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Unformatted text preview: Review Notes For Exam 4 SAR Physics AM Kolchin I. Circular Motion 1. Circular motion is done about an axis : All of our discussions in class have made reference to some axis about which rotation takes place. When talking about rotational velocity, rotational accelera- tion, torque, angular momentum, or rotational inertia we are always referring to some axis of rotation. 2. Rotation : Takes place about an objects internal axis. 3. Revolution : Takes place about an external axis. 4. Rotational speed ( ): angle swept out per unit time. (units: rpm, degrees/s, radians/s) For a rigid object, rotating about an axis, the rotational speed is the same at all points in the object. 5. Linear speed or tangential speed ( v ): distance cov- ered per unit time. The linear speed is not the same everywhere on a rigid object, rotating about an axis. Rather, the linear speed of a point on the object is di- rectly proportional to the rotational speed ( ) and the distance ( r ) of the point from the axis of rotation: v r . So if you double r , you double v ; if you double , you double v ; if you double both, then v quadruples. (Units include: m/s, mph,). 6. Centripetal Force : An object in circular motion is always changing direction. Its speed may be constant, 1 but then it is always experiencing a force that is perpen- dicular to its velocity. This force is directed inward in the radial direction . We call this force centripetal force no matter what its source. Centripetal force can be caused by gravitational attraction or a string pulling inward on a whirling ball. For an object in circular mo- tion about an axis, the centripetal force is related to the objects: mass ( m ), speed ( v ), and distance ( r ) from the axis by the equation: F c = mv 2 r What happens to F c if I double the objects speed, mass, or distance from the axis of rotation? Be able to an- swer these and similar questions. (Units are good old fashioned newtons (N)). Note : Since F c is always perpendicular to the direc- tion of v for a mass in circular motion, it never does any work on the object. Work is only done on an ob- ject by the component of a force in the direction of the objects motion. Since F c is to the motion, it has no...
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