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ROTATIONAL MOTION

ROTATIONAL MOTION. Sec. 7.4 Torque, Equilibrium & Stability. ROTATIONAL MOTION. This motion is related to circular motion except the object is rigid. It deals with the spin of an object, not the linear motion! Torque : measures the ability of a force to rotate an object around some axis.

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ROTATIONAL MOTION

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  1. ROTATIONAL MOTION Sec. 7.4 Torque, Equilibrium & Stability

  2. ROTATIONAL MOTION • This motion is related to circular motion except the object is rigid. It deals with the spin of an object, not the linear motion! • Torque: measures the ability of a force to rotate an object around some axis. • The greater the torque, the greater the change in rotational (spinning) motion. • We measure the perpendicular distance from the axis of rotation to a line drawn along the direction of the force. This is called the lever arm.

  3. TORQUE • Torque is a vector quantity which is measured in mN. It is symbolized by Greek letter, . • The direction of the torque is always in the direction of the force! • Net torque is the sum of all the torques acting on the object. FORMULA Torque = Lever Arm x Force  = rF The units for torque are expressed as mN to avoid confusion with the units for work.

  4. Equilibrium • We know that when we reach equilibrium, the forces are balanced (translational equilibrium). • When we say that torques are balanced, we reach rotational equilibrium. • When an object is in mechanical equilibrium, the object has both rotational and translational equilibrium. • When a rigid body remains at rest, it is called static equilibrium and is stable.

  5. Stability • An object becomes unstable when either condition is met: • The object’s center of gravity (or center of mass) lies outside the base of support. • The line of action of the weight does not intersect the original base of support. • Knowing the object’s center of mass helps in determining stability.

  6. Moment of Inertia • The resistance of an object to changes in its rotational motion is called the moment of inertia (I). • While this is similar to mass (they both measure inertia), I depends not only on the object’s mass but the distribution of the mass around the axis of rotation. • See page 905 in your text for I for various rigid objects.

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