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. A

    Calculating tension in rotational kinematics

    Homework Statement A block with mass m is revolving with linear speed v1 in a circle of radius r1 on a frictionless horizontal surface (see the figure ). The string is slowly pulled from below until the radius of the circle in which the block is revolving is reduced to r2. Calculate the...
  2. A

    Rotational kinematics using energy

    Homework Statement A 45.0-cm diameter wheel, consisting of a rim and six spokes, is constructed from a thin rigid plastic material having a linear mass density of 25.0 g/cm. This wheel is released from rest at the top of a hill 52.0m high. a.) How fast is it rolling when it reaches the...
  3. V

    Rotational mechanics question?

    One more question on rotation. first of all see attachment i want to say that if friction is too large then net force is in direction of inclined in upward direction but from common experience we can say that it will move in downward direction so where is it going wrong. why it moves in...
  4. V

    Rotational and translational motion hybrid quetion. How to digest it?

    Homework Statement If two circular discs A and B of mass m and 3m and radii R,2R, respectively. are placed from the top of a rough inclined plane, which disc will reach bottom first.2. The attempt at a solution NOTE: u is coefficient of friction (both kinetic and static). g is acceleration due...
  5. D

    Rotational Inertia: Front end of an airborn bike drops when brake is applied

    So every mountain biker or motocross rider knows to never grab the front brake in the air. When you do the front end drops and can potentially ruin your day in a hurry. I figure that due to the loss of rotational inertia gravity accelerates the front wheel more rapidly, but I would like to see...
  6. V

    Finding the Torque of Normal Force for a Sliding Block on an Inclined Plane

    Homework Statement A cubical block of mass 'm' and edge length 'a' slides down a rough inclined plane of inclination alpha radian with a uniform speed. Find the torque of the normal force acting on the block about its center. On the back of the book answer is \frac{1}{2}mga sin(alpha) 2. The...
  7. A

    Pulley question / rotational kinematics

    Homework Statement Summary: The pulley in the figure has radius (R) and a moment of inertia (I). The rope does not slip over the pulley, and the pulley spins on a frictionless axle. The coefficient of kinetic friction between block A and the tabletop is \muk. The system is released from...
  8. X

    Understanding Rotational Motion: Explained by a Scientist

    Can anybody please explain to me the concept of rotational motion?? I'm so lost in that chapter... ><
  9. A

    Moment of Inertia for Rotational Kinematics

    Homework Statement A thin uniform rod of mass (M) and length (L) is bent at its center so that the two segments are now perpendicular to each other. a. Find its moment of inertia about an axis perpendicular to its plane and passing through the point where the two segments meet...
  10. J

    Rotational Kinetic Energy and Conservation of Momentum

    There are several references to “lost” kinetic energy when trying to analyze motion based on the Conservation of Momentum and the Conservation of Energy laws. Generally, the answer to apparent discrepancies include references to ‘elastic’ or ‘in-elastic’ collisions and whether or not they are...
  11. B

    Substended angle, rotational kinematics

    The moon has a diameter of 3.48 x 10^6 m and is a distance of 3.85 x 10^8m from the earth. The sun has a diameter of 1.39 x 10^9 m and is 1.50 x 10^11 m from the earth. (a.) What are the angles (in radians) subtended by the moon and the sun, as measured by a person standing on the earth...
  12. B

    Substended angle, rotational kinematics

    A jet is circling an airport control tower at a distance of 18.0 km. An observer in the tower watches the jet cross in front of the moon. As seen from the tower, the moon substends an angle of 9.04 x 10-3 radians. Find the distance traveled (IN METERS) by the jet as the observer watches the nose...
  13. S

    Converting Rotational Work to Linear Motion for a Crossbow

    I don't know if I'm using the terms properly, since I am no mechanical engineer, but I am attempting to find the best way to convert rotational work from an electric motor to draw a long, high-torque linear stroke, followed by a very fast return stroke. To clarify my question, I trying to...
  14. B

    Rotational and vibrating energy levels - find energy separation

    Homework Statement The equilibrium separation of the atoms in H35CI equals 1.27 x 10-10m. Calculate the energy separation between adjacent lines in the rotational-vibrational spectrum Homework Equations E = (n + 1/2)h\omega + h2/ 2I * l(l+1) I = \mur02 \mu = m1m2/ m1 + m2 The...
  15. Philosophaie

    Comparing Angular Rates of Star Rotation at Different Latitudes and Longitudes

    The Earth's rotational velocity at the Equator is 1,674.4 km/h or 465.1 m/s. The stars at the equator rotate at that same rate taking in account Precession at that particular time of the year and the longitude & latitude they are viewed from. Do the stars at the poles rotate faster then at the...
  16. Z

    Torques in rotational equilibrium

    Homework Statement Why, for any system that is in rotational equilibrium, the torque about 1) any point on the object or 2) any point in space, must be zero.Homework Equations N/AThe Attempt at a Solution What I do not understand is, why the torque about ANY point on the object is zero...
  17. A

    Theoretical Rotational-Linear Kinetic Energy Ratio of Spherical Projectile

    For my investigation regarding the aerodynamic forces on a spherical projectile, I really need to know the theoretical ratio of rotational kinetic energy to linear kinetic energy of a spherical projectile (assuming the only spin is forward spin and there is no Magnus effect). Can someone please...
  18. S

    Express h in Terms of r and theta: Rotational Physics

    So the question is this: A disc is released from rest. A block is causing it to rotate. After a time t the block has fallen a height h and the disc has rotated through an angle theta. (in rad) Express h in terms of r (the radius of the part of the hub around which the string is wound) and...
  19. E

    Why rotational kinetic energy of a closed system is not conserved?

    A simple rotating system with no external forces acting on it carries a fixed angular momentum and an associated rotational kinetic energy. If the system changes its internal configuration, such as a spinning skater retracting or extending his/her arms, the angular momentum remains constant...
  20. F

    Questions on Rotational Kinematics/Dynamics

    Some background information: I'm doing some reading up for PhO (so it's beyond what I'm supposed to learn), and so I'll post all my questions here (regarding both concepts and actual practice questions). Sorry if you feel there is a lack of effort on my part, but sometimes I'm really lost and...
  21. A

    Rotational Motion/Projectile motion of a ball on a circular ramp.

    Homework Statement A small sphere of radius r0 = 1.4 cm rolls without slipping on the track shown in the figure whose radius is R0 = 40.0 cm. The sphere starts rolling at a height R0 above the bottom of the track. Assume that it leaves the track after passing through an angle of 135° as...
  22. Femme_physics

    Can you do sum of all moments on a point that's not a rotational axis?

    Can you do "sum of all moments" on a point that's not a rotational axis? For instance, in this structure, with the two wires holding the beam http://img228.imageshack.us/img228/3687/twowiresj.jpg Can I do sum of all moments on B? Or is it not allowed because it's not a pivot point?
  23. A

    Rotational Motion of a car on a curve

    Homework Statement A 600 kg car is going around a banked curve with a radius of 110m at a speed of 24.5 m/s. What is the appropriate banking angle so that the car stays on its path without the assistance of friction? Homework Equations N cos{theta} = mg N sin{theta} = mv^2/r The...
  24. S

    Energy of Falling Dominoes - Rotational and Gravitational

    First of all, hi! I'm new here. Homework Statement Rather than a specific problem, my friend and I are doing an Extended Experimental Investigation on the energy transfer of dominoes for our grade 12 assignment. 2. The attempt at a solution We have approached it using loss of gravitational...
  25. A

    Block of mass Rotational Problem

    Homework Statement A block of mass m1 = 2.00 kg and a block of mass m 2 = 6.00 kg are connected by a massless string over a pulley in the shape of a disk having radius R = 0.250 m and mass M = 10.0 kg. These blocks are allowed to move on a fixed block – wedge of angle 30.0°, as shown in...
  26. L

    Rotational Collisions: Putty Wad

    Homework Statement Two 2.1 kg balls are attached to the ends of a thin rod of negligible mass, 63 cm in length. The rod is free to rotate in a vertical plane about a horizontal axis through its center. With the rod initially horizontal as shown, a 50 gm wad of wet putty drops onto one of the...
  27. M

    How do rotational and vibrational energies in molecules depend on masses?

    Hi! I'm wondering what the effects of a variable mass of elementary particles on the rotational and vibrational energy-transitions would be like? Would they increase, decrease or stay the same? Thank you for your help! Regards
  28. M

    Rotational kinematics of a ball

    Homework Statement A ball attached to a string starts at rest and undergoes a constant angular acceleration as it travels in a horizontal circle of radius 0.30 m. After 0.65 sec, the angular speed of the ball is 9.7 rad/s. Determine the magnitudes of the ball’s tangential, centripetal, and...
  29. F

    Right hand grip rule in rotational kinematics

    Simple question: Is there a reason behind the right hand grip rule, or is it just like that (inexplicable)? How do we know that for an object with counter-clockwise rotation (e.g. on the table), the angular velocity is upwards?
  30. Femme_physics

    Acceleration & Rotational Motion: What's the Link?

    They're both this? http://img37.imageshack.us/img37/4795/acccelerationrotational.jpg
  31. A

    Does Rotating an Object Absorb Energy?

    I took physics in school, but our class kind of skipped over rotational motion, so I was just reading about it myself. I was wondering about whether rotating an object absorbed some of the energy put into it and, if so, how to calculate how much is transferred to the rotation.
  32. S

    Rotational Loading Homework: Find Force Pulling Arm away from Pivot Point

    Homework Statement I'm not using numbers because I only want to understand the mathematical relationship. This is not a coursework question, but I imagine it stems from coursework level physics. I am trying to design a rotating arm for my own use and I want to calculate bracing requirements...
  33. M

    Rotational Motion, Rolling Motion

    Im looking over an example that was given in class that I jotted down in my notes; the question posed was...A 320 kg motorcycle includes two wheels each of which is 52 cm in diameter and has rotational inertia 2.1 kg*m^2. The cycle and its 75 kg rider are coasting at 85km/hr on a flat road when...
  34. M

    Rotational Inertia problem: Getting different results from two solving methods

    Homework Statement A disk of radius r meters has a string wrapped around its perimeter. A mass of m kilograms is attached to the end of the string and is allowed to descend freely from a height of h meters, it takes t seconds for the mass to travel the distance. Find the moment of inertia (I)...
  35. A

    Unraveling the Mystery of Rotational Motion and Moment of Inertia

    This a question that has been haunting me for some time now. Regarding the rotational motion of rigid bodies why wasn't the moment of inertia defined as the integral sum elements of infinitesimal mass time the radius from the axis of rotation rather than the radius squared. In this case the...
  36. M

    Angular velocity and rotational equilibrium

    Homework Statement A solid rectangle of uniform density has one corner at the origin. It has a mass of 50 kg. The rectangle has a length of 4 m in the z-direction, a length of 3 m in the y-direction, and a length of 2 m in the x-direction. The pivot is at the center of mass. There is a 50 N...
  37. 0

    Spring energy to rotational energy

    How do i convert spring energy to rotational energy? I am building a 'green device' that allows a bicycle to move forward using stored energy (compressed spring energy). So i am figuring out how do i change the spring energy stored to rotational energy so that th bike will be able to propel...
  38. D

    How can I design a linear to rotational crank mechanism without using gears?

    I'm looking for a crank that can give me linear movement to rotation (not the other way around). There is a component that I'm aware of called a uni-directional drive that turns rotation movement in either direction into movement in only on direction... stick a rack in the mix and there is...
  39. L

    Rotational kinetic energy of a fly wheel.

    Homework Statement The moment of inertia of a fly wheel about it's axis is 20 kg m2. A constant torque of 40 N m is applied to the initially stationary fly wheel. Find it's rotational KE after 3 seconds assuming there is no friction in the system? Homework Equations...
  40. A

    Rotational Motion and magnitude

    Im studying for a final, there are a few questions about rotational motion that i have, can anyone please help? A 5.2 kg disk of radius 2.3 m initially rotates about its axis at 6.4 rad/s. A tangential 7.4-N frictional force is applied to the rim of the disk. How many revolutions does the...
  41. C

    Uniform linear, circular and rotational motion

    In uniform linear motion, an object moves at a constant speed v0. And as long as no acceleration is applied onto the object, it will continue move at the same speed and direction forever. For uniform circular motion to happen, an object must move at a constant speed v0 with a constant...
  42. W

    How friction affects rotational inertia of a hollow cylinder

    How does friction affect the rotational inertia and angular velocity of a hollow cylinder rolling down an inclined plane? Assuming the cylinder isn't slipping
  43. U

    Group Theory: Rotational Symmetries

    Homework Statement Show that the group R of rotational symmetries of a dodecahedron is simple and has order 60. The Attempt at a Solution I see how to get order 60 using the orbit stabilizer theorem. Letting R act in the natural way on the set of faces, we find the size of the orbit...
  44. J

    Calculating Rotational Speed for 2g Experience in a Rotating Machine

    Homework Statement A NASA astronaut is placed in a rotating machine to see how well their bodies withstand G Force. What rotational speed is needed in a device that has a 6.25 m radius to allow a 75.8 kg astronaut to experience a force that is twice his normal weight (or 2g)?Homework Equations...
  45. A

    Rotational Inertia: Solve for Acceleration of Suspended Masses

    Homework Statement Two masses are suspended from a pulley system. The pulley itself has a mass of .2kg, a radius of .015m, and a constant torque of .35Nm due to friction between the rotating pulley and its axle. What is the magnitude of acceleration of the suspended masses if m1=.4kg and m2 =...
  46. E

    Rotational Force Homework: Finding Angular Velocity

    Homework Statement A sheet of material, mass 2kg, uniform density, is in the shape of a right triangle bounded by the lines y=0, x=0, and y=4-x. Its attached to the origin, and free to rotate about the y axis. A force of 1000 Newtons is applied perpendicular to the plane, located at the x=4...
  47. E

    Net Torque / Rotational Dynamics Pulley System help

    Homework Statement A uniform, solid cylinder with mass M and radius 2R rests on a horizontal tabletop. A string is attached by a yoke to a frictionless axle through the center of the cylinder so that the cylinder can rotate about the axle. The string runs over a disk-shaped pulley with mass M...
  48. M

    Calculating KE in rotational motion

    Homework Statement A 20kg solid disk (I=1/2Mr^2) rolls on a horizontal surface at the rate of 4.0m/s Calculate its total kinetic energy The Attempt at a Solution I think that simply equating the KE to (0.5)(m)(v^2) would be a wrong solution because then I would not use the moment of...
  49. F

    Rotational Motion of a wheel turning

    Homework Statement A wheel starting from rest accelerates at 1.2 rad/s² counterclockwise. A) How long does it take to turn through 60 revolutions? B)What is the rotational speed at that time? 2. The attempt at a solution I converted 1.2rad/s² to 1.885 rev/s² by dividing by 2πrad (2...
  50. K

    How Does Tension Direction Affect Torque in a Pulley System?

    Block 1 has mass m1 = 460g, block 2 has mass m2 = 500g, and the pulley, which is mounted on a horizontal axle with negligible friction, has radius R = 5.00cm. When released from rest, block 2 falls 75.0 cm in 5.00 s without the cord slipping on the pulley. (a) What is the magnitude of the...
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