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# General/Notes 8.2

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### General/Notes 8.2

1. 1. ROTATIONALDYNAMICSSection 8. 2
2. 2. Rotational Dynamics• How can I describe torque?• What factors influence torque?
3. 3. Opening A Door• Easiest Way to do it?• Why is the handle where it is?
4. 4. Opening a Door
5. 5. Rotational Dynamics• A force must be exerted to open a door• Doorknob is near the outer edge, as far from the axis of rotation as possible• Pull at a right angle to the door to apply the most force
6. 6. Rotational Dynamics• So where we apply the force changes how effective it is• In this section we will be measuring how effectively an applied force turns something• Pertains to doors, pedals, wrenches…..
7. 7. Rotational Dynamics• The change in angular velocity depends on: • the magnitude of the force • the distance from the axis to the point where the force is exerted • the direction of the force
8. 8. Lever Arm• Lever Arm: The perpendicular distance from the axis of rotation to the point where the force is exerted
9. 9. Lever Arm • For the door, it is the distance from the hinges the point where you exert the force • If the force is perpendicular to the radius of rotation then the lever arm is the distance from the axis, r
10. 10. Reduced Lever Arm• If force is not perpendicular, lever arm is reduced• Lever Arm (L)• L = r sin ɵ• ɵ = angle between force and the radius from the axis of rotation to the point where the force is applied
11. 11. Torque•
12. 12. Example• A bolt on a car engine needs to be tightened with a torque of 35 Nm.• You use a 25 cm long wrench to pull on the end of the wrench at an angle of 60.0° from the perpendicular.• How long is the lever arm and how much force do you have to exert?
13. 13. Homework• Page 203• # 11 - 15
14. 14. Balancing Torques• http://phet.colorado.edu/en/simulation/balancing-act When we played with the balance, what two things did we change?
15. 15. Moment of Inertia•
16. 16. Moment of Inertia• Hold your book at the bottom and rotate• Hold your book in the middle and rotate• Which one has a lower torque?
17. 17. Moment ofInertia• When something is spinning, we have not taken into account shape
18. 18. Homework• Read Page 205-207