Rotational Dynamics Chapter 8 Section 3. Torque Direction  A net positive torque causes an object...

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Transcript of Rotational Dynamics Chapter 8 Section 3. Torque Direction  A net positive torque causes an object...

Rotational Dynamics

Rotational DynamicsChapter 8 Section 3Torque DirectionA net positive torque causes an object to rotate counterclockwise.

A net negative torque causes an object to rotate clockwise.+-Newtons Second Law for RotationT = I

Net Torque = (Moment of Inertia)(Angular Acceleration)

Translational vs. RotationalNewtons 2nd LawTranslational: F = maForce = Mass x Acceleration

Rotational: T = ITorque = Moment of Inertia x Angular Acceleration Example Problem #1A toy flying disk with a mass of 165.0 grams and a radius of 13.5 cm that is spinning at 30 rad/s can be stopped by a hand in 0.10 sec. What is the average torque exerted on the disk by the hand?Example Problem #1 AnswerT = IT = (mr2)((f-i)/t)T = ()(0.165kg)(0.135m)2 ((0rad/s 30rad/s)/0.10s)

T = -0.45 NmResistance to ChangeSwinging a sledge hammer, or a similarly heavy object, takes some effort to start rotating the object.The same can be said about stopping a heavy object that is rotating.

MomentumTranslational Momentum A vector quantity defined as the product of an objects mass and velocity.Also known as, Inertia In Motion

Angular Momentum The product of a rotating objects moment of inertia and angular speed about the same axis.

Angular Momentum EquationL = I

Angular Momentum = (Moment of Inertia)(Angular Speed)Angular Momentum The variable used for Angular Momentum.Capital letter L

The SI units for angular momentum.Kgm2/sTranslational vs. RotationalMomentumTranslational: p = mvLinear Momentum = mass x velocity

Rotational: L = IAngular Momentum = moment of inertia x angular speedConservation of Angular MomentumThe Law of Conservation of Angular Momentum - When the net external torque acting on an object is zero, the angular momentum of the object does not change.

Li = LfIi = IfAngular Momentum ExampleAngular momentum is conserved as a skater pulls his arms towards their body, assuming the ice they are skating on is frictionless.

During an ice skaters spin, they will bring their hands and feet closer to the body which will in turn decrease the moment of inertia and as a result increase the angular speed.Example Problem #2A 0.11kg mouse rides on the edge of a rotating disk that has a mass of 1.3 kg and a radius of 0.25m. If the rotating disk begins with an initial angular speed of 3.0 rad/s, what is its angular speed after the mouse walks from the edge to a point 0.15m from the center? What is the tangential speed of the disk at the outer edge?Example Problem #2 Answer = 3.2 rad/sv = 0.8 m/s

Kinetic EnergyRotational Kinetic Energy Energy of an object due to its rotational motion.

Greater angular speeds and greater moment of Inertia, yields greater rotational kinetic energyRotational Kinetic Energy EquationMomentum vs. EnergyExample Problem #3A car tire has a diameter of 0.89m and may be approximated as a hoop. How fast will it be going starting from rest to roll without slipping 4.0m down an incline that makes an angle of 35 degrees with the horizontal?Example Problem #3 Diagramd = 4.0mh = ?0.89mVi = 0 m/sVf = ? Example Problem #3 Answer