What is Rotational: Definition and 1000 Discussions

A rotation is a circular movement of an object around a center (or point) of rotation. The geometric plane along which the rotation occurs is called the rotation plane, and the imaginary line extending from the center and perpendicular to the rotation plane is called the rotation axis ( AK-seez). A three-dimensional object can always be rotated about an infinite number of rotation axes.
If the rotation axis passes internally through the body's own center of mass, then the body is said to be autorotating or spinning, and the surface intersection of the axis can be called a pole. A rotation around a completely external axis, e.g. the planet Earth around the Sun, is called revolving or orbiting, typically when it is produced by gravity, and the ends of the rotation axis can be called the orbital poles.

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  1. B

    Torque and rotational kinetic energy

    This is problem from a textbook whose solution I don't understand. A small rock of mass 0.5kg is attached to a 1.5m string, then it is whirled for 5 seconds until it achieves a near horizontal orbit at 120 rpm. What is the torque required? I used the equation : τ=Iα (moment of inertia x...
  2. C

    Question on rotational transform matrix, I

    Hi, I'm reading a book called Robotics, written by Tadej Bajd on my own to learn about robotics and have no one else to put my questions other than to the forums. Here the writer on 11th page writes: "By considering the similarity of triangles in Figure 2.3, it is not difficult to derive...
  3. T

    Rotational Edge Drop: Angular Velocity & Translation

    So here is the problem I have. There is a box that is 27.5" tall and 41" tall with a mass of 480lbs. One edge is put on a 3.5" tall block while the other is raised 12" and dropped. The question is to see if the block will continue to rotate and fall on it's side. I have been able to find the...
  4. A

    Rotational Inertia equation help

    Homework Statement Derive the equation for the linear acceleration of a falling mass m suspended by a string from the rim of a uniform disk of mass M and radius R that is free to rotate without friction about its principal axis. Homework Equations a=rα linear acceleration = radius*angular...
  5. F

    Troubleshooting Tension: A Closer Look at Rotational Motion Calculations

    Homework Statement The tension answer I got is different from the answer. Can someone help me take a look at my working? Really appreciate your time in doing this. Thanks alot.
  6. J

    Rotational Motion, angular velocity

    Homework Statement Two people are on a ride where the inside cars rotate at constant angular velocity three times the constant angular velocity of the outer cars. If the two cars are in line at t = 0, and moving at 3ω and ω respectively, at what time will they next pass each other? The...
  7. L

    Calculating Rotational Energy Levels for a Caroussel: How Do I Proceed?

    Hello forum members... I have a problem in calculating the energy of rotational levels. Kindly help me...The question goes like this: A Caroussel(merry-go-round) of diameter 20 m and weight 1 ton( =1000 kg that is evenly distributed on the discs) rotates 10 times per minute. Calculate the...
  8. D

    How does the distribution of mass affect the rotational inertia of a disc?

    Hey all, I've been thinking about this all night because it been bothering me I need a proper answer for it... Now I have a disc that has mass on each corner(N,W,S,E) it has a total mass of 1 Killo now with the small object spread on its edges. I want to increase the speed of that disc with...
  9. S

    Rotational mechanics sphere collision problem

    Homework Statement Hello everyone, Please help me with this problem, as I am getting stuck solving this problem. A solid sphere of Mass M and Radius R rolling (pure) encounters a protrusion on the surface of height R/4. Its in the form of a step. What can be the maximum velocity V of...
  10. S

    Rotational Mechanics: Sphere collision problem

    Hello everyone, Please help me with this problem, as I am getting stuck solving this problem. A solid sphere of Mass M and Radius R rolling (pure) encounters a protrusion on the surface of height R/4. Its in the form of a step. What can be the maximum velocity V of...
  11. F

    Kinematics of Rotational Motion Part II

    Homework Statement I cannot get the answer for part c, -13.02rad/s^2 The Attempt at a Solution 52.08=260.42+α(10) -208.34=10α α= -20.83rad/s^2 (Correct) 0=52.08+α(10) -52.08=10α α=-5.208rad/s^2 (Wrong) Please help, thanks..
  12. J

    Rotational Mechanics: Comparing Cylinder P & Q

    Two solid cylinders p and Q of same mass and same radius start rolling down a fixed inclinded plane from the same height at the asme time.cylinder P has most of its mass concentrated near its surface while Q HAS MOST of its mass near the axis... THEN 1)WHICH HAS MORE MOMENT OF INERTIA AND...
  13. F

    Kinematics of Rotational Motion (Gears)

    Homework Statement Since Gear A -------- Vi=0 t=12s θ=40∏ as given in the question, can I assume that Vi=0 t=12s for Gear B/Drum C as well? If not why? Thanks..
  14. F

    Does GR Consider Angular Momentum & Rotational Stress?

    Forgive ne if this sounds stupid but does GR take into account angular momentum, torsion, and other effects of rotation? The stress energy tensor doeznt appear to have any terns that are meant two accommodate angular momentum abd rotational stress.
  15. R

    Rotational Equilibrium: Finding Force at Point B and D

    Hello I've attached the problem. Any help would be greatly appreciated. I understand that by giving us the clue of rotational equilibrium, it means net T has to be 0. The force at 10N will cause clockwise torque (T= rF, so T = .04m * 10N). The thing that confuses me is the force at point...
  16. T

    The Physics of Fishing and Rotational Motion

    Homework Statement I am writing a large paper on the physics of fishing and would like to know what equation would prove the statement that a longer fishing pole would result in a higher velocity of the lure? My physics book gives me a gamut of equations for rotational motion but I can't...
  17. Nono713

    Rotational mechanics, angular momentum revision

    Homework Statement The problem is attached as an image. Note this is from a past exam. Homework Equations Conservation of angular momentum. Rotational kinetic energy. The Attempt at a Solution a) The moment of inertia of the man and stool is given as 3 kg m^2, and the dumbells...
  18. azizlwl

    Rotational motion, frictionless.

    http://img444.imageshack.us/img444/8148/rollwx.jpg A rope is wound around a cylinder of mass 4kg, and I=0.02kg.m2 about the cylinder axis. The frictional force between table and cylinder is negligible. Solution: a=20/4=5.0m/s2 α=ar is not applicable when slippage occurs. My...
  19. C

    Conservation of angular momentum and change in rotational kinetic energy

    I have a question regarding angular momentum and rot kinetic energy. For example, if angular momentum is conserved, and the radius is cut in half, then moment of inertia is reduced by a fourth, which will result in increase in angular velocity by factor of 4. My question is why is the...
  20. T

    Rotational Motion and Coefficient of Friction

    Hey guys! Ok, so I don't have that much to work with, and I am sorry for my lack of detail, but my memory is a bit fuzzy. I had to do a problem like this for my physics test yesturday: On the test we were given a problem involving a roller coaster (Rotating Drum of Death was its name)...
  21. F

    Solve Rotational Motion Homework: Pulleys A, B, C

    Homework Statement Attached 1st pic is the question, 2nd pic is the formula PS: I am sorry if i have posted on the wrong section. The Attempt at a Solution My understanding for this question is for compound pulley, angular velocity is the same, linear velocity is different. For pulleys in...
  22. F

    Solving Doubts on Rotational Motion

    Hi guys, I have a few doubts on Rotational Motion and hope someone can help me out. This 3 formula is for finding angular velocity, angular acceleration and angular displacement right? ω=ωi+αt ω²=ωi²+2αδ δ=ωit+1/2αt² And this 3 formula is for finding the linear...
  23. Z

    Question About Rotational And Translational Motion

    1) When there is no friction , a wheel with a Vcm = X m/s will only be slipping across the surface at a constant velocity.It won't be rotating because there will not be any forces creating torque. 2) The system ( composed of the rotating object ) always tries to find a balance between...
  24. N

    Rotational dynamics - System in Equilibrium

    Hi all, first year MechEng student here, final question of the final assignment of the year, would appreciate any assistance: Homework Statement A 0.150 kg yo-yo has an outer radius R that is 5.60 times greater than the radius r of its axle. The yo-yo is in equilibrium if a mass m is...
  25. T

    Is there a limit on the rotational speed of fast spinning pulsars?

    And if so, what is the limit? Some pulsars spin very fast. more than 700 times per second, or even faster? So the centrifugal force would be very strong at the equator, threatening to rip them apart. Is there a limit on how fast a pulsar can spin before the centrifugal force would...
  26. fluidistic

    HCl molecule, rotational and vibrational energy question

    Since the HCl molecule has a non vanishing dipole moment, it is allowed to have pure rotational spectrum. If I understand this correctly then it's possible to excite the molecule in such a way that its vibrational energy stays the same while its rotational energy increases by some amount. This...
  27. N

    Rotational motion of a spinning disk

    Homework Statement A spinning disk is rotating at a rate of 30 rad/s in the counterclockwise direction. The disk is slowing down at a rate of 6 rad/s2. Find the angle through which the disk has turned after 3 s in radians, degrees, and revolutions. Homework Equations θ=θi + ωi*t + αt^2...
  28. R

    Rotational kinetic energy with no friction

    Homework Statement Engineers are designing a system by which a falling mass m imparts kinetic energy to a rotating uniform drum to which it is attached by thin, very light wire wrapped around the rim of the drum. There is no appreciable friction in the axle of the drum, and everything starts...
  29. D

    Rod rotational inertia - axis at end of rod

    I want to ask why a rod's rotational inertia with rotational axis at its end is not 1/2 mR2 but 1/3 mR2?
  30. P

    Rotational Velocity and Acceleration

    1. Electric toothbrushes can be effective in removing dental plaque. One model consists of a head 1.1 cm in diameter that rotates back and forth through a 70.0 angle 7600 times per minute. The rim of the head contains a thin row of bristles. (See the figure )Assuming that the toothbrush has...
  31. P

    Rotational and Kinetic Energy - Down a Hill

    Homework Statement 15. Three objects with the same total mass and radius, a hoop, a disk, and a sphere each roll without slipping down an incline. Which of the following statements is FALSE? A) The hoop has the largest moment of inertia, so it arrives at the bottom first. B) The...
  32. T

    How Does Wheel Radius Affect Rotational Kinetic Energy?

    Homework Statement You attach a mass M to a string and wind the string around the circumference of a wheel. The wheel has two hubs with radii R1 and R2 with R2 > R1. You perform two experiments. First you wind the string around the smaller radius wheel and allow the mass to fall a distance...
  33. Y

    Mastering Physics 10.85 - Dynamics of Rotational Motion

    This is my first post on here; I hope it will be worth it. I haven't been able to find an adequate solution to this elsewhere. Homework Statement A 5.30 kg ball is dropped from a height of 11.5 m above one end of a uniform bar that pivots at its center. The bar has mass 8.50 kg and is 9.00 m...
  34. Z

    What is the angular acceleration of a falling rod with two attached balls?

    Homework Statement A thin, uniform 3.60kg bar has two balls glued on either end, each with a weight of 2.5kg. The length of the rod is 90cm and is balanced in the center. When one ball falls off calculate the angular acceleration just after this happens. (e.g the rod is falling in a...
  35. W

    Conservation of Rotational Energy Question

    Homework Statement A string is wrapped several times around the rim of a small hoop with radius 8.00 and mass 0.180 . The free end of the string is held in place and the hoop is released from rest. After the hoop has descended 95.0 , calculate the angular speed and speed of its center...
  36. S

    Rotational Plus Translational Motion for Sphere of Yarn

    A massless string is wrapped around the equator of a solid sphere (mass M = 76.2 kg, radius R = 0.211 m). A girl holds the free end of the string, and the sphere is released from rest, Assume: - the sphere is always parallel to the floor - the string is always perpendicular to the radius of...
  37. M

    Orbital velocity and rotational velocity

    Hi Is "orbital velocity" supposed to be the same as "rotational velocity"? it seems that a "rotation curve" is supposed to plot the rotational velocity of a star, but then some articles e.g. http://en.wikipedia.org/wiki/Galaxy_rotation_problem claim "orbital speed" is plotted. The equation...
  38. M

    Oribtal and rotational velocities

    Hi Is "orbital velocity" supposed to be the same as "rotational velocity"? it seems that a "rotation curve" is supposed to plot the rotational velocity of a star, but then some articles e.g. http://en.wikipedia.org/wiki/Galaxy_rotation_problem claim "orbital speed" is plotted. The equation...
  39. D

    Circular rotational motion (kinematics)

    Homework Statement A wheel starts from rest and accelerates uniformly to 200rpm in 6s. After it has been rotating for some time at this speed, the brakes are applied and it takes 5 minutes to stop the wheel. If the total number of revolutions is 3100, calculate the total time of rotation...
  40. C

    Rotational energy of an object.

    Homework Statement An object, consisting of two point masses with mass m are connected by a rigid shaft with negligible mass. They are lying horizontally at rest. One of the point masses is instantly given a speed of v. Calculate the total energy of the system once the system has stopped...
  41. I

    How much rotational kinetic energy does the disk have ?

    A string is wrapped around a disk of mass 2.1 kg (it's density doesn't have to be uniform). From rest, you pull the string with a constant force of 9 N. At this instant, the center of mass has moved 0.11 m, and your hand has moved 0.28 m. 1. At this instant, how much rotational kinetic...
  42. A

    How Do You Calculate the Moment of Inertia for a Rotating Wheel?

    Homework Statement You are asked to measure the moment of inertia of a large wheel, for rotation about an axis through its center. You measure the diameter of the wheel to be 0.740 m and find that it weighs 280 N (note, that's not the mass!). You mount the wheel, using frictionless...
  43. C

    Equation relating rotational velocity and output electrical power

    What is the equation that relates the angular velocity of an alternator rotor with its output electrical power, voltage and frequency (not necessarily just one equation) . And for a DC generator? I know that when you calculate the power generated by a turbine you multiply its efficiency of...
  44. V

    Advanced Rotational Velocity Question

    Homework Statement A particle of mass m rotates with a uniform speed on the inside of a bowl's parabolic frictionless surface in a horizontal circle of radius R=0.4 meters as shown below. At the position of the particle the surface makes an angle θ=20 degrees with the vertical. The angular...
  45. C

    Rotational and vibrational spectroscopy

    Hello there! Im studying microwave spectroscopy, and I'm stuck because I don't see why the spacing between rotational energy levels increases as J does. Can anyone explain please? Also, about IR, I see everywhere that the selction rule is Δv=±1. Correct me if I'm wrong but I'm guessing...
  46. C

    Calculating Rotational Energy of Disk After Forces Applied

    Homework Statement A uniform disk with mass m = 9.32 kg and radius R = 1.37 m lies in the x-y plane and centered at the origin. Three forces act in the +y-direction on the disk: 1) a force 340 N at the edge of the disk on the +x-axis, 2) a force 340 N at the edge of the disk on the –y-axis...
  47. D

    Some doubts regarding rotational dynamics

    a) suppose we tie a ball with a string and rotate it keeping its other end tied with one of our fingers then the only external force acting on it is the tension. Which force then cancells 'mg' acting vertically downward? b) if a mass is rotating in a circular path then in which direction does...
  48. B

    Analyzing Rotational Motion of Massless Rod and Object

    Homework Statement A mass-less rod of length, Ls, is rotating clockwise about a point, P1 in the horizontal plane. Attached by a cable of length, Ls, to the end of the rod and at angle θ is an object of mass, M. Starting at rest and then with the rod having and angular acceleration α, describe...
  49. C

    Rotational Quantum Number J: Understanding Its Role in Microwave Spectroscopy

    Is it the same as the total angular momentum quantum number ?
  50. S

    Rotational and translational equilibrium

    When a ruler is placed on a pivot and I push it down from one side, then there is a moment. But then is there any linear movement? Since there is no translational equilibrium then won't there be a linear motion? Then how does rotational equilibrium come into play here?
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