LOW OR FAST - NCERT

1 motion and Time13In Class VI, you learnt about differenttypes of motions. You learnt that amotion could be along a straight line,it could be circular or periodic. Can yourecall these three types of motions?Table gives some commonexamples of motions. Identify the typeof motion in each SLOW OR FASTWe know that some vehicles move fasterthan others. Even the same vehicle maymove faster or slower at different a list of ten objects moving alonga straight path. Group the motion ofthese objects as slow and fast. How didyou decide which object is moving slowand which one is moving fast?If vehicles are moving on a road inthe same direction, we can easily tellwhich one of them is moving faster thanthe other. Let us look at the motion ofvehicles moving on a at Fig.

2 It shows the positionof some vehicles moving on a road inthe same direction at some instant oftime. Now look at Fig. It shows theposition of the same vehicles after sometime. From your observation of the twofigures, answer the following questions:Which vehicle is moving the fastestof all? Which one of them is moving theslowest of all?The distance moved by objects in agiven interval of time can help us todecide which one is faster or slower. Forexample, imagine that you have gone tosee off your friend at the bus you start pedalling your bicycleat the same time as the bus begins toTable Some examples ofdifferent types of motionExample ofType of motionmotionAlong a straightline/circular/periodicSoldiers in amarch pastBullock cartmoving on astraight roadHands of anathlete in a racePedal of a bicyclein motionMotion of the Eartharound the SunMotion of a swingMotion of apendulumIt is common experience that themotion of some objects is slow while thatof some others is 22 SCIENCE144move.

3 The distance covered by you after5 minutes would be much smaller thanthat covered by the bus. Would you saythat the bus is moving faster than thebicycle?We often say that the faster vehiclehas a higher speed. In a 100-metre raceit is easy to decide whose speedis the highest. One who takesshortest time to cover thedistance of 100 metres has thehighest SPEEDYou are probably familiar withthe word speed. In theexamples given above, a higherspeed seems to indicate that agiven distance has beencovered in a shorter time, or alarger distance covered in agiven most convenient way tofind out which of the two ormore objects is moving fasteris to compare the distancesmoved by them in a unit , if we know the distancecovered by two buses in onehour, we can tell which one isfaster.

4 We call the distancecovered by an object in a unittime as the speed of the we say that a car ismoving with a speed of 50kilometres per hour, it impliesthat it will cover a distance ofFig. Position of vehicles shown inFig. after some timeFig. Vehicles moving in the samedirection on a road50 kilometres in one hour. However, acar seldom moves with a constant speedfor one hour. In fact, it starts movingslowly and then picks up speed. So,when we say that the car has a speed of50 kilometres per hour, we usuallyconsider only the total distance coveredby it in one hour. We do not botherwhether the car has been moving with2021 22 motion AND TIME145We can determine the speed of a givenobject once we can measure the timetaken by it to cover a certain Class VI you learnt how to measuredistances.

5 But, how do we measuretime? Let us find MEASUREMENT OF 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 , 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 we need to measure intervalsof time which are much shorter than aday. Clocks or watches are perhaps themost common time measuring you ever wondered how clocks andwatches measure time?

6 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 everyday life we seldom find objectsmoving with a constant speed over longdistances or for long durations of 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 Some common clocks(b) Table clock(c) Digital clock(a) Wall clocka 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.

7 So, for us the speedis the total distance covered dividedby the total time taken. Thus,Total distance coveredSpeed = Total time taken2021 22 SCIENCE146A simple pendulum consists of asmall metallic ball or a piece of stonesuspended from a rigid stand by athread [Fig. (a)]. The metallic ballis called the bob of the (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. (b)]. The to and fromotion of a simple pendulum is anexample of a periodic or an pendulum is said to havecompleted one oscillation when its bob,starting from its mean position O, movesTo set the pendulum in motion ,gently hold the bob and move it slightlyto one side.

8 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 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 observationsFig. (b) Differentpositions of the bob of anoscillating simple pendulumFig. (a) A simplependulumto 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 up a simple pendulum asshown in Fig.

9 (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 measure the time period ofthe pendulum we will needa stopwatch. However, if astopwatch is not available, a tableclock or a wristwatch can be 22 motion AND TIME147 Table Time period of a simplependulumLength of the string = 100 taken for 20 Time periodoscillations (s)(s) These clocks are called quartzclocks. The time measured by quartzclocks is much more accurate than thatby the clocks available of time and speedThe basic unit of time is a second. Itssymbol is s. Larger units of time areminutes (min) and hours (h).

10 Youalready know how these units are relatedto one would be the basic unit ofspeed?Since the speed is distance/time, thebasic unit of speed is m/s. Of course, itcould also be expressed in other unitssuch as m/min or must remember that thesymbols of all units are written insingular. For example, we write 50 kmand not 50 kms, or 8 cm and not 8 is wondering how manyseconds there are in a day and howmany hours in a year. Can you helphim?in Table The first observationshown is just a sample. Yourobservations could be different fromthis. Repeat this activity a few times andrecord your observations. By dividingthe time taken for 20 oscillations by 20,get the time taken for one oscillation, orthe time period of the the time period of your pendulumnearly the same in all cases?