DOCSLIB.ORG

Chapter 4 Rigid Body Dynamics

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Chapter 4 Rigid Body Dynamics Chapter 4 Rigid Body Dynamics In Chapter 3 we developed the equations of motion for attitude kinematics. The main results of that chapter involve the description of attitude motion using attitude vari- ables, such as rotation matrices, Euler angles, Euler axis/angle sets, or quaternions. Knowing the values of any of these variables at a particular instant of time allows the attitude analyst to visualize the orientation of one reference frame with respect to another. Examples of mission-oriented analysis include determining the location of a target on the Earth, the direction to the Sun, and the local direction of the Earth’s magnetic field. We also developed in Chapter 3 the differential equations describing how these variables depend on the angular velocity of the reference frame. We did not investigate how the angular velocity, ~ω, is determined, and that is the subject of this chapter. In orbital dynamics, we typically model a spacecraft as a point mass or as a solid, rigid sphere, ignoring the rotational motion of the spacecraft about its mass center. This model is an excellent simplification for studying the motion of a satellite in its orbit — it works well for describing the motion of Europa about Jupiter, of Jupiter about the Sun, and of artificial satellites about the Earth. For artificial satellites, however, we usually need to know the orientation of the satellite with some degree of accuracy, so we need a more detailed model than the point mass model. A real spacecraft, of course, is a complicated piece of machinery with moving parts, flexible appendages, and partially filled fluid containers. However, even with all this complexity, a useful model is the rigid body, and that is the model we develop in this chapter. We begin with a rigorous development of the equations of motion for a rigid body, both translational and rotational. Because the torques applied to spacecraft are typically quite small, we find it useful to consider the special case of torque- free motion. As noted above, all spacecraft include non-rigid elements. The most notable effect of non-rigidity is the effect of energy dissipation, which has important implications for spacecraft design. We also present analysis of two important examples of rigid body dynamics: the Lagrange top, and the unbalanced rotor. 4-1 Copyright C. D. Hall September 20, 2005 4-2 CHAPTER 4. RIGID BODY DYNAMICS 4.1 Newton’s Second Law Newton’s laws are applicable to a particle, or point mass, m. As noted previously, Newton’s Second Law may be expressed as one second-order vector differential equa- tion m~¨r = ~f (4.1) or as two first-order vector differential equations ~p˙ = ~f (4.2) ~r˙ = ~p/m (4.3) Here ~r is the position vector of the particle with respect to an inertially fixed point, ~p is the linear momentum of the particle, and ~f is the total applied force acting on the particle. Either of these equations describe the dynamics of a particle. Integration of these equations requires selection of specific coordinates or position variables. For example, with respect to an inertial reference frame, we might represent ~r and ~f by T ~r = r1ˆi1 + r2ˆi2 + r3ˆi3 = r ˆi (4.4) T n o ~f = f1ˆi1 + f2ˆi2 + f3ˆi3 = f ˆi (4.5) In this case, the scalar second-ordern equationso of motion are mr¨1 = f1 (4.6) m¨r = f mr¨ = f (4.7) ⇔ 2 2 mr¨3 = f3 (4.8) and the scalar first-order equations of motion are p˙1 = f1 (4.9) p˙ = f p˙ = f (4.10) ⇔ 2 2 p˙3 = f3 (4.11) r˙1 = p1/m (4.12) r˙ = p/m r˙ = p /m (4.13) ⇔ 2 2 r˙3 = p3/m (4.14) One approach to developing the equations of motion of a spacecraft is to model the spacecraft as a system of point masses, mi, i = 1, ..., n and then develop the set of 3n second-order equations of motion or the set of 6n first-order equations of motion. This approach involves summations of the mass particles. For example, the n ~ n ~ total mass is m = i=1 mi, and the total applied force is f = i=1 fi. However, if we are to model a complicatedP spacecraft as a system of particles,P the number n will be large and the system of equations will be quite unwieldy. In the following section, we develop the rigid body as an approximate mathematical model for spacecraft attitude motion. Copyright C. D. Hall September 20, 2005 4.2. THE RIGID BODY MODEL 4-3 4.2 The Rigid Body Model Here we introduce the concept of a continuum of differential mass elements, dm.A differential mass element is quite a different concept from a mass particle. Whereas a mass particle has zero volume but finite mass, a differential mass element has infinitesimal volume and infinitesimal mass. The mathematical approach to collecting mass particles is summation, whereas the approach to collecting differential mass elements is integration. A common problem in studying rigid body motion is the problem of integrating over the volume of the body. This operation may have a scalar integrand, a vector integrand, or a tensor integrand, and all three of these types of integrands are impor- tant in rigid body motion. Integrating over the volume of the body is a triple integral operation, but we use the following shorthand notation: r1max r2max(r1) r3max(r1,r2) f(~r) dV = f(~r) dr3 dr2 dr1 (4.15) ZB Zr1min Zr2min(r1) Zr3min(r1,r2) Let us consider two examples, using a rectangular prism shape with a reference frame coinciding with one corner of the prism as shown in Fig. 4.1. The first example is the scalar integral corresponding to the total mass, and the second is the vector integral corresponding to the first moment of inertia. a c bˆ3 bˆ o 2 bˆ1 b Figure 4.1: A rectangular prism for computing mass and first moment of inertia Total mass. To compute the total mass of a rigid body, we need to know the density of the body, which we denote by µ. Units for µ are mass/length3; for example, aluminum has density µ = 2.85 103 kg/m3. In general, the density could vary from point to point within the body, or× it could vary as a function of time. Thus, generally Copyright C. D. Hall September 20, 2005 4-4 CHAPTER 4. RIGID BODY DYNAMICS µ = µ(~r,t). However, we generally assume that µ is constant, both spatially and temporally. Also, we usually use dm = µ dV as the variable of integration, even though m is not an independent variable. The meaning should always be clear. The total mass may then be expressed as m = µ dV = dm (4.16) ZB ZB Carrying out the indicated operations for the prism in Fig. 4.1, we obtain the expected result: a b c m = µ dr3 dr2 dr1 = µabc (4.17) Z0 Z0 Z0 Note that sometimes the total mass is called the zeroth moment of inertia, because it is a generalization of the first and second moments of inertia. First moment of inertia. This vector quantity is defined as ~co = ~ro dm (4.18) ZB Clearly this quantity depends on the origin from which ~r is measured, which motivates use of the superscript o to denote that ~r is measured from o, and the superscript o to denote that ~c is the first moment about o. The matrix version of this equation is co = ro dm (4.19) ZB To compute the first moment, we must choose a reference frame, and the compo- nents depend on that choice of frame. For this example, we use the body frame and corner origin of Fig. 4.1. Thus, ~c = c T bˆ , ~r = r T bˆ , and b { } b { } a b c o T µ 2 2 2 T cb = µ [r1 r2 r3] dr3 dr2 dr1 = a bc ab c abc (4.20) 0 0 0 2 Z Z Z h i Center of mass. We can use this result to determine the center of mass, c, which is defined as the point about which the first moment of inertia is zero; i.e. ~cc = ~0. Mathematically, this definition leads to ~cc = ~rc dm = ~0 (4.21) ZB The position vector from o to any point may be written as ~ro = ~roc +~rc (4.22) where ~roc is the position vector of c with respect to o, which is what we want to find, and ~rc is the position vector from c to a point in the body. The matrix version of this expression is ro = roc + rc (4.23) Copyright C. D. Hall September 20, 2005 4.2. THE RIGID BODY MODEL 4-5 where we omit the subscript b since the reference frame is clear. Substituting this expression into Eq. (4.19), we find co = (roc + rc) dm = roc dm = roc dm = mroc (4.24) ZB ZB ZB Thus, 1 µ roc = co = a2bc ab2c abc2 T (4.25) m 2m h i Using the expression for the mass m developed in Eq. (4.17), we obtain 1 roc = [abc] T (4.26) 2 which is obvious from the figure. However, this same procedure is applicable to more complicated geometric figures and to cases where µ is not constant. We now continue with the development of equations of motion for a rigid body, beginning with the linear momentum equations, and then developing the angular momentum equations. Linear momentum. The linear momentum of a rigid body is defined as ~p = ~v dm (4.27) ZB where ~v is the time derivative of the position vector ~r of a differential mass element dm = µ dV .
Load more

Recommended publications
  • The Experimental Determination of the Moment of Inertia of a Model Airplane Michael Koken [email protected]
    The University of Akron IdeaExchange@UAkron The Dr. Gary B. and Pamela S. Williams Honors Honors Research Projects College Fall 2017 The Experimental Determination of the Moment of Inertia of a Model Airplane Michael Koken [email protected] Please take a moment to share how this work helps you through this survey. Your feedback will be important as we plan further development of our repository. Follow this and additional works at: http://ideaexchange.uakron.edu/honors_research_projects Part of the Aerospace Engineering Commons, Aviation Commons, Civil and Environmental Engineering Commons, Mechanical Engineering Commons, and the Physics Commons Recommended Citation Koken, Michael, "The Experimental Determination of the Moment of Inertia of a Model Airplane" (2017). Honors Research Projects. 585. http://ideaexchange.uakron.edu/honors_research_projects/585 This Honors Research Project is brought to you for free and open access by The Dr. Gary B. and Pamela S. Williams Honors College at IdeaExchange@UAkron, the institutional repository of The nivU ersity of Akron in Akron, Ohio, USA. It has been accepted for inclusion in Honors Research Projects by an authorized administrator of IdeaExchange@UAkron. For more information, please contact [email protected], [email protected]. 2017 THE EXPERIMENTAL DETERMINATION OF A MODEL AIRPLANE KOKEN, MICHAEL THE UNIVERSITY OF AKRON Honors Project TABLE OF CONTENTS List of Tables ................................................................................................................................................
    [Show full text]
  • Rotational Motion (The Dynamics of a Rigid Body)
    University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Robert Katz Publications Research Papers in Physics and Astronomy 1-1958 Physics, Chapter 11: Rotational Motion (The Dynamics of a Rigid Body) Henry Semat City College of New York Robert Katz University of Nebraska-Lincoln, [email protected] Follow this and additional works at: https://digitalcommons.unl.edu/physicskatz Part of the Physics Commons Semat, Henry and Katz, Robert, "Physics, Chapter 11: Rotational Motion (The Dynamics of a Rigid Body)" (1958). Robert Katz Publications. 141. https://digitalcommons.unl.edu/physicskatz/141 This Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Robert Katz Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. 11 Rotational Motion (The Dynamics of a Rigid Body) 11-1 Motion about a Fixed Axis The motion of the flywheel of an engine and of a pulley on its axle are examples of an important type of motion of a rigid body, that of the motion of rotation about a fixed axis. Consider the motion of a uniform disk rotat­ ing about a fixed axis passing through its center of gravity C perpendicular to the face of the disk, as shown in Figure 11-1. The motion of this disk may be de­ scribed in terms of the motions of each of its individual particles, but a better way to describe the motion is in terms of the angle through which the disk rotates.
    [Show full text]
  • Chapter 3 Dynamics of Rigid Body Systems
    Rigid Body Dynamics Algorithms Roy Featherstone Rigid Body Dynamics Algorithms Roy Featherstone The Austrailian National University Canberra, ACT Austrailia Library of Congress Control Number: 2007936980 ISBN 978-0-387-74314-1 ISBN 978-1-4899-7560-7 (eBook) Printed on acid-free paper. @ 2008 Springer Science+Business Media, LLC All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Science+Business Media, LLC, 233 Spring Street, New York, NY 10013, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks and similar terms, even if they are not identified as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. 9 8 7 6 5 4 3 2 1 springer.com Preface The purpose of this book is to present a substantial collection of the most efficient algorithms for calculating rigid-body dynamics, and to explain them in enough detail that the reader can understand how they work, and how to adapt them (or create new algorithms) to suit the reader’s needs. The collection includes the following well-known algorithms: the recursive Newton-Euler algo- rithm, the composite-rigid-body algorithm and the articulated-body algorithm. It also includes algorithms for kinematic loops and floating bases.
    [Show full text]
  • Pierre Simon Laplace - Biography Paper
    Pierre Simon Laplace - Biography Paper MATH 4010 Melissa R. Moore University of Colorado- Denver April 1, 2008 2 Many people contributed to the scientific fields of mathematics, physics, chemistry and astronomy. According to Gillispie (1997), Pierre Simon Laplace was the most influential scientist in all history, (p.vii). Laplace helped form the modern scientific disciplines. His techniques are used diligently by engineers, mathematicians and physicists. His diverse collection of work ranged in all fields but began in mathematics. Laplace was born in Normandy in 1749. His father, Pierre, was a syndic of a parish and his mother, Marie-Anne, was from a family of farmers. Many accounts refer to Laplace as a peasant. While it was not exactly known the profession of his Uncle Louis, priest, mathematician or teacher, speculations implied he was an educated man. In 1756, Laplace enrolled at the Beaumont-en-Auge, a secondary school run the Benedictine order. He studied there until the age of sixteen. The nest step of education led to either the army or the church. His father intended him for ecclesiastical vocation, according to Gillispie (1997 p.3). In 1766, Laplace went to the University of Caen to begin preparation for a career in the church, according to Katz (1998 p.609). Cristophe Gadbled and Pierre Le Canu taught Laplace mathematics, which in turn showed him his talents. In 1768, Laplace left for Paris to pursue mathematics further. Le Canu gave Laplace a letter of recommendation to d’Alembert, according to Gillispie (1997 p.3). Allegedly d’Alembert gave Laplace a problem which he solved immediately.
    [Show full text]
  • Magnetism, Angular Momentum, and Spin
    Chapter 19 Magnetism, Angular Momentum, and Spin P. J. Grandinetti Chem. 4300 P. J. Grandinetti Chapter 19: Magnetism, Angular Momentum, and Spin In 1820 Hans Christian Ørsted discovered that electric current produces a magnetic field that deflects compass needle from magnetic north, establishing first direct connection between fields of electricity and magnetism. P. J. Grandinetti Chapter 19: Magnetism, Angular Momentum, and Spin Biot-Savart Law Jean-Baptiste Biot and Félix Savart worked out that magnetic field, B⃗, produced at distance r away from section of wire of length dl carrying steady current I is 휇 I d⃗l × ⃗r dB⃗ = 0 Biot-Savart law 4휋 r3 Direction of magnetic field vector is given by “right-hand” rule: if you point thumb of your right hand along direction of current then your fingers will curl in direction of magnetic field. current P. J. Grandinetti Chapter 19: Magnetism, Angular Momentum, and Spin Microscopic Origins of Magnetism Shortly after Biot and Savart, Ampére suggested that magnetism in matter arises from a multitude of ring currents circulating at atomic and molecular scale. André-Marie Ampére 1775 - 1836 P. J. Grandinetti Chapter 19: Magnetism, Angular Momentum, and Spin Magnetic dipole moment from current loop Current flowing in flat loop of wire with area A will generate magnetic field magnetic which, at distance much larger than radius, r, appears identical to field dipole produced by point magnetic dipole with strength of radius 휇 = | ⃗휇| = I ⋅ A current Example What is magnetic dipole moment induced by e* in circular orbit of radius r with linear velocity v? * 휋 Solution: For e with linear velocity of v the time for one orbit is torbit = 2 r_v.
    [Show full text]
  • Lie Group Formulation of Articulated Rigid Body Dynamics
    Lie Group Formulation of Articulated Rigid Body Dynamics Junggon Kim 12/10/2012, Ver 2.01 Abstract It has been usual in most old-style text books for dynamics to treat the formulas describing linear(or translational) and angular(or rotational) motion of a rigid body separately. For example, the famous Newton's 2nd law, f = ma, for the translational motion of a rigid body has its partner, so-called the Euler's equation which describes the rotational motion of the body. Separating translation and rotation, however, causes a huge complexity in deriving the equations of motion of articulated rigid body systems such as robots. In Section1, an elegant single equation of motion of a rigid body moving in 3D space is derived using a Lie group formulation. In Section2, the recursive Newton-Euler algorithm (inverse dynamics), Articulated-Body algorithm (forward dynamics) and a generalized recursive algorithm (hybrid dynamics) for open chains or tree-structured articulated body systems are rewritten with the geometric formulation for rigid body. In Section3, dynamics of constrained systems such as a closed loop mechanism will be described. Finally, in Section4, analytic derivatives of the dynamics algorithms, which would be useful for optimization and sensitivity analysis, are presented.1 1 Dynamics of a Rigid Body This section describes the equations of motion of a single rigid body in a geometric manner. 1.1 Rigid Body Motion To describe the motion of a rigid body, we need to represent both the position and orien- tation of the body. Let fBg be a coordinate frame attached to the rigid body and fAg be an arbitrary coordinate frame, and all coordinate frames will be right-handed Cartesian from now on.
    [Show full text]
  • Rigid Body Dynamics 2
    Rigid Body Dynamics 2 CSE169: Computer Animation Instructor: Steve Rotenberg UCSD, Winter 2017 Cross Product & Hat Operator Derivative of a Rotating Vector Let’s say that vector r is rotating around the origin, maintaining a fixed distance At any instant, it has an angular velocity of ω ω dr r ω r dt ω r Product Rule The product rule of differential calculus can be extended to vector and matrix products as well da b da db b a dt dt dt dab da db b a dt dt dt dA B dA dB B A dt dt dt Rigid Bodies We treat a rigid body as a system of particles, where the distance between any two particles is fixed We will assume that internal forces are generated to hold the relative positions fixed. These internal forces are all balanced out with Newton’s third law, so that they all cancel out and have no effect on the total momentum or angular momentum The rigid body can actually have an infinite number of particles, spread out over a finite volume Instead of mass being concentrated at discrete points, we will consider the density as being variable over the volume Rigid Body Mass With a system of particles, we defined the total mass as: n m m i i1 For a rigid body, we will define it as the integral of the density ρ over some volumetric domain Ω m d Angular Momentum The linear momentum of a particle is 퐩 = 푚퐯 We define the moment of momentum (or angular momentum) of a particle at some offset r as the vector 퐋 = 퐫 × 퐩 Like linear momentum, angular momentum is conserved in a mechanical system If the particle is constrained only
    [Show full text]
  • Finite Element Model Calibration of an Instrumented Thirteen-Story Steel Moment Frame Building in South San Fernando Valley, California
    Finite Element Model Calibration of An Instrumented Thirteen-story Steel Moment Frame Building in South San Fernando Valley, California By Erol Kalkan Disclamer The finite element model incuding its executable file are provided by the copyright holder "as is" and any express or implied warranties, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose are disclaimed. In no event shall the copyright owner be liable for any direct, indirect, incidental, special, exemplary, or consequential damages (including, but not limited to, procurement of substitute goods or services; loss of use, data, or profits; or business interruption) however caused and on any theory of liability, whether in contract, strict liability, or tort (including negligence or otherwise) arising in any way out of the use of this software, even if advised of the possibility of such damage. Acknowledgments Ground motions were recorded at a station owned and maintained by the California Geological Survey (CGS). Data can be downloaded from CESMD Virtual Data Center at: http://www.strongmotioncenter.org/cgi-bin/CESMD/StaEvent.pl?stacode=CE24567. Contents Introduction ..................................................................................................................................................... 1 OpenSEES Model ........................................................................................................................................... 3 Calibration of OpenSEES Model to Observed Response
    [Show full text]
  • Moment of Inertia
    MOMENT OF INERTIA The moment of inertia, also known as the mass moment of inertia, angular mass or rotational inertia, of a rigid body is a quantity that determines the torque needed for a desired angular acceleration about a rotational axis; similar to how mass determines the force needed for a desired acceleration. It depends on the body's mass distribution and the axis chosen, with larger moments requiring more torque to change the body's rotation rate. It is an extensive (additive) property: for a point mass the moment of inertia is simply the mass times the square of the perpendicular distance to the rotation axis. The moment of inertia of a rigid composite system is the sum of the moments of inertia of its component subsystems (all taken about the same axis). Its simplest definition is the second moment of mass with respect to distance from an axis. For bodies constrained to rotate in a plane, only their moment of inertia about an axis perpendicular to the plane, a scalar value, and matters. For bodies free to rotate in three dimensions, their moments can be described by a symmetric 3 × 3 matrix, with a set of mutually perpendicular principal axes for which this matrix is diagonal and torques around the axes act independently of each other. When a body is free to rotate around an axis, torque must be applied to change its angular momentum. The amount of torque needed to cause any given angular acceleration (the rate of change in angular velocity) is proportional to the moment of inertia of the body.
    [Show full text]
  • Angular Momentum and Magnetic Moment
    Angular momentum and magnetic moment Introduction to Nuclear Science Simon Fraser University Spring 2011 NUCS 342 | January 12, 2011 NUCS 342 (Lecture 1) January 12, 2011 1 / 27 2 Conservation laws 3 Scalar and vector product 4 Rotational motion 5 Magnetic moment 6 Angular momentum in quantum mechanics 7 Spin Outline 1 Scalars and vectors NUCS 342 (Lecture 1) January 12, 2011 2 / 27 3 Scalar and vector product 4 Rotational motion 5 Magnetic moment 6 Angular momentum in quantum mechanics 7 Spin Outline 1 Scalars and vectors 2 Conservation laws NUCS 342 (Lecture 1) January 12, 2011 2 / 27 4 Rotational motion 5 Magnetic moment 6 Angular momentum in quantum mechanics 7 Spin Outline 1 Scalars and vectors 2 Conservation laws 3 Scalar and vector product NUCS 342 (Lecture 1) January 12, 2011 2 / 27 5 Magnetic moment 6 Angular momentum in quantum mechanics 7 Spin Outline 1 Scalars and vectors 2 Conservation laws 3 Scalar and vector product 4 Rotational motion NUCS 342 (Lecture 1) January 12, 2011 2 / 27 6 Angular momentum in quantum mechanics 7 Spin Outline 1 Scalars and vectors 2 Conservation laws 3 Scalar and vector product 4 Rotational motion 5 Magnetic moment NUCS 342 (Lecture 1) January 12, 2011 2 / 27 7 Spin Outline 1 Scalars and vectors 2 Conservation laws 3 Scalar and vector product 4 Rotational motion 5 Magnetic moment 6 Angular momentum in quantum mechanics NUCS 342 (Lecture 1) January 12, 2011 2 / 27 Outline 1 Scalars and vectors 2 Conservation laws 3 Scalar and vector product 4 Rotational motion 5 Magnetic moment 6 Angular momentum in quantum mechanics 7 Spin NUCS 342 (Lecture 1) January 12, 2011 2 / 27 Scalars and vectors Scalars Scalars are used to describe objects which are fully characterized by their magnitude (a number and a unit).
    [Show full text]
  • Newton Euler Equations of Motion Examples
    Newton Euler Equations Of Motion Examples Alto and onymous Antonino often interloping some obligations excursively or outstrikes sunward. Pasteboard and Sarmatia Kincaid never flits his redwood! Potatory and larboard Leighton never roller-skating otherwhile when Trip notarizes his counterproofs. Velocity thus resulting in the tumbling motion of rigid bodies. Equations of motion Euler-Lagrange Newton-Euler Equations of motion. Motion of examples of experiments that a random walker uses cookies. Forces by each other two examples of example are second kind, we will refer to specify any parameter in. 213 Translational and Rotational Equations of Motion. Robotics Lecture Dynamics. Independence from a thorough description and angular velocity as expected or tofollowa userdefined behaviour does it only be loaded geometry in an appropriate cuts in. An interface to derive a particular instance: divide and author provides a positive moment is to express to output side can be run at all previous step. The analysis of rotational motions which make necessary to decide whether rotations are. For xddot and whatnot in which a very much easier in which together or arena where to use them in two backwards operation complies with respect to rotations. Which influence of examples are true, is due to independent coordinates. On sameor adjacent joints at each moment equation is also be more specific white ellipses represent rotations are unconditionally stable, for motion break down direction. Unit quaternions or Euler parameters are known to be well suited for the. The angular momentum and time and runnable python code. The example will be run physics examples are models can be symbolic generator runs faster rotation kinetic energy.
    [Show full text]
  • Rotation: Moment of Inertia and Torque
    Rotation: Moment of Inertia and Torque Every time we push a door open or tighten a bolt using a wrench, we apply a force that results in a rotational motion about a fixed axis. Through experience we learn that where the force is applied and how the force is applied is just as important as how much force is applied when we want to make something rotate. This tutorial discusses the dynamics of an object rotating about a fixed axis and introduces the concepts of torque and moment of inertia. These concepts allows us to get a better understanding of why pushing a door towards its hinges is not very a very effective way to make it open, why using a longer wrench makes it easier to loosen a tight bolt, etc. This module begins by looking at the kinetic energy of rotation and by defining a quantity known as the moment of inertia which is the rotational analog of mass. Then it proceeds to discuss the quantity called torque which is the rotational analog of force and is the physical quantity that is required to changed an object's state of rotational motion. Moment of Inertia Kinetic Energy of Rotation Consider a rigid object rotating about a fixed axis at a certain angular velocity. Since every particle in the object is moving, every particle has kinetic energy. To find the total kinetic energy related to the rotation of the body, the sum of the kinetic energy of every particle due to the rotational motion is taken. The total kinetic energy can be expressed as ..
    [Show full text]