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www.GOALias.blogspot.coma constant speed or not during thathour. The speed calculated here isactually the average speed of the car. Inthis book we shall use the term speedfor average speed. So, for us the speedis the total distance covered dividedby the total time taken. Thus,Speed =In everyday life we seldom find objectsmoving with a constant speed over longdistances or for long durations of time.If the speed of an object moving alonga straight line keeps changing, itsmotion is said to be non-uniform. Onthe other hand, an object moving alonga straight line with a constant speedis said to be in uniform motion. Inthis case, the average speed is thesame as the actual speed.We can determine the speed of a givenobject once we can measure the timetaken by it to cover a certain distance.In Class VI you learnt how to measuredistances. But, how do we measuretime? Let us find out.13.3 MTotal distance coveredTotal time taken13.3 MEASUREMENTOF TIMEIf you did not have a clock, how wouldyou decide what time of the day it is?Have you ever wondered how our elderscould tell the approximate time of theday by just looking at shadows?How do we measure time interval ofa month? A year?Our ancestors noticed that manyevents in nature repeat themselves afterdefinite intervals of time. For example,they found that the sun rises everydayin the morning. The time between onesunrise and the next was called a day.Similarly, a month was measured fromone new moon to the next. A year wasfixed as the time taken by the earth tocomplete one revolution of the sun.Often we need to measure intervalsof time which are much shorter than aday. Clocks or watches are perhaps themost common time measuring devices.Have you ever wondered how clocks andwatches measure time?The working of clocks is rathercomplex. But all of them make use ofsome periodic motion. One of the mostwell-known periodic motions is that ofa simple pendulum.(a) Wall clock(c) Digital clock(b) Table clockFig. 13.3 Some common clocks145MOTIONAND TIME145