotating about itsenergies. Thus, rotational K.E. own axis (line perpendicular to its plane and passing through its centre), its entire mass M1 m1r12Z 2 1 m2r22Z 2 }} 1 mN rN2Z 2 is practically at a distance equal to its radius2 2 2 R form the axis. Hence, the expression for the moment of inertia of a uniform ring of mass M? Rotational K.E. and radius R is I = MR2. 1 1 22 m1r12 m2r22 } mN rN2 Z2 IZ 2 N¦Where I m1r12 m2r22 }} mN rN2 mi ri2 i1 131.5.2 Moment of Inertia of a Uniform Disc: of inertias of objects of several integrable geometrical shapes can be derived. Some of Disc
2025-04-23Added. W N mN rN2DThus, magnitude of angular momentum of the If the rotation is restricted to a singlebody is given by plane, directions of all these torques are theL m1r12Z m2r22Z } mN rN2Z same, and along the axis. Magnitude of the m1r12 m2r22 } mN rN2 Z IZ resultant torque is then given byWhere, I m1r12 m2r22 } mN rN2 is the W W1 W2 } WN IDmoment of inertia of the body about the given m1r12 m2r22 } mN rN2 D 17where, I m1r12 m2r22 } mN rN2 is the Examples of conservation of angular momentum: During some shows of balletmoment of inertia of the object about the given dance, acrobat in a circus, sports like ice skating, diving in a swimming pool, etc., theaxis of rotation. principle of conservation of angular momentum is realized. In all these applications the product The relation W ID is analogous tof = ma for the translational motion if the L IZ I 2S n is constant (once the playersmoment of inertia I replaces mass, which is acquire a certain speed). Thus, if the moment of inertia I is increased, the angular speed andits physical significance. hence the frequency of revolution n decreases. Also, if the moment of inertia is decreased, the1.10 Conservation of Angular Momentum: frequency increases. (i) Ballet dancers: During ice ballet, theIn the article 4.7 of XIth Std. we have dancers have to undertake rounds of smaller and larger radii. The dancers come togetherseen the conservation of linear momentum while taking the rounds of smaller radius (near the centre). In this case, the moment of inertiawhich says that linear momentum of an of their system becomes minimum and the frequency increases, to make it thrilling. Whileisolated system is conserved in the absence outer rounds, the dancers outstretch their legs and arms. This increases their moment ofof an external unbalanced force. As seen inertia that reduces the angular speed and hence the linear speed. This is essential toearlier, torque and angular momentum are prevent slipping. (ii) Diving in a swimming pool (duringthe respective analogous quantities to force competition): While on the diving board, the divers stretch their body so as to increase theand linear momentum in rotational dynamics. moment of inertia. Immediately after leaving the board, they fold their body. This reducesWith suitable changes this can be transformed the moment inertia considerably. As a result, the frequency increases and they can completeinto the conservation of angular momentum. more rounds in air to make the show attractive. Again, while entering into water they stretchAs seen in the section 1.8, angular their body into a streamline shape. This allows them a smooth entry into the water.romwofhotaaemtrsieeoynnsrttueaimmnsdtoihsreptghpiievosesmtinhtoiebomylnienvnLeetacortformlriunofrempoaemrnmttuhomema.exnitsuomf ?DNddioLtfddwfLte,reddddntrtt riar=utuivFnpg awmnidtrhvudrdueddtpsvptp=e cFdtdrtt.outipme, we get, ?BNuoddtwLtr vuv 0?WThus, r u F ×dLF is the moment of force or torque τ . constant. Example 1.8: A spherical water balloon idf tW 0, dL 0 or L is revolving at 60 rpm. In the course of dt time, 48.8 % of its water leaks out.
2025-03-28Is a two dimensional circular object those are given in the Table 3 at the end of the topic.of negligible thickness. It is said to be uniform 1.6 Radius of Gyration:if its mass per unit area and its composition is As stated earlier, theoretical calculation m mass of moment of inertia is possible only forthe same throughout. The ratio V A area mathematically integrable geometrical shapes.is called the surface density. However, experimentally we can determine the moment of inertia of any object. It depends Consider a uniform disc of mass M and upon mass of that object and how that mass is distributed from or around the given axisradius R rotating about its own axis, which is of rotation. If we are interested in knowing only the mass distribution around the axis ofthe line perpendicular to its plane and passing rotation, we can express moment of inertia of any object as I = MK 2 , where M is massthrough its centre ?V M of that object. It means that the mass of that S R2 . object is effectively at a distance K from the As it is a uniform circular object, it can given axis of rotation. In this case, K is defined as the radius of gyration of the object aboutbe considered to be consisting of a number the given axis of rotation. In other words, if K is radius of gyration for an object, I = MK 2 isof concentric rings of radii increasing from the moment of inertia of that object. Larger the value of K, farther is the mass from the axis.(practically) zero to R. One of such rings ofmass dm is shown by shaded portion in theFig. 1.14. Fig .1.14: Moment of Inertia of a disk. Consider a uniform ring and a uniform Width of this ring is dr, which is so small disc, both of the same mass M and samethat the entire ring can be considered to be radius R. Let Ir and Id be their respective moment of inertias.of average radius r. (In practical sense, dr isless than the least count of the instrument that If Kr and Kd are their respective radiimeasures r, so that r is constant for that ring). of gyration, we can write, dmArea of this ring is A = 2πr.dr ?V 2S r.dr Ir = MR2 = MK 2 ∴Kr = R and 1 r = R 2∴ dm = 2πσr.dr. Id = 2 MR2 = MK 2 ∴ Kd ∴ Kd d As it is a ring, this entire mass is at a It shows mathematically that K isdistance r from the axis of rotation. Thus, the decided by the distribution of mass. In amoment of inertia of this ring is Ir = dm (r2) ring the entire mass is distributed at the Moment of inertia (I) of the disc can now distance R, while for a disc, its mass isbe obtained by integrating Ir from r = 0 to distributed between 0 and
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