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www.GOALias.blogspot.comAFig. 13.4 (a) A simplependulumFig. 13.4 (b) Differentpositions of the bob of anoscillating simple pendulumA simple pendulum consists of asmall metallic ball or a piece of stonesuspended from a rigid stand by athread [Fig. 13.4 (a)]. The metallic ballis called the bob of the pendulum.Fig. 13.4 (a) shows the pendulum atrest in its mean position. When the bobof the pendulum is released after takingit slightly to one side, it begins to moveto and fro [Fig. 13.4 (b)]. The to and fromotion of a simple pendulum is anexample of a periodic or an oscillatorymotion.The pendulum is said to havecompleted one oscillation when its bob,starting from its mean position O, movesOBto A, to B and back to O. Thependulum also completes oneoscillation when its bob movesfrom one extreme position A to theother extreme position B andcomes back to A. The time takenby the pendulum to complete oneoscillation is called its timeperiod.Activity 13.2Set up a simple pendulum asshown in Fig. 13.4 (a) with athread or string of length nearlyone metre. Switch off any fansnearby. Let the bob of thependulum come to rest at itsmean position. Mark the meanposition of the bob on the floorbelow it or on the wall behind it.To measure the time period ofthe pendulum we will needa stopwatch. However, if astopwatch is not available, a tableclock or a wristwatch can be used.To set the pendulum in motion,gently hold the bob and move it slightlyto one side. Make sure that the stringattached to the bob is taut while youdisplace it. Now release the bob from itsdisplaced position. Remember that thebob is not to be pushed when it isreleased. Note the time on the clockwhen the bob is at its mean position.Instead of the mean position you maynote the time when the bob is at one ofits extreme positions. Measure the timethe pendulum takes to complete 20oscillations. Record your observations146SCIENCE