Physics lesson 4 - 2026

Added: 2026-05-30

[00:00:00]hello guys welcome to the physics lesson this is the physics lesson four so in today's lesson we'll be looking at the measurement of time so let's define what time is so time is defined as the measure of duration of events so time is actually the measure of duration of events all right so the sr unit of time is this seconds and other units in which time is measured are we have minutes hours days month year and the century so the sr unit for time is the second and these are other units in which time is a measured so common instruments used in a laboratory to measure time let's look at those common instruments used in the laboratory to measure time all right so let's squeeze here so these are the watch the stop clock the ticker tip timer the electronic clocks the pendulum and so forth and so on so we'll just consider to hear the ticker tip timer and the pendulum all right so let's look at the ticker tape timer so a take a tape timer measure time of an event by making dots on a carbon paper it makes it about 50 dots in one second all right so that's what it does it is used to actually measure time by doing what by making actually 50 dots on a carbon paper in one second all right let's say see so this is the formula for finding time on a ticker table timer all right let's squeeze a bit here excuse me so the formula says time is equal to 0.02 dots so this is the formula for example if a ticker tip timer makes see 20 dots and then they ask you to say what is the time taken in making those 20 dots so you just say 0.02 times 20 dots then you'll get the time that's how the ticker tip timer works all right so let's move a bit and look at it the simple pendulum all right the simple pendulum as another instrument that is used to measure time known as period all right so a simple pendulum is a suspended weightless string with a small mass so a ticker tape timer is actually a suspended weightless string with a small mass all right so that's what it is it is so for instance if we have a support here and then we have a string here with a small mass here then we have what we call a simple pendulum so a simple pendulum has got these parts here or these characteristics that you always see in books and also maybe exam paper so it has this distance here which is the distance from the center i mean yeah the center of the a a bob this one should be a bob i didn't put push it properly here it should be from the center of the bob so the distance from the center of the bob to the fixed point is known as the length that is the length of the pendulum then we have this part here which is known as the amplitude which is the angle between this position here the straight position here is called the equilibrium so the angle between the equilibrium and the maximum position that can be made maybe this one is displaced up to here then allowed to swing so this one here now can be called the maximum displacement all right so that maximum displacement is actually the amplitude all right then it has these points like point a point b point e uh c which actually helps us to understand the better about how the pendulum or the simple pendulum weeks so let's say this one is the oscillation these are the terms related to the simple pendulum known as the oscillation so what is an oscillation it is a one complete two and fro movement of the bulb from point a to point b to point c and back to a so it is one complete two and four movement of the bulb from point a to point b to point c and back to point e and like this one if the bob here moves from point a to point b to point c back to point b and then into pointy a that one complete uh to and fro movement is what we call oscillation all right so for instance if this one moves like that and comes back it has made the complete oscillation another complete oscillation another one another one another one so it has actually made maybe four or five v oscillation all right so let's talk about also the period all right the period another term is the period represented by t so what is the period the sum it is the time taken for one complete oscillation so the period is the time taken for one complete oscillation so for instance if this one oscillates from here and back to there the time that it takes for this one to oscillate here and then back that diameter takes is what we call the period and then the amplitude i already talked about it it is the distance between the rest position this rest position of the bob point b to the extreme end of the oscillation extreme end of the oscillation either a or c so either from here or to there that displacement there is what we call the amplitude all right then we have frequency so frequency it is the total number of oscillations made in one second so the total number of oscillations made in one second maybe if you have a a watch then you time one second you count how many oscillation it is making then you are trying to find the what we call frequency it is opposite of um the period so it is given by this formula frequency is equal to one over t where f is a frequency and t is period all right so let's look at how the simple pendulum works so if this one is a simple pendulum if let's say the same find the period of the pendulum when it makes 20 oscillations so we can use this pendulum to find the period that it takes to make 20 oscillations so to do so you need a temple like this one where you have number of swings or number of yeah experiments of the swings that you do then here you have time then you need this formula now to calculate there a period which says the period is equal to total time taken over number of full swings then a period since he represented by t so it is written in short form as t is equal to time small letter t and the number of swings small letter n so this is the formula for calculating now the period we can use to calculate the period here now in an experiment for example in a school laboratory or in a university you you may use this one to find the period of the pendulum so you have the table and then they say find the period of 20 oscillations so you need to do maybe about five trials of 20 to get the correct results because if you want to find the total i mean the the time period you need to time you need the stopwatch and then you need the same pendulum then you're allowed to swing you count 20 oscillation then you check the time that has been taken on the stopwatch then you record but if you do it once you you are likely to get an error so you repeat it five times like i've done here so that you you say the first time here you let it swing here so you count 20 oscillation like if it starts to goes and it comes back it's one oscillation like so let's try you say you have a stopwatch when you do like you let it go you start to the stopwatch so you let it go and you start one two three four five six seven eight nine ten 11 12 13 14 15 16 17 18 19 20 you even stop the stop watch then you record the time on the stopwatch here for instance maybe the time recorded is 10.

[00:08:43]uh two you record it as such then you go for the next one again you try another one the same to make sure that you do it correct so again you you start when you allow it to swing immediately you start to the stopwatch so you say start then 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 stop the stopwatch also then you record whatever time is indicated on the stopwatch in this case we are zooming in 10.21 then you do another one here three then again you start again one two three four 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 again you record the time may be 10.0 again you reset the stopwatch make sure it goes to zero yeah that's how it works then you again you let it swing you start together with the swing and the stop what you say start all right so it says start one two three four five six seven 8 9 10 11 12 13 14 15 16 17 18 19 20.

[00:10:30]again you record maybe the time is 10.2 again you do another one you reset the stopwatch until it is zero then you start together with it the swing again you start again start all right when you record five you say stat 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20.

[00:11:06]then you recorded the time on your stopwatch maybe 10 23 so these are the times we have indicated so the reason why we do a lot like five because you are assuming you are making errors in your starting and stopping of the stopwatch and in the counting so by collecting many values then get the average of it then the average value is 30 to be correct to the actual number so you get the average of this time so you say average time with a line on top there a bar on top which means average is equal to you add this 10.2 plus 10.1 plus 10.0 plus 10.2 plus 10.3 then you divide by the total number of these values you are adding there how many one two three four five so divide by five then you said average time is equal to then you add you get fifty point eight divided by five which will give you ten point two two seconds so this this will be now the time taken to make 20 oscillation all right now you need now to calculate the period so this one is just the time now you need to calculate the period all right so you have now time here you have found the time which is 10.2 so remember time is 10.2 seconds so move and then we'll say period is equal to time here 10.2 to now the oscillation number of swings how many swings have it made in that 10 seconds 20 so you say 20 then you say that time or period is equal to 10.2 divided by 20 which will give you 0.5 1 seconds i see your time period all right so let's move a bit to another section here so let's look at the distance displacement speed velocity and the acceleration so we have moved it to this so here we start with the definitions first so look at the definition of distance so distance is defined as a one dimension space between two points so distance is one dimension space between two points it is measured in meters so the s i units for distance is a meters all right so it is a scalar quantity so distance is a scalar quantity a scalar quantity is a quantity which has just magnitude and no direction like mass mass is a scalar quantity if you're asking me about my mass maybe i tell you i weigh 60 kg i will not tell you the direction because mass has got a no direction so yeah so distance is a scalar quantity then let's talk about displacement so displacement we are now here at the displacement so displacement is the distance moved in the specific direction so distance moved in a specific direction is also known as a displacement or distance moved in a straight line between two points of course i like the second definition so displacement is distance moved in a straight line between two points so if you have two points the distance in the uh between two points in the straight line is a displacement or it is the shortest distance between two points like we talk of shortcuts so all those shortcuts uh qualified to be [Music] a displacement because direct from the beginning to the end all right so see distance moved in a straight line between two points or uh or distance moved in the specific direction all right so we are saying it is also measured in meters just like a distance and displacement is a vector quantity so what is a vector quantity a vector quantity is a quantity which has both direction and the magnitude all right so for instance if you uh okay let me say in which situation do we talk about vectors so vectors are directions when you are giving someone direction to say okay where is the let's say uh usa i'm just giving an example okay from where we are in zambia usa is maybe um uh let's say maybe 150 degrees uh uh west the knee yeah west then from zambia maybe to usa maybe you say it is maybe uh 10 000 kilometers all right or 100 000 kilometers so you have taught the person the direction in which usa is and then also the distance between the zambia and the usa so that's how actually vectors work so whenever you are telling a person no you move like this from there to there it is this distance we are going to cover your face in the east direction or waist direction those direction you are giving we call them vectors so vectors are direction where you tell someone the direction and the distance between that place where they are going all right so let's look at it uh some examples here on the distance and the displacement so we have example one here which says a car traveled from point a to e uh sorry here it's supposed to be d because i edited this part so a cat traveled from point a to uh d along a curved path abcd so ignore e what is the total distance and the displacement traveled so we have this situation here a car has moved from pointy a to b then it to c they need to d then they are saying what is the total distance and the displacement traveled so for us as here for distance distance would be like this so the distance traveled by the car is the total distance abcd so it will be just distance from a to b you add then from b to c you add then from c to d u r that would be what that would be the distance that the the the car would have covered however for displacement is tricky so displacement is considered as the distance between the two points in a straight line so between the beginning and the end the distance between two points in a straight line like this you draw like this that's what we call a vector so say under the vector say displacement traveled by the car is a distance a d so it is distance a d right of a so you have even given the direction so the car has moved the a d right of a so it is on the right of a a is this side d is this side so it is on d d is on the right of a so that's how you say displacement you tell the distance and also the direction button in straight from the starting point to the end point all right let's move a bit and look at the example two so here we have a car moves four kilometers to the east and the three kilometers to the north find the its distance and the displacement also we need distance and displacement so we have a car here and this is the route it uses so it moves like that all right then here it also moves now to the east like that all right so now they want us to find the distance 80 uh they find its distance and the displacement so what is its distance at the displacement so we know that here this car has moved this four kilometer four kilometers in going to the east and then at two or three kilometers going to the north because this is the north it goes like that then if we are to find the distance distance would be equal to actually four kilometers here plus it three kilometers here so which would be 70 kilometers now if we talk of displacement displacement is another story now it will be a distance from the beginning straight to the end point so it will be this line here from the beginning to the straight point that would be it's a displacement so our diagram changes like this so let's say we have now this distance it moved four kilometers to the east and then three kilometers to there uh north and then there's an angle also involved which gives a direction so displacement is this now line here that would be the displacement so it will be found by d let's say d is a displacement so we use pythagoras theorem in mathematics so say to find this one so in mathematics this one becomes a right angle d triangle this long line here or the light the long side here is called the hypotenuse then the other one is called the adjacent and then this one is called the vertical one is cody the opposite then the horizontal one is called the adjacent so adjacent squared plus uh the opposite is squared then it gives you the displacement which is the hypotenuse here so the d for displacement squared is equal to it will be now four squared here four squared plus three squared which would be d squared is equal to four squared sixteen plus 3 squared 9 the d squared is equal to 16 plus 9 which is 25 then you square this one to get rid of the square even in this one so you get rid of square you have a d then your square root 25 you're getting five kilometers so this is five kilometer however you haven't done you haven't finished you need to give direction so direction now you need to give this angle so from a soccer tower we have this part here we caught our soccer tour so in this one we are using tower because we have this area with this side we are given uh here we are given this side which is the adjacent this side which is a torah meaning tan opposite adjacent so tan opposite adjacent so opposite we have been given is this side adjacent to this side so this simply means tan theta tan theta is equal to opposite over adjacent we are now put opposite we have three then adjacent we have four so substitute three four then we divide three divided by four we'll get zero point seventy five so 0.75 then it will now get the inverse of this which would be like theta is equal to 10 inverse sorry there's one turn here supposed to be just one 1092 tons there so we are supposed to have just one turn there not two tons where is the color that i'm trying to pick so just one time okay so the inverse tan inverse of that value you found here then you let's squeeze a bit here then you'll find that the um theta here is equal to 36.9 degrees so now you put here your answer you say uh 50 5 kilometers 36.9 degrees northeast so this one is north east so it is in between there this one east this one the north then in between that's why you say north east all right so you dive inside that way you have told the distance moved from here and also the direction so i think 6.9 degrees north east all right so let's look at now speed what is speed speed is the rate of change of distance moved with the time all right so that's what the distance is it is the rate of change of distance moved with the time so let me just see explain a little bit here when they say speed is the rate of change of distance moved with the time then it is just people when they hear speed they think as if something moving fast then it is that is speed no speed is anything even my working can be said to be speed as long as we are trying to measure the time i've taken it to work between certain two points and then measure that that that distance i've moved maybe if i move between the two points and i record the distance and i record the time taken to move between those two points then i get that distance then it divided by the time then i'm finding speed that's why we are saying speed is the rate of change of distance moved with it time or it is just a comparison of distance covered with the time taken that is it in that way also you can explain it better it is a comparison between distance move and the time taken to move this that specific distance so it is expressed by the equation below here so we have speed is equal to distance over time all right so we are saying where is sc is the speed that's how we represent it in meters per second so the sr unit for speed is meters per second the day here is distance in meter z per second and the time is in t is time here in seconds so speed is supposed to be in meters per second the distance is supposed to be in meters then the time is supposed to be in seconds then we are saying the s i unit for speed is meters per second i've already explained and it is a scalar quantity so speed again is a scalar quantity so like when you see a vehicle moving speeding or whatever you just say oh i saw a car maybe it was moving at 20 kilometers per hour i don't know or maybe you are moving you're a driver you are driving then you're looking at the speedometer of maybe 80 kilometers per hour that is just a speed it is not even telling you where the car is going no so speed is just a scalar quantity all right let's now look at the velocity so velocity we are saying it is the rate of change of displacement with the time or rate of change of distance moved with time in a specific direction so velocity now is a scalar i mean a vector quantity so it is the rate of change of displacement with the time i like this one but other books they define it as the rate of change of distance moved with it time in a specific direction that is also good so for example if you are moving north east northeast then you are i tell you i was moving northeast at 10 kilometers per hour then whatever i'm describing now that is the vector because i've told you where i was going north east i've told you direction and i've also told you the amount of the speed i was having so that one now becomes the velocity all right so that's how it is then we are saying it is the same as the speed except that the velocity requires a speed to fully describe it no no no requires a direction here sorry so here we have said the speed sorry so it's supposed to be direction all right so i'm going to edit and put the direction here all right so we are saying uh it is the same as the speed except that the velocity requires a a direction to fully describe it it is a vector quantity and its sr units are meters per second so the s i units are the same as for the speed here so they only differ in that when you are dealing with the velocity you should talk about the direction but when you are dealing with the speed you only talk about it the quantity no need to talk about it the direction all right so let's look at an example here which is saying a car moved due east a distance of 40 kilometers in one hour find it a its speed and a b its velocity so solution here or same air you know the formula for speed here so speed is equal to distance over time so we need information or data to replace in this formula to calculate for speed s is speed d is a distance t is time so we have data or the information needed to help us to solve this question because they are asking us to to find the speed and this is the formula so we know we have been given we need distance here which is the 40 kilometer it's here and then we need the time we have been given time is what one hour and then they're asking us to find the speed so we put it in that way then say okay distance is equal to uh speed is equal to distance here it is the 40 so we have to put 40 where there is d we put 40 then where there is a tt we put it uh one hour there then we divide okay so one hour in 240 it will be 40 so kilometer per hour so that's our speed the next look at the velocity velocity again is see it will be the same so velocity and speed are the same except with the velocity you need to talk about direction so the way we have calculated here it's good we can use this one for velocity so same velocity is equal to 40 kilometer per hour due east now you're explaining where that speed is going so whenever you say due east now you have added the direction now it has become also velocity all right so let's push a bit and look at uh this expression where we are saying since both velocity and speed are measured in meters per second then we are saying you need to know how to change from kilometers per hour to meters per second yes it's very important that you know how to change from meters per i mean kilometers per second to meters per second because you remember speed is in meters per second but you need to know how to change in kilometers per hour to meters per second so you know that one kilometer is equal to 1000 meters so again you know that one hour is equal to 60 minutes all right then you know that this custom minutes also can give us 60 seconds all right so now it means that if you want to get one hour in two seconds first you multiply one hour by six day which is it minutes and then by 60 second seconds which will give you one hour which will give you 3600 this seconds so one hour is equal to 3600 seconds all right so now you can now change from kilometer to uh meters per second that value we found 40 kilometers per hour excuse me can be changed into seconds meters per second as shown so you can change from kilometer per hour to meters per second as shown so you have 40 kilometers per hour uh which can be now like this so say 40 over or i mean this 40 now you're changing it into kilometers i many meters now so you say 40 you know times all right so if you remember prefixes it's as good as saying this one is a thousand here so times one thousand meters over yeah because this one is just a thousand kilo a thousand so if you press a number here then it would have changed to meters so 40 times 1000 meters divided by this hour here per hour one hour one hour is equal to 3 600 seconds then it will be equal to you multiply that one you're getting 40 000 hey what is this one 40 000 meters all right then divided by the same 3600 seconds so when you you you divide this one by this you get 11.1 meters per second as your value as you convert it so this one any kilometers it is 40 kilometers per hour in a second it's 11.1 meters per second so we have come to the end of this lesson i hope you enjoyed the lesson and you learnt a lot in this lesson see you in the next lesson as for now guys bye