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Rotational Motion Chapter 8

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**Describing Rotational Motion**

8.1

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**Describing Rotational Motion**

Radian Angular Displacement change in an angle

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Practice Problem What is the angular displacement in radians of a wagon that moves 135 around a circular path?

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Angular Velocity

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Practice Problem A playground merry-go-round makes two complete rotations in 9.6s. What is its angular velocity?

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Angular Acceleration

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Practice Problem A car increases in speed from rad/s to rad/s in 5.0 s. What is the angular acceleration of the car?

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Homework Page 200 5-9

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Rotational Dynamics 8.2

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**Starting Rotation of Object**

lever arm perpendicular distance from the axis of rotation to the point where the force is exerted L= rsinθ θ=angle at which the force is applied!!! r=distance to point where force is applied to axis

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Torque measure of how effectively a force causes rotation N*m τ (tau)

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Practice Problem How much torque is applied if a wrench applies a force of 240N at an angle of 60.0° from the perpendicular? The wrench is 32cm long.

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Net Torque Net=Sum decide on a positive & a negative rotation direction ∑τ=τ1+τ2

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Practice Problem Ashok, whose mass is 43kg, sits 1.8m from the center of a seesaw. Steve, whose moss is 52 kg, wants to balance Ashok. How far from the center of the seesaw should Steve sit?

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Homework Page 203: 12-15 Page 205: 17-20

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**The Moment of Inertia resistance to rotation I kg*m2**

different for different shapes Point Mass I=mr2

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Practice Problem What is the moment of inertia of a hockey puck that has a mass of 170g, a thickness of 2.5 cm, and a diameter of 7.6 cm? The formula for the moment of inertia of a cylinder is I=1/2mr2.

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Homework Page 208: problems 21-24

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**Newton’s Second Law of Rotation**

Replace Newton’s Second Law (F=ma) with angular terms a=α F=τnet m=I

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Practice Problem A solid steel wheel has a mass of 15kg and a diameter of 0.44m. It starts at rest. You want to make it rotate at 8.0 rev/s in 15s. What torque must be applied to the wheel? If you apply the torque by wrapping a strap around the outside of the wheel, how much force should you exert on the strap? If the force on the strap were twice as great, what would be the speed of rotation of the wheel after 15s?

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Equilibrium 8.3

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Center of Mass the point on the object that moves in the same way that a point particle would move Locating Suspend the object from a point, center of mass is along the vertical line drawn intersection of 2 vertical lines will give a point Center of mass for a person different male & female

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**Center of Mass and Stability**

Stable an external force is required to tip object Higher center of gravity less force needed to upset Equilibrium translational equilibrium rotational equilibrium

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Practice Problem A 5.8kg ladder, 1.80m long, rests on two sawhorses. Sawhorse A is 0.60m from one end of the ladder, and sawhorse B is 0.15m from the other end of the ladder. What force does each sawhorse exert? What if the mass of the ladder is 11.4kg?

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Homework Page 210: 26-29 Page 215: 37-39

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**Rotating Frame of Reference**

Centrifugal Force apparent force (not real) pulls toward the outside of the rotation Coriolis Effect deflection of a moving object

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Homework Page 217: 40-45 Page 223: 75, 78, 81, 84, 88 MAKE SURE YOU STUDY FOR YOUR TEST TOMORROW!!! ***IF YOU HAVE QUESTIONS ASK***

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