2LA_Lab_5_CentripetalForce3 - Physics 2LA Centripetal Force...

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Physics 2LA Centripetal Force (**Includes Pre Lab Assignment) Introduction: This lab has a different style then your previous labs. It is meant to lead you through the process of testing for yourself the physical theory of centripetal force. A good scientist (or good doctor, or prudent person) will always, when told some new idea, ask whether or not it is actually true. S ETUP AND OPERATION OF THE APPARATUS: The apparatus provided is a Centripetal Force Accessory (trolley track, trolley, weight hanger and strings) mounted on the Introductory Rotational Apparatus as shown in Figure 1. The trolley track is mounted along a diameter of the platter of the Rotational Apparatus as shown in the Figure. In order for the trolley to rotate with the platter, a centripetal force is required, accelerating the trolley toward the axis of rotation. This is applied by a hanging mass, suspended by a pulley through a hole in the axis of the spinning platter. Figure 1 - Centripetal Force Apparatus To begin, the platter is rotated by hand just fast enough to push the trolley lightly against the stop at the end of the track, so the centripetal force is supplied, in part by the push of the stop. Friction slows the rotation of the platter, gradually decreasing the rotational velocity of the trolley. Eventually the centripetal force provided by the hanging mass exceeds the force required to maintain the circular motion of the trolley, and the trolley is pulled inward. At the instant before the trolley begins to move inward, the acceleration of the trolley is just balanced by the centripetal
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force of the hanging mass. The Smart Pulley with its built-in photogate can be used to measure the angular velocity of the platter, and therefore of the trolley, at this point of balance. When using the Smart Pulley to measure the angular velocity, the point where the trolley begins to move inward is easily seen in the computer plot of angular velocity versus time as shown in Figure 2. The change in position of the trolley reduces the moment of inertia of the rotating system (the platter, the track, and the trolley). Since there is no torque (other than friction – which is small) acting on the rotating system, the angular velocity must increase so that angular momentum is conserved. The sudden increase in angular velocity is easily seen on the computer graph. With the angular velocity measured, all the variables of the equation of motion are known and can be used to test the relationship: F c = m T R ω 2 . F c = the weight of the hanging mass = m h g m T = the mass of the trolley plus any additional mass that has been added to it. R = the distance to the trolley center of mass from the axis of the platter, when the clip is pulled up just against the spindle of the rotational apparatus. ω
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This note was uploaded on 09/30/2009 for the course PHYSIC2LA phy taught by Professor Robertclare during the Spring '09 term at UC Riverside.

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2LA_Lab_5_CentripetalForce3 - Physics 2LA Centripetal Force...

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