What is Rigid bodies: Definition and 127 Discussions

In physics, a rigid body (also known as a rigid object ) is a solid body in which deformation is zero or so small it can be neglected. The distance between any two given points on a rigid body remains constant in time regardless of external forces or moments exerted on it. A rigid body is usually considered as a continuous distribution of mass.
In the study of special relativity, a perfectly rigid body does not exist; and objects can only be assumed to be rigid if they are not moving near the speed of light. In quantum mechanics, a rigid body is usually thought of as a collection of point masses. For instance, molecules (consisting of the point masses: electrons and nuclei) are often seen as rigid bodies (see classification of molecules as rigid rotors).

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  1. F

    Is a Wall Considered a Rigid or Non-Rigid Body?

    Is a wall rigid body or a non-rigid body? As per definition, a rigid body does not change any inter-molecular space on application of force but moves. But a wall does neither. So what is it - rigid or non-rigid?
  2. M

    How Does Gear Size Affect Torque in a Truck Transmission?

    Homework Statement PS:The compound gear to the right is located inside a truck, whose engine rotates the driving gear with torque of 816 N*m. Assume that in the figure to the right, each of the smaller gears have a radius of 5.0 cm, and the larger gears have a radius of 25cm. Also, assume...
  3. E

    Equilibrium of Rigid Bodies: Solving a Complex Crane Boom Problem

    Homework Statement The upper portion of the crane boom consists of the jib AB, which is supported by the pin at A, the guy line BC, and the backstay CD, each cable being separately attached to the mast at C. If the 5-kN load is supported by the hoist line, which passes over the pulley at...
  4. E

    A problem in rigid bodies equillibrium

    Homework Statement The platform assembly has weight W1 and center of gravity at G1. If it is intended to support a maximum load W2 placed at point G2,,determine the smallest counterweight W that should be placed at B in order to prevent the platform from tipping over. Homework Equations...
  5. L

    Why Do Different Reference Points in Moment Calculations Affect the Result?

    Homework Statement Determine the reaction at C. Homework Equations The Attempt at a Solution i know i have to take the moment about line DB and thus resulting in ƩM(DB)= λ(DA X F) + λ(DC X RC) =0 where λ = DB/ DB however the book solved it differently. they instead do...
  6. L

    Why Does Object A Reach the Bottom First in a Rigid Body Dynamics Experiment?

    Homework Statement Three objects of equal mass, A, B and C, are released from rest at the same instant from the same height on identical ramps. Objects A and B are both blocks, and they slide down their respective ramps without rotating. Object C rolls down the ramp without slipping. Its...
  7. L

    Statics- equilibrium of rigid bodies

    Homework Statement An opening in a floor is covered by a 1×1.2-m sheet of plywood of mass 18 kg. The sheet is hinged at A and B and is maintained in a position slightly above the floor by a small block C. Determine the vertical component of the reaction (a) at A, (b) at B, (c) at C.Homework...
  8. K

    How to apply force to individual rigid bodies based on impact?

    Hello, SCENARIO : I am a game developer and I am playing around with rigidbodies. I have a piece of wall which might get impact by an external force. The wall has in turn several other broken pieces (hidden for the moment) put up together to resemble this huge wall. Upon impact I switch...
  9. L

    Inverse Dynamics - 2 Rigid Bodies connected with hinge

    I am trying to simulate and animate inverse dynamics. So I need to calculate torques. I started out by just taking a rod which rotates about its COM, applied a torque and then just used \tau=I\alpha to make sure I get back the same torque I applied. Next, I attached the rod to the ground with...
  10. J

    Engineering Statics (Equilibrium of Rigid Bodies)

    I have problems understanding the sense of reactive forces generated at pins / connections. Book is by Hibbler, I am posting links to images which will better explain my questions. The following links are concerning reactive forces. 1# http://i.imgur.com/0x62lp2.png 2#...
  11. T

    Planar kinematics of rigid bodies

    Homework Statement Homework Equations I'm dumbfounded by this problem. I can solve the (I think) harder problems with two fixed points and three rod segments. All the examples given in this book are of such "harder" problems and I have absolutely no example that resembles this...
  12. R

    Rigid Bodies Moment, Rotation & Angular Velocity Basics

    Hello, I am first year mechanical engineering student and I am struggling with the concept of moment, rotation and angular velocities, accelerations of rigid bodies. Can someone recommend an article with problems or an excerpt from a book or something similar to help me. It would be great if...
  13. P

    Moments & equilibrium of rigid bodies

    The underlined red part: I don't particularly understand where they got this from? The yellow highlight: Why are they finding the vertical distance, instead of the perpendicular distance?
  14. P

    Angular Momentum of Rigid Bodies

    I'm trying to calculate the rotation of a rigid body due to a force applied to a single point on the body. This application is for 3D game programming. I understand how to find the axis of rotation by calculating the cross product of the point of intersection & the vector between the center of...
  15. binbagsss

    Angular acceleration direction, rotating rigid bodies

    The direction of rotating bodies about a fixed axis. I am confused on how you should regard the the direction of mg sin x, the angular acceleration and the force exerted on the body by the axis, perpendicular to OG ( where O is the fixed axis location and G is the centre of mass of the body)...
  16. binbagsss

    What are the equations of motion for a rotating disc on a fixed axis?

    -See attached diagram - A uniform disc of mass m and radius a is free to rotate in a vertical plane about a fixed smooth horizontal axis, the axis passes through the mp A of the radius of the disc. - It then asks you to dervie equations of motion for when AO makes an angle pheta with the...
  17. T

    Dealing with angular momentum of rigid bodies

    I'm taking dynamics of rigid bodies, I'm having some trouble with impulse and momentum. Basically ,I know that angular momentum abt. a point ,is the linear momentum multilied by the moment arm . BUT ,I'm not feeling at all comfortable appliying it For example , When is H abt. a pt. p =...
  18. D

    Kinematics of Rigid Bodies question

    Homework Statement The uniform 3.6m pole is hinged to the truck bed and released from the vertical position as the truck starts from rest with than acceleration of 0.9m/s^2. If the acceleration remains constant during the motion of the pole, calculate the angular velocity of the pole as it...
  19. L

    Need help Dynamic Problem: Plane Kinetic of rigid bodies.

    Homework Statement The central moments of inertia of the gears A and B as shown are 10 kg.m2 and 20 kg.m2, respectively. If the mass of the block C is mC = 50 kg and the system is released from rest, determine alpha A of the gear A fig is attachedHomework Equations The Attempt at a Solution...
  20. Y

    Kinematics of Rigid Bodies in 3D

    Homework Statement Problem 15.18: I've defined the positive J (fixed) axis along the pole pointing upwards and the positive X (fixed) axis going to the right, with both centered at the point of contact between the hoop and the pole, which I will refer to as point O. The rotating axes...
  21. I

    Plane kinetics of rigid bodies

    Homework Statement The uniform 3.6m pole is hinged to the truck bed and is released from the vertical position as the truck starts from rest with an acceleration of 0.9m/s^2. If the acceleration remains constant during the motion of the pole as it reaches the horizontal position what is the...
  22. R

    How Do You Calculate the X and Y Components of Forces in a Pully System?

    Hi guys, Got a general question here about sum of forces that keeps coming up for me, and so I want to try and nail it this time. If you have a pully system, and there is a weight attached to a rope at one end of say 10 kg and the other end is is attached to a force of 1000 N that is...
  23. R

    Kinetics of Rigid Bodies: Torque and Acceleration Calculation

    Homework Statement The rotor of an electric motor of mass 200 kg has a radius of gyration of 150 mm. Calculate the torque required to accelerate it from rest to 1,500 rev/min in 6 seconds. Friction resistance may be neglected. Homework Equations ƩMO = Iα I = 1/2 (mr2 I am really really...
  24. S

    Rigid bodies and infinitely stiff springs ?

    Hi, guys! I've managed to implement a decent enough rigid body simulator for my 2D game using my sketchy high-school physics knowledge. Now I'm trying to pin some rigid bodies together. Connecting them with springs doesn't quite do the job. Too soft, the bodies drift apart. Too stiff, the...
  25. C

    Solving Dynamics of Rigid Bodies with Connected Loads and Inclined Forces

    Dynamics of Rigid bodies. Help How so solve this? Help Loads 1, 2 and 3 are connected to each other with light unstretchable rope and are on a horizontal rough (friction μ = 1 / 3) plane. Loads weights are: G, 2G, 3G. Force F acts at first load with intensity F = 10G√2 inclined at 45 °...
  26. C

    How Do You Calculate Speed and Angular Acceleration in Rigid Body Kinematics?

    What is solution to this problem any help is appreciated. Thanks Around wheel that can rotate around a horizontal axis is coiled (reeled) light unstretchable rope whose other end is tied load P. The position of the body is determined at all time with coordinate z =(pi)t³/3. Determine current...
  27. A

    Angular Velocities of rigid bodies at plane motionHelp me

    Angular Velocities of rigid bodies at plane motion..Help me please Please Help for solve this problem.I really need it.It looks like is but not for me ... For the given positions OA=0,3m ; AB=0,6m ; BC=0,3m ; O1B=0,5m ; ω[SUB]OA[SUB]=2 s-1 of the part OA from the mechanism , shown in the...
  28. C

    What is the Magnitude of Acceleration for Point B in a Moving Piston System?

    Homework Statement The piston is moving up at constant velocity 2.2m/s, the distance p is 1m, the distance q is 0.1m, the angle ɵ is 52 degrees. Find the magnitude of the acceleration of B Homework Equations Vb = Va + Vb/a Ab = Aa + Ab/a The Attempt at a Solution In...
  29. S

    Rigid, Non-Rigid and System of Rigid Bodies?

    Rigid, Non-Rigid and System of Rigid Bodies? Homework Statement Classify each of the following items as a rigid body, a system of rigid bodies, or a non-rigid body: a fishing pole, a diving board, a mechanical spring, a bowling ball, a baseball bat, a system of gears, a slider block system...
  30. N

    Moments Rigid Bodies: Understanding x-axis Forces

    This is not really a homework question but i am having dificullty visualizing the problem and needed some help understanding, the solutions say " The moment about the x-axis due to the forces can be found using the z components of each force acting at their interesction with the xy plane (A &...
  31. N

    Minimizing in the 6 dof of rigid bodies

    Hello -- I want to solve a very general type of minimization problem. I have 2 rigid bodies (e.g. molecules) which are exerting forces on each other, and I want to minimize the interaction energy (e.g. from a molecular forcefield with terms for things like Coulomb, van der Waals, etc). In...
  32. D

    Modeling climate with rigid bodies

    Okay, please don't ask me why I would want to do the following; I'll try to explain.. I am writing a program and already have a module for computing 2D rigid-body and spring-mass (with only 32-bit integers, no less) which I've modified from Box2DLite. Now, for my application, I would like...
  33. M

    Equivalent spring constant of rigid bodies

    Hello everyone. I have a problem...I'm calculating natural frequency of a standing cylinder. In order to do that, I need the ke - equivalent spring constant of that cylinder. ke = (EY * s) / (h) EY is Young's elastic modulus for material cylinder's made of s is the cross section of...
  34. B

    How to Calculate Force Applied on a Ladder Supported by a Wall and Ground?

    Hi everybody!How is it going! I'm having kind of a trouble trying to understand how to solve this following simple problem.If somebody is willing to help me i'll be very greatfull! I'll try to translate it from portuguese and hope everything written is correct. Homework Statement...
  35. H

    Calc Equilibrium Forces in Rigid Bodies

    Hi, Given a rigid body, to keep the body equilibrium, multiple upward forces act on the body with each a known displacement from the center of mass. How can we calculate the upward forces? Now imagine we have a table, with 3 legs of neglible mass, the center of mass is at the center of the...
  36. M

    Equilibrium of Rigid Bodies Problem

    Homework Statement A hand truck is used to move two barrels, each weighing 80 lb. Neglecting the weight of the hand truck, determine (a) the vertical force P which should be applied to the handle to maintain equilibrium when α = 35°, (b) the corresponding reaction at each of the two...
  37. 3

    Finding the Instantaneous Center of Rotation for Constant Angular Velocity

    Homework Statement At the instant shown, bar AB has a constant angular velocity of 25 rad/s counterclockwise. Dtermine at that instant a) the angular velocity of the rectangular plate FBDH, b) the velocity of point F Homework Equations Va = Vb + Wba x Ra/b The Attempt at a Solution...
  38. A

    Rigid bodies: generalized forces according to the Euler-Lagrange formalism

    I am trying to derive the dynamic equations of an aerial vehicle with 6 degrees of freedom (a quadrotor to be precise). I am using - two coordinate systems: the Earth frame and the body frame; - the Euler-Lagrange formalism: generalized coordinates {x,y,z,phi,theta,psi}, respectively, the...
  39. M

    Stability & Balance of rigid bodies

    Homework Statement A steel girder with a mass of 1000kg and a length of 10m rests on a concrete slab, with 4m overhanging the edge. How far can a 100kg man walk on the girder? The Attempt at a Solution I supposed that the right end of the slab is the fulcrum (rotating point), and...
  40. A

    Planar Kinematics of Rigid Bodies - Relative Motion

    Homework Statement http://www.hdimage.org/images/bgkkpt63ckx96gu3xgh.jpg Homework Equations aA = aA/B + aB VA = VA/B + VB The Attempt at a Solution After staring at this question for some time, I finally had a go and got an answer of 1.25 rad/s, which is incorrect. I am totally...
  41. A

    What is the net force acting on a wheel with a cord wrapped around it?

    Homework Statement A cord is wrapped around the rim of a solid uniform wheel 0.250 m in radius and of mass 9.20 kg. A steady horizontal pull of 40.0 N to the right is exerted on the cord, pulling it off tangentially from the wheel. The wheel is mounted on frictionless bearings on a...
  42. A

    What Causes Rotation of Rigid Bodies?

    Hi All, I don't know whether this question makes any sense or not anyhow i will ask. For example, consider a ball which is attached at the end of the string and let us assume that this ball rotates in the uniform circular motion. Then the reason it rotates in uniform circualar motion is...
  43. C

    How Can You Determine the Kinematics of Rigid Body BD in a Rotating System?

    Homework Statement "Knowing that crank AB has a constant angular velocity of 160rpm counterclockwise, determine and plot, for values of \theta from 0 to 360 degrees, (a) the angular velocity and angular acceleration of rod BD, (b) the velocity and acceleration of collar D...
  44. A

    How Is the Force on the Boom Hinge Calculated in a Wrecking Ball Setup?

    Homework Statement A wrecking ball weighing 4800N is supported by a uniform boom of wieght 3600N. A support cable runs from the top of the boom to the tractor. the angle between the support cable and the horizontal is 32degrees and the angle between the boom and the horizon is 48degrees...
  45. C

    Two rigid bodies combined center of gravity

    Hello all. I'm trying to find the equation for combined Center of Gravity (CoG) when you combine two objects with different weight. I know how to find the centroid, which is what the CoG would be if both objects were the same weight. But what about two objects with different weight...
  46. atyy

    Rigid bodies in special & general relativity

    It's often said there are no rigid bodies in relativity. Is the notion of "angle" primary or derived in relativity? Does "angle" require rigid bodies, at least "locally"? For example, to use light rays to measure the metric, do we need mirrors which are "rigid"?
  47. A

    What is the acceleration of a cylindrical shell rolling down an inclined plane?

    Homework Statement A hollow cylindrical shell with mass M = 100 g and radius R = 5 cm rolls without slipping down an inclined plane making an angle \alpha = 30° with the horizontal. (a) If the initial speed of the shell is zero, what will be the speed of its center of (c) Calculate...
  48. S

    Dynamics problem - Kinematics of Rigid Bodies

    Homework Statement There is a slider-crank mechanism. Express the angular velocity omegaAB and angular acceleration alphaAB of the connecting rod AB in terms of the crank angle theta for a given constant crank speed omega naught. Take omegaAB and alphaAB to be positive counterclockwise. The...
  49. J

    Problems in Statics Involving Moments related to rigid bodies

    I have a couple problems that I can't seem to get. The first I'm really close, I can tell, because my professor did one that was similar in class. The answer the book gives is 250 lbs, I'm coming up with 225. The second I think is similar to another problem he did in class but I'm not...
  50. P

    Rotations of Earth and other Rigid Bodies

    Does anyone know of a good mathematical reference covering the use of one-parameter group actions to model rotations of planets and/or other rigid bodies? Thanks in advance!
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