Rotational Definition and 1000 Threads

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

    Kinetic Energy rotational and translational conceptual question.

    Homework Statement A ball rolls without slipping down incline A, starting from rest. At the same time, a box starts from rest and slides down incline B, which is identical to incline A except that it is frictionless. Which arrives at the bottom first? (a) The ball arrives first. (b) The...
  2. M

    Qn. on general rotational mechanics

    Homework Statement a uniform rod of mass m nd length 2l lies on smooth horizontal surface. a particle of mass m is connected to a string of length l whose other end is connected to one of the ends of the rod. initially string is taut and both string and rod lie in same plane with 90 angle...
  3. T

    Conservation of energy with springs and rotational motion

    Homework Statement Homework Equations T1 + V1 = T2 + V2 T = kinetic energy V = potential energy The Attempt at a Solution I am only confused about how to get T2. Here is how the answer key gets T2: I don't get why Va = Vb = 0. The wheels have some velocity so it doesn't...
  4. T

    Conservation of energy with rotational motion problem

    Homework Statement Alright so I am confused about why the solution does not include the kinetic energy due to rotational energy from I_g2 as indicated in red in the following image. I don't understand why they would neglect it... I calculated the value and it makes a big difference.
  5. W

    Rotational Motion of Masses Problem

    Homework Statement Hey all - I'm doing a physics lab on rotational motion, and I can not make sense of the last part, the additional analysis. The question is: Since our masses are not actually point-masses, the theoretical model I = MR^2 is not very accurate, so for a better theoretical...
  6. J

    Pulling a plank across cylinders - Rotational Dynamics

    Homework Statement A plank with a mass M = 6.10 kg rides on top of two identical, solid, cylindrical rollers that have R = 4.10 cm and m = 2.00 kg. The plank is pulled by a constant horizontal force F of magnitude 4.80 N applied to the end of the plank and perpendicular to the axes of the...
  7. B

    Struggling with Rotational Dynamics Homework?

    Homework Statement I've been trying to attempt this problem for the past day but I don't understand how to answer the questions. It's frustrating. If someone could point me in the right direction that would be nice. My TA did the problem in class but I still don't quite understand it. This is...
  8. L

    4 Point masses form a body - Inertia & Rotational Kinetics

    Homework Statement Four point masses form a rigid body (they are connected by massless rigid rods) given the positions M1= 3kg (2m, 4m, 0m) M2= 2kg (1m, -4m, 0m) M3= 1kg (10m, 2m, 0m) M4= 5kg (-5m, 2m, 0m) Find: A) Moment of inertia of this system when it rotates about x axis B) Moment of...
  9. I

    Which components of the force create Rotational motion?

    Which components of the force create Rotational motion?
  10. V

    How Do You Calculate Average Angular Acceleration?

    Homework Statement A rolling wheel moving to the right initially has an angular speed of 7.0 rad/s but after making 3 complete revolutions the wheel has a final angular speed of 11.0 rad/s. What is the magnitude and direction of the average angular acceleration of the wheel? Homework...
  11. J

    Rotational Kinetic Energy of disc brakes

    The disc brakes of a high performance car are often made of carbon fiber instead of iron, thereby reducing the mass. If both types of discs are of the same size and shape, and each iron disc has a mass of 4 kg and each carbon disc has a mass of 1 kg, what is the reduction in rotational kinetic...
  12. A

    Building a rotational matrix transformation

    I am trying to build a rotational transformation matrix both for counterclockwise and clockwise angles. The first matrix in the picture is for counterclockwise angles and the second one for clockwise angles. The first matrix I built corresponds to the one given in my linear algebra book so it...
  13. M

    What Is the Speed of a Rolling Disk's Center of Mass?

    A solid, uniform disk of radius 0.250 m and mass 57.7 kg rolls down a ramp of length 3.70 m that makes an angle of 14° with the horizontal. The disk starts from rest from the top of the ramp. (a) Find the speed of the disk's center of mass when it reaches the bottom of the ramp. (b) Find the...
  14. M

    Rotational Dynamics, Torque, and Tension

    Homework Statement A man drags a 71.5-kg crate across the floor at a constant velocity by pulling on a strap attached to the bottom of the crate. The crate is tilted 22.5° above the horizontal, and the strap is inclined 64.0° above the horizontal. The center of gravity of the crate coincides...
  15. B

    Rotational kinetic energy / moment of inertia

    Homework Statement A thin, cylindrical rod = 27.0 cm long with a mass m = 1.20 kg has a ball of diameter d = 10.00 cm and mass M = 2.00 kg attached to one end. The arrangement is originally vertical and stationary, with the ball at the top as shown in the figure below. The combination is...
  16. M

    Rotational dynamic equilibrium problem on an angle

    A man drags a 67.0-kg crate across the floor at a constant velocity by pulling on a strap attached to the bottom of the crate. The crate is tilted 24.0° above the horizontal, and the strap is inclined 64.0° above the horizontal. The center of gravity of the crate coincides with its geometrical...
  17. N

    Rotational problem another time (this time not the center of mass)

    Homework Statement A chimney (length L = 82.6 m, mass M = 2280 kg) cracks at the base, and topples. Assume: - the chimney behaves like a thin rod, and it does not break apart as it falls - only gravity (no friction) acts on the chimney as if falls - the bottom of the chimney tilts but...
  18. N

    How Do You Calculate the Speed of a Falling Chimney's Center of Mass?

    2Homework Statement A chimney (length L = 82.6 m, mass M = 2280 kg) cracks at the base, and topples. Assume: - the chimney behaves like a thin rod, and it does not break apart as it falls - only gravity (no friction) acts on the chimney as if falls - the bottom of the chimney tilts but...
  19. M

    How Long Does It Take for a Flywheel to Reach 125 Rev/Min from Rest?

    A flywheel 1.2 m in diameter is uniformly accelerated from rest and revolves completely 68 times in reaching a speed of 125 rev/min. Find the time taken to reach this speed i have been given the correct answer as 65.29 seconds Wht i have up to now ω1 = 0 ω2 = 125 r/min = 125/60 =...
  20. J

    Rotational motion pendulum problem

    Homework Statement Homework Equations ƩF = ma The Attempt at a Solution I know how to break this problem up into the x and y direction Y Direction: Ft cosθ - Fg = 0 X Direction: Ft sinθ - Fg = mv2 / r The hint says to find the inward radial component. I'm guessing this...
  21. J

    Rotational motion roller coaster problem

    Homework Statement Homework Equations ƩF = ma = (mv2) / r The Attempt at a Solution For part A: Fn - Fg = (mv2) / r Fn = ((mv2) / r ) + mg Fn = (500x20^2) / 10 + 500x9.8 Fn = 24900 This seems too high to be true. But I'm unsure of what I did wrong. I know the normal force...
  22. L

    The Equations of Rotational Kinematics help

    [b]1. Consider a fan that has rotated by a total of 2 complete revolutions from where it started. This quantity (2 revolutions) is analogous to what quantity in linear kinematics? Position Displacement Velocity Speed Time Time Homework Equations I am...
  23. C

    Moment of inertia and rotational kinetic energy

    I can't seem to understand moment of inertia. What does it mean and how is it derived ? How does it relate to rotational kinetic energy.
  24. C

    Rotational Motion to Linear Motion

    This is a question for a rather odd purpose. I came across a working LEGO model of a Russian tank and I'm trying to model it using CAD software. It has a motorized system to elevate and lower the main cannon. This is where putting it into words might get weird: It uses a small motor which...
  25. A

    Combining translational and rotational velocities to get rotational velocity

    Homework Statement Suppose you are standing uptight with your right arm stretched out straight in front of you, palm down, the upper arm rotating rightward relative to space at 2 radians/s and the elbow flexing in the horizontal plane at 2 radians/s. What is the rotational velocity of the...
  26. A

    Combining rotational and translational velocities

    Hi, I need help with a homework problem, because I am unsure in what way rotational and translational velocity combine. Also, I can not really imagin ethe movement described in the problem, so I am not sure how to start Homework Statement At some instant a person’s forearm has a rotational...
  27. N

    Forces Producing Rotational & Translatory Motion: Exploring Mechanics

    Why does a force applied exactly perpendicular to one end of a rigid rod produce translatory motion along with rotational motion about center of mass? I know the mathematics F=m(center of mass)*a but I want a mechanical answer. Please help me analyze the mechanics of it. I think analyzing the...
  28. S

    Problem 10.30 (with Hints) - Mastering Physics - Rotational Inertia

    Homework Statement Three 2.30kg masses are located at the vertices of an equilateral triangle 55.0cm on a side, connected by rods of negligible mass. Find the rotational inertia, I_2, of this object about an axis that passes through one vertex and the midpoint of the opposite side Homework...
  29. azizlwl

    Calculating Rotational Inertia of Flat Plate

    How to derive the Rotational Inertia for a flat plate. I do not see any symmetry about the axis of rotation as in a thin rod.http://imageshack.us/a/img845/1134/53433231.jpg
  30. X

    Rotational Kinetic Energy of Cl2?

    Homework Statement In a crude model of a rotating diatomic molecule of chlorine (Cl2), the two Cl atoms are 2.00 10^-10 m apart and rotate about their center of mass with angular speed ω = 3.80 10^12 rad/s. What is the rotational kinetic energy of one molecule of Cl2, which has a molar mass...
  31. G

    Does the Direction of an Object's Velocity Affect its Rotational Kinetic Energy?

    suppose we are sitting on earth.now we threw a object with velosity greater than escape velosity.will the kinetic energy remain same if we throw the object in different directions?how
  32. V

    Rotational mechanics. cylinder over a plank

    Homework Statement a solid cylinder of mass m and radius R and a plank of mass M are placed on a smooth horizontal surface. there is sufficient friction between the cylinder and the plank's surface to prevent sliding of the cylinder. the fixed pulley is smooth. if the plank is pulled with a...
  33. Gh778

    Can rotational speed increase gas density in a cylinder?

    Hi, I would like to know if it's possible to turn a cylinder full of gas (air for example) at W rd/s for have more density up ? The first drawing show the basic system turn around the center. The second drawing show the density of gas like I think it can be, black=more density, white=less...
  34. K

    Rotational Dynamics about an axis parellel to the axis at the center of mass

    Homework Statement A uniform sign is nailed into a wall at its top two corners. The nail on the upper-right corner breaks, and the sign begins to rotate about the remaining nail in the upper-left corner. The mass of the sign is 69 kg, L (length) = 1.6 m, H (height) = 2.2 m, and the moment of...
  35. F

    Impulse Momentum Method for Rotational

    Homework Statement I have abit of problem understanding this 2 question, I would appreciate if someone can guide me along. For diagram 1 (Left) I interpret the condition as: At 0sec, initial velocity is 0, At 5sec final velocity is unknown. For diagram 2 (Right) I interpret the...
  36. F

    Impulse Momentum Method for Rotational

    Homework Statement For impulse momentum, I know that collision between two object, impulse = 0. Can somebody explain to me how come for this question the impulse is 0 "shaft A engaged to shaft b" has the same meaning as collision? And I assume that it is right for me to add shaft A and shaft...
  37. A

    Neutron velocity selector and rotational speed of shaft

    Homework Statement Suppose that there is a beam of neutrons with a range of wavelengths. One intends to produce neutrons of single wavelength by the use of neutron mechanical velocity selector. The question is, if the slot in the disk B (second disk) of the selector lags the slot in the disk...
  38. F

    Work Energy Method for Rotational Motion

    Homework Statement I don't how how to work out for the Gravitational Energy "H". Info given is 1.5rev converting to 9.42rad/s S=rδ S=0.2x9.42 =1.884 *Should i multiply the axle radius or wheel? The Attempt at a Solution E1=K1+G1+S1 =1/2(8.5)202+(20)(9.81)(H)+0Thanks
  39. R

    A few Angular Momentum and Rotational Kinematics Conceptual Questions

    Homework Statement Each of the questions are either increase, decrease, equal, or undetermined. 1) You are sitting on a rotating chair, holding a 2 kg mass in each arm outstretched. When the masses are dropped, your angular momentum -------. 2) You sit in a rotating chair holding 2 kg...
  40. K

    What Causes a Sphere to Rotate on a Ramp with Negligible Friction?

    If I have a sphere at rest on top of a ramp, if friccion is negligible, the only forces acting on it would be gravity and the normal force. Both of these go through it's axis of rotation which would be the spheres center. What would cause the rotation then so τ≠0?
  41. F

    Work Energy Method for Rotational Motion

    Homework Statement Reference to part C of the question, I would like to ask for work done U1to2, how come linear motion is not included? I thought both linear and rotational exist since the parcel "move downwards" (linear) and the wheel is rotating (rotational) Thanks...
  42. K

    Introductory rotational dynamics/energy conservation question

    Homework Statement Hey guy's I am kind of struggling with this i would appreciate any help A potter's wheel having a radius 0.49 m and a moment of inertia of 12.1 kg · m2 is rotating freely at 52 rev/min. The potter can stop the wheel in 6.0 s by pressing a wet rag against the rim and...
  43. F

    Work Energy Method for Rotational Motion

    Homework Statement Hi guys, I would like to ask for this particular question, The GPE is 0 for all position is because I set datum as the original position, GPE = 0 and when the string got detached from the axle, there is no GPE for datum 2. Am I right? Thanks
  44. B

    Two Conceptual questions about rotational mechanics

    Hi, I see in many of these physics problems it is explicitly stated that "the cord on the pulley does not slip." [1] What is the implication of this? Why is it important? Also, there is a problem here with a diagram that I will try to explain in words and then have a question about...
  45. Saitama

    Solving Rotational Motion Homework: a2 = 5m/s^2

    Homework Statement http://i46.tinypic.com/548uox.pngHomework Equations The Attempt at a Solution My teacher said that we will take the axis at the point which is in contact with the ground (i don't get why we need to do so?). I followed the teacher's hint. Angular acceleration α=a/R, where a is...
  46. Saitama

    Rotational Motion Homework: Find Kinetic Energy of Rod

    Homework Statement A thin rod of length L and mass m is suspended freely from one end. It is pulled to one side and then allowed to swing like a pendulum, passing through its lowest position with angular speed ω. In terms of these symbols and g, and neglecting friction and air resistance, find...
  47. P

    Rotational speed of a coil in a magnetic field

    This is for self-study. Homework Statement The armature of a motor has 97 turns each of area 190 cm² and rotates in a uniform magnetic field of 0.33 T. A potential difference of 24 V is applied. If no load is attached and friction is neglected, find the rotational speed at equilibrium...
  48. N

    Calc Centripetal Acceleration: Rotational Motion & Vinyl Record Player

    In the days before compact discs and MP3 players (ancient history!), music was recorded in scratches in the surface of vinyl-coated disks called records. In a typical record player, the record rotated with a period of 3.6 s. Find the centripetal acceleration of a point on the edge of the record...
  49. N

    Rotational Motion and speed of a disc

    A 36.5-cm diameter disk rotates with a constant angular acceleration of 2.00 rad/s2. It starts from rest at t = 0, and a line drawn from the center of the disk to a point P on the rim of the disk makes an angle of 57.3° with the positive x-axis at this time. (a) Find the angular speed of the...
  50. K

    Calculating Linear and Rotational Motion

    Homework Statement In this given pole, mass is "M" and length is "L" and it is on a frictionless surface as the picture describes. "G" is the center of gravity and "I" is the inertia. A ball with a mass of "m" comes as the picture and hits the pole with a velocity of "v" and turns the...
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