Full transcript of “NECT Physical Sciences - Gr 11 Verification of Newtons Second Law”. Click a timestamp to open the video at that moment. Need another video? Use the YouTube transcript generator.
0:01 [Music] so [Music] hi grade 11's i'm john mcbride and i'm joyce poco today we're working through the practical on newton's second law joyce will demonstrate the procedures and together we'll discuss points of interest as we work through this investigation newton's second law states the acceleration of an object is in the direction of the net force the acceleration is directly proportional to
0:47 the net force and inversely proportional to its mass the formula f net is equals to m a expresses the relationships between the net force the mass of the object and its acceleration the basic setup is a trolley on a track attached with a light inextensible string to weight that hangs over a pulley when the trolley is released the weight pulls the trolley down the trap the string must be light so it has almost no mass compared with the weights and the trolley it must not stretch we say it must be inextensible the force of the weight is exerted
1:32 hundred percent on the trolley there is no stretching of the string whenever something moves it experiences kinetic friction or forces which resist its motion such as air friction this holds true for the motion of the trolley on the track the trolley tends to slow down a little as it travels along the track we compensate for friction and resistive forces by raising the track slightly at one end until the trolley runs down the track at a constant speed how we are measuring the speed of the trolley we'll use the ticker timer to measure the trolley speed ticker timer is a gadget which has a vibrating point that makes marks on carbonized tape we call the tape ticker tape here's the
2:20 tape if i scratch the tape with my nail you can see a mark on the tape so if the pointer hits the tape hard at regular time intervals while we pull it through the ticket timer we can see dots on the tape let's try let's pull the tape through the ticker timer while the timer is switched on right the tape is fed under the pointer we are ready to switch the machine on pull the tape at constant speed here is the sample of the tape the first part of the tape is rather messy so we'll exclude that part here is a
3:05 section of the tape where the dots are spaced at equal time intervals from each other the tape was running through the machine at a constant rate we can even measure the speed of the tape measure the distance it moved in say five intervals of time and calculate the average speed as distance divided by time that raises another question how many seconds are there between each vibration or let's phrase it another way what is the frequency of the vibrations the frequency of vibration is the same as frequency of vibration of the escom main supply in south africa electric power is applied at a frequency of 50 hertz 50 cycles per second the period of vibration is equal to the inverse of its
3:53 frequency t equals 1 over f t equals 1 over 50. therefore t equals 0.02 seconds we have marked off a section of five gaps and measured the distance the tape traveled during these five intervals it traveled 19 millimeters in this time the time taken is five times zero comma zero two seconds which gives us zero comma one seconds distance traveled equals nineteen millimeters which equals zero comma zero one nine meters the average speed equals distance divided by time which equals zero comma zero one nine meters divided by zero one seconds and that gives us an
4:41 answer of zero comma one nine meters per second the tape traveled at a constant velocity of zero comma one nine meters per second our first task is to raise the tracks efficiently for the trolley to run freely down the track at constant velocity while it pulls the ticker tape through the ticket timer when we achieve this we have set up a friction compensated track [Music]
5:32 let's check that the dots are evenly spaced on the step it looks like they're evenly spaced and the trolley is running down the track at constant velocity i'm going to measure the different segments now the first segment has a length of 20 millimeters and the second segment also has a length of 20 millimeters we have set up a friction compensator track why is it important that the track is friction compensated newton's second law deals with the net
6:17 force of the object if there is friction acting on the object the pulling force of the volume will be applied force not the net force we won't actually be able to record results for newton's second law because we won't know the net force of the trolley the friction compensator track allows us to use the pulling force of falling weights as the net force acting on the trolley that's really cool my next question is how do we measure the acceleration of the trolley acceleration is the rate of change of velocity it is a change in velocity divided by the change in time acceleration is the final velocity minus the initial velocity divided by the time we know how to measure the average velocity
7:03 so all we have to do is measure the average velocity at two different time intervals to get the initial and final velocities we also have to work out delta t that is how much time passed while the velocity changed here's a section of tape for the truly accelerating uniformly on the track i've marked off three consecutive segments of five gaps between the dots i've measured the distance traveled in each time interval remember each time interval takes 5 times 0.02 seconds which is 0.1 seconds calculate the average initial and average final velocity average velocity equals distance divided by time the time is zero comma one
7:51 seconds because we have used five gaps between the dots the distance is 38 millimeters and must be converted to meters so it is zero comma zero three eight meters the velocity is zero comma three eight meters per second and we can calculate the other two values of average velocity in the same way [Music] okay done that now we calculate the change in velocity [Music] the final velocity minus the initial
8:36 velocity that is v3 minus v1 but how long does it take to change the velocity take a look at the tape the average initial velocity occurred exactly halfway through the time interval and the average final velocity occurred exactly halfway through the last time interval so the time gap is one half plus one whole plus one half which gives us two times zero point one second the time is 0.2 seconds [Music] let's complete the calculation acceleration is equal to the change of velocity divided by delta t or delta
9:23 time that is zero comma four eight meters per second divided by zero comma two seconds the acceleration is two comma four meters per second per second in the direction of the net force another little point to make before we start taking results the net force is equal to the weight of the slotted mass pieces which will hang over the pulley so we make a note of the total mass of the slotted mass hanger this mass hanger has a mass of 5 grams and the slotted mass piece has a mass of 5 grams so the total mass is 10 grams the net force equals the mass of the hangar times the acceleration due to
10:10 gravity we must convert the grams to kilograms we divide 10 grams by 1000 grams per kilogram and get zero comma zero one zero kilograms so the net force in this case is zero comma zero nine eight newton now we are set up ready to begin taking measurements very nearly there's one last and very important point to note the mass of the system must remain constant this means that the mass of the trolley and the slotted mass pieces must remain constant yes we are working with acceleration and net force so the total mass of the system must be kept constant while we change
10:57 the net force and measure the acceleration there's a very neat way of doing this the trolley carries all the mass pieces which are not hanging from the mass hanger take a look and see in this way the total mass of the accelerating system remains constant okay that's it we are ready learners you need a few minutes to read through the procedures in part a which we've already gone through with you and part b which we are about to start doing get yourself a pen or a pencil so you can take down the results as we're going along hit the pause button now
11:44 okay folks we're back on track first we'll set up the ticket timer with a new tape thanks joyce mark the tape as tape number one we feed the tape through the ticket timer and connect it to the trolley there we go we also make sure the trolley remains stationary on the track we'll do this by holding the tape to keep it still next we hang the mass piece the slotted mass holder and we hang it over the trolley connected to the string this has a mass of 10 grams record this mess in your tabular results on the row for tape number one don't
12:30 forget that we need to keep the mass of the whole system constant the four slotted mass pieces are on the trolley to keep the mass constant now we need to act swiftly to get all of this running smoothly joyce will start the ticket timer and release the trolley right john let's go we need a ruler and a sharp pencil to mark the segment of the tape remember that we are counting five gaps between the dots and we are marking of three consecutive segments of five dots learners are you ready to record these results segment one has a length of 39 millimeters
13:15 segment two has a length of 41 millimeters segment three has a length of 44 millimeters and that is the first set of readings done just a hint here the lines marking the segment of the tape must be exactly perpendicular to the length of the tape every millimeter counts in these readings the pencil must be sharp so you only draw a thin line on the tape we are ready to set up for the next set of values of net force on the trolley we
14:01 take a mass piece from the trolley and place it on the hanger so let me do that there we go we take the mask piece put it on the hanger connect the hanger over the pulley the mass of the hanger is now 20 grams we prepare a new length of tape and label it as tape two feed it through the ticket timer and attach it to the trolley and away we go let's analyze tape two first we must find a section of the tape where the dots are well defined and easy to see use the ruler draw a line perpendicular
14:48 to the tape count five gaps between the dots and make another line do this twice more until we have three consecutive segments now we measure the length of each segment results for tape 2 the accelerating mass is 20 grams segment 1 54 millimeters segment 261 millimeters segment three sixty six millimeters [Music] okay tape three coming up here's the tape let's label it tape three now we need to move another mass piece onto the mass hanger
15:33 the accelerating mass is now 30 grams ready to roll joyce oh and we analyze tape three just like we did for the other two tapes results for tape 3 the accelerating mass is 30 grams segment 1 62 millimeters segment 2 68 millimeters segment 3 79 millimeters [Music]
16:22 we're setting up tape for now there are three mass pieces on the mass hanger the accelerating mass is now 40 grams and off she goes and now we analyze tape 4. the accelerating mass is 40 grams segment 1 86 millimeters segment 2 98 millimeters segment 3 millimeters
17:10 the last round of the trailer is coming up we're taking five sets of measurements so that we can confirm a trend in the variation of acceleration with net force if we only take three measurements we have less data available to us if we had time to take 10 readings we would do even more certainty and have more reliability of our results five sets of results are sufficient for our purpose so we're going to move the last mass piece from the trolley onto the mass hanger we've prepared a new tape and we're ready to go for the last time we analyzed the last tape here are the
17:57 results for tape number five the accelerating mass is 50 grams segment 1 90 comma 5 millimeters segment 2 105 millimeters segment 3 118 millimeters right now leonis you should have taken down these results on your worksheet there are number of questions for you to answer such as naming the independent and dependent variables you must also name the variables which remained fixed during this experiment we'll leave you to complete the worksheet for part b before we begin demonstrating part c of this practical investigation press the pause button until you're ready to start part c
18:44 [Music] part c investigates the relationship between acceleration and mass while the net force remains constant in this experiment we ask you to write down the method but we don't need you to explain everything in detail again the details have been included in part b so for instance you can say prepare a new length of ticker tape thread the ticker tape through the ticker timer and attach it to the trolley you don't have to explain how to measure the length of the tape as we did in part b now we won't give the game away by
19:30 showing you the method here what we have done is to give you the results of an experiment so that you can analyze these and come to your conclusions we will leave this work up to you [Music] you
