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Ex 13 to 18 p 120

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Subido el 24 de marzo de 2020 por Segismundo P.

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Hi guys, how are you today? We are going to continue solving exercises from unit 5. 00:00:01
We are currently on page 120 and I would like to continue with exercise number 13 00:00:08
and the wording says that you leave home and take three minutes to reach a store 00:00:16
15 meters away. If you spend 12 minutes shopping and return the same way in eight minutes, 00:00:22
what was the mean speed of your return and mean of the total journey 00:00:29
okay so we are going to the notebook to start solving this exercise 00:00:34
so let me change it in the tool okay here we are here we are so it's 00:00:41
exercise reset 13 page 120. 00:00:48
So we write the important data we have in this statement, and the first thing is that 00:00:55
in the first motion you take three minutes to cover a space of 500 meters. 00:01:06
That's when you are going in one direction. 00:01:22
Now you stay still without moving for twelve minutes, so in that period of time the distance 00:01:25
that you cover is zero, because you are stopped, and then you return home in eight minutes, 00:01:38
the time for the third part is eight minutes and you cover back the 500 00:01:48
meters right so the first thing we need to know is what is the speed of the 00:01:56
return this is the return okay and to do that so we are going to write the 00:02:05
unknown we have two unknowns in this one is the velocity of the return that is 00:02:13
what what we call v3 and the mean or average speed for all the journey okay 00:02:21
what equation are we going to use okay we will use the equation that defines 00:02:31
the mean speed. Okay, as you know the mean speed is the distance traveled over the 00:02:38
time taken. Okay, the time taken you can write just the t or delta t if it's a 00:02:47
period of time, doesn't matter really. So the solution for the space when you are 00:02:52
returning, so v3 is going to be the distance covered when returning over the 00:03:01
time taken so it's 500 meters over eight minutes but careful before continuing 00:03:07
with these eight minutes you have to convert that into seconds okay to use 00:03:16
international system units okay so we'll do that now that conversion so we write 00:03:22
we divide between minutes and multiply by seconds and we know that one minute 00:03:30
equals 60 60 sorry 60 seconds okay so it's 480 seconds so if you 00:03:35
substitute here you will get a value of 500 over 480 and this is 1.04 meters per 00:03:50
second I'm sorry for that I will modify that okay so 1.04 if we take two decimal places meters per 00:04:03
second so that's the answer to the first question and the second is the mean speed for for the 00:04:15
journey okay so we'll write the mean speed is the total distance covered over 00:04:25
the total time taken and what about the total distance traveled so if going is 00:04:32
500 meters and returning is other 500 meters in total the distance covered is 00:04:40
1000 meters and whatever the time the time is going to be the t1 plus t2 plus 00:04:48
t3 so the three times in total so this is three minutes plus 12 plus 8 okay so 00:04:57
you sum all this up you get the value of 23 minutes and once more we have to 00:05:09
convert this into seconds so we use a conversion factor so minutes seconds 160 00:05:18
we get rid of the minutes and we obtain a value of 1380 seconds and we write 00:05:27
this in the denominator okay so hope you are not getting lost this value is going 00:05:43
to be here and we finally reach to a result of 0.72 meters per second and 00:05:52
this is the total average speed okay so this is the end of exercise number 13 00:06:07
we will proceed now with exercise number 14 next so let me check it okay 00:06:14
exercise number 14 this one you say it says that an airplane takes 2 hours and 00:06:26
18 minutes to complete the journey if its mean speed is 720 kilometers per 00:06:32
how how far did it travel ok so we go to the notebook ok we delete the previous 00:06:38
exercise and we start with exercise we set 14 on page 120 so we have an 00:06:55
airplane the data is that it takes two hours and 18 minutes to cover a distance 00:07:10
and the mean speed of traveling is 720 kilometers per hour okay we have 00:07:22
different choices for this the one choice would be to use only international 00:07:32
system units but this is going to complicate much more the result but you 00:07:43
know it is ok but then you have to to convert everything to kilometers or any 00:07:48
other unit or other thing that we can do is that is to convert this into hours 00:07:55
and then as we have the speed in kilometers per hour we can operate 00:08:03
operate them directly so to do that you know that this is two hours plus 18 00:08:07
minutes agreed so you have two hours and 18 minutes and now we will convert this 00:08:13
18 minutes into hours so this is two hours plus 18 minutes as we want to get 00:08:20
rid of the minutes we will write the minutes in the denominator and the hours 00:08:27
in the numerator as you know one hour is 60 minutes so we write a 1 next to the 00:08:32
hour and a 60 next to the minutes now we can get rid of the minutes and the 00:08:40
number of hours is 18 over 60 which is 0.3 so this is 2 hours plus 0.3 hours so 00:08:46
So, in other words, this is 2.3 hours, right? Did you follow what I did? If you have any doubt, please, you can go to the doubts section and ask or write your question there so that any other student can see your question and benefit from the answer of that question. 00:08:58
Okay, so the unknown is the distance covered and the equation we're going to use is the one that describes the average speed is the distance traveled over the time taken. 00:09:20
As we want to calculate the distance traveled, this time is going to pass to this side multiplying, so you will have the distance traveled equals the velocity or the speed times the time taken. 00:09:35
the velocity is 720 kilometers per hour and as we have the time expressed in 00:09:52
hours now we can cancel hours with hours and we will get the result in kilometers 00:09:59
so 720 times 2.3 and this is 1656 kilometers and that's all okay if you 00:10:05
decide to do it by converting everything into the international system units you 00:10:20
should convert this into seconds and this into meters per second proceed and 00:10:25
then you will get this result in meters which is also right okay next exercise 00:10:30
is exercise number 15 okay we have a graph okay and it says according to the 00:10:38
graph which car moves faster the one represented by the purple line or by the orange line okay so 00:10:52
we are going to solve it in our notebook i'm going to delete this to start from scratch 00:11:00
and we write as usual exercise 15 page 120 okay so we will start by 00:11:14
By drawing the graph, because the graph is the data we have, we don't need to draw it 00:11:23
very accurately because we are not being asked to answer with precise numbers, just to give 00:11:30
a general idea. 00:11:38
So we have a graph where we have a time in hours and the distance covered, or the space, 00:11:39
in kilometers. 00:11:51
have a purple well this is the closer to 00:11:52
purple that I have in these two so a 00:11:56
purple vehicle moving that far and an 00:11:59
orange vehicle moving that far and the 00:12:06
first question is well the first and 00:12:10
only question is which car is moving 00:12:14
faster so we have distance versus time so if you look at at any moment imagine 00:12:18
we check this both at the beginning of the motion were in the origin and after 00:12:29
a certain period of time the orange car has moved this far while the purple car 00:12:35
has moved this far so the purple has covered this distance while the orange 00:12:47
has covered this distance so the purple car is moving faster okay that's one way 00:12:55
of looking at this the other way is as they are moving at a constant speed the 00:13:03
graph of space or distance cover versus time is a straight line and when the 00:13:09
slope okay the slope of of this straight line is larger in the purple one that 00:13:16
means that the purple car is moving faster so the purple moves faster right 00:13:26
now we proceed to exercise number 17 okay so we are going to read the 00:13:40
statement before continuing okay exercise 17 says what is the change in 00:13:48
speed if the acceleration is minus 2.5 meters per second squared for 6 seconds 00:13:57
okay by the way when you have a negative acceleration that means that you're 00:14:03
braking or the speed of the of the mobile is being slower and slower 00:14:08
ok so we have exercise 17 page 120 so we write the data we have is firstly we 00:14:21
We have an acceleration of minus 2.5 meters per second squared, and the time taken for 00:14:35
the braking in this case is 6 seconds, and the unknown that we have to discover or to 00:14:46
calculate is delta V, right? 00:14:55
We know the definition of acceleration. 00:15:01
So that's the equation we are going to use, is the definition of the acceleration. 00:15:04
The acceleration is the change in speed over, or divided by, the time taken in the braking 00:15:10
or acceleration. 00:15:19
So as what is the unknown here is delta v, we need to isolate delta v, or solve for delta 00:15:21
Delta t will pass to the other side of the equality multiplying, and that's how we are 00:15:31
going to solve this. 00:15:39
Okay, regarding the units, we have all the units in the international system, so we won't 00:15:40
have any problem. 00:15:47
And we write that delta v equals the acceleration times delta t as we passed this to this side 00:15:49
multiplying. 00:15:56
Delta V is going to be the acceleration, minus 2.5 meters per second squared, times 6 seconds, 00:15:58
so we cancel the 6 with the square, and then we have 2.5 times 6, this is 15, so minus 00:16:08
15 meters per second. 00:16:16
So in those 6 seconds, the speed decreased 15 meters per second. 00:16:18
So the final speed is 15 meters per second slower than the initial speed. 00:16:28
So this is the exercise 17, and now we are going to cover number 18, that is the last, 00:16:36
but not the least. 00:16:44
we go to the book and then in this exercise 18 you have a graph where we 00:16:47
represent the speed versus the time and we have to describe the motion 00:16:58
represented in the following graph of speed as a function of time 00:17:04
okay by the way this part about simple machines will not be covered this year 00:17:09
because you have covered that part in technology 00:17:14
ok so with this this unit would be finished 00:17:18
ok well so we are back in our notebook and we're going to collect this last 00:17:23
exercise for today. So, we write again, this is exercise 18, page 121, and we 00:17:33
have a graph, so we have a graph, you know that when we have a graph, usually the 00:17:48
graph is the data, and we have a velocity in meters per second represented versus 00:17:54
time in seconds, and we have three different segments. For a segment that is 00:18:07
the velocity is increasing, a second segment where the velocity is steady is 00:18:15
not changing is constant and a third segment where the velocity is 00:18:22
decreasing at a constant rate so with a constant acceleration and well we don't 00:18:29
really need the figures or the numbers for this motion because we just want to 00:18:38
have a rough description of the motion. So in the first part you know that the 00:18:46
velocity is increasing at a constant rate, so the acceleration is constant and 00:18:53
positive because the speed is increasing. In the second segment you can see that 00:19:13
the velocity is constant, it's not changing, it's constant, so the acceleration is zero, 00:19:21
we don't have any acceleration, and eventually the mobile is going to reduce the speed, so 00:19:31
So in the third segment, the velocity is decreasing, I'm going to lower this, is decreasing, so 00:19:41
the acceleration is constant, because it's decreasing at a constant rate, acceleration 00:19:53
constant and is negative sorry for that writing constant and negative okay so 00:19:59
that's all for today don't forget to correct your exercises in your notebook 00:20:17
and upload to the virtual classroom picture of your work so that I can 00:20:23
correct it and check that you are working properly okay so keep on working 00:20:30
hope to see you soon and take care see you bye 00:20:35
Autor/es:
Segismundo Peláez
Subido por:
Segismundo P.
Licencia:
Reconocimiento - No comercial - Compartir igual
Visualizaciones:
53
Fecha:
24 de marzo de 2020 - 15:47
Visibilidad:
Público
Duración:
20′ 43″
Relación de aspecto:
1.78:1
Resolución:
1364x768 píxeles
Tamaño:
33.14 MBytes

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