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hi guys good morning I'm going to help you to solve the exercises from page 120

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we will start with exercise number four but as you can see in your book on page

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number 20 I'm going to make a zoom in just to be able to see it properly okay

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it says what mass would you hand with your hand have to hold to apply the

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force of one Newton and given that result would you say that the Newton is

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a large unit of force relative to the human scale okay we are going to start

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solving this so I'm going to change to a different kind of board okay so we

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start with exercise 4 page 120 and we have a force of one Newton and we are

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want to compare it with a mass so the data we have is that we have to imagine

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something with a weight of one Newton and the unknown if we want to compare

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that weight to have an idea to hold it with our hand what would we what is

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familiar for us is the mass and that's going to be the unknown okay what is the

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equation we will have to use. The equation is that the weight equals the

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mass times the acceleration of the gravity, and you know that the acceleration

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of the gravity is 9.8 meters per second squared, okay? So having all this in mind,

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we can start trying to analyze what is the mass. So the idea is if we have a

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hand okay well let's imagine that this is a hand I am not very good at drawing

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hands okay and we have a mass and this is the weight of one Newton we have to

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know what is the mass that we are holding with our hand okay this is our

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hand it's supposed to be our hand okay so to do that the unknown is the mass

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and we know the weight and we know the acceleration of the gravity so we have

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solve a solution we have to solve for mass in other words we have to isolate

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the mass so the mass as the G is going to pass this G is going to pass to the

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denominator in this side so it's going to pass dividing the mass is going to be

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the weight over the acceleration of the gravity. So, the mass is 1 newton divided

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by 9.8 meters per second squared. Just something that is important that you

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know is that 1 newton equals 1 kilogram per meter over second squared. So, if we

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cancel meter over so this is don't worry too much with this okay just to clarify

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why at the end of the day these are newtons okay meters per second square we

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cancel these with these and we have kilograms if you didn't understand this

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this part about units just skip it okay don't worry the important point is that

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you that you use international system units here so the mass that you are

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going to obtain is in kilograms that is the unit of mass for the international

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system and this is 0.102 kilograms or the result in your book this mass is 102

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grams okay but this result would be okay because in fact it's in kilograms so

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So, based on that, could you say that a Newton is a large unit or is a small unit? Well, to hold something 100 grams is not a huge force, so we could say that when Newton is, or the Newton, sorry, the Newton better, okay, I will modify this.

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the Newton is a small unit

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ok and that's the second part

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ok this is the end of exercise number 4

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now we are going to proceed to solve

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exercise number 6 so I'm going to switch

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to your book back just one second

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ok so here we are now we're going to

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solve exercise number six okay this one okay and it says identify the forces

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acting on the two objects green and red in the following image okay so we are

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going to try to do it on our special board so let's go back sorry because

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it's not really professional yet but we all are learning to do things in this

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way so I will delete last exercise and now we continue with exercise 6 page 120

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so there is we have a table okay sorry because my drawing on this kind of

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device is not really good but I hope it helps you to to solve it okay well we

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have this device we have a pulley and a string or a rope okay and we have a

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green one so the green is this green one and we have a red one here okay and we

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have to draw the different forces acting on these two devices so the first force

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that we can identify is the weight and is pointing down or is pointing towards

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the center of the earth okay then when something is on a surface the force that

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that is acting the surface on the on the object is the normal force so we have a

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normal force upwards okay and the force exerted or made by a string is called

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tension so you have here tension and you have here tension okay and finally the

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last but not the least is the friction force due to the friction of this object

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against and the surface of the of the table against the boot and this is

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friction force okay so that's all for this that's all for this exercise six

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now we're going to proceed with exercise eight I will change back to the book to

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your book okay so we don't know why I can't move it down now one second please

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okay exercise sorry now here we are exercise number eight it says if we had

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a 20 kilogram from the spring in the previous exercise so sorry we have to do

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before eight we have to do exercise number seven how much will the spring

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stretch so first we're going to do exercise 7 to find what is the the

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constant of the spring okay to understand what is the meaning of that

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and then we will do exercise number 8 based on the data of exercise 7 okay so

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I'm going to switch to the board okay this card but we have exercise number

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seven we will start with it size number seven page 120 and it says that we have

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a spring okay with a K with a constant that is 250 newtons per meter okay

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So, and it says what is the meaning of that constant, of that elastic constant. So, the value of the constant is giving us the force we have to exert on the spring to extend or to expand the spring one meter.

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so the the the larger is the constant the stronger is the spring and the

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smaller is the constant the weaker is the spring okay I hope this is clear for

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all of you and now we continue with exercise 8 okay exercise 8 says if we

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have if we hand so we have data we have the constant taken from exercise seven

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so is 250 Newton per meter and we hand a mass of 20 kilograms and from the spring

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and now is how much the spring will stretch so the unknown is how much the

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spring will stretch what we what is being seen as the X in Hooke's law and

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second how much might well we will do the second part later so we are going to

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solve this before so what equation we will use okay we are going to use Hooke's

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law which says that F the force that we exert on a spring equals the constant

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times the extension and in this case the force that we are exerting as we are

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hanging so imagine that we have the spring without no mass hanging from it

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and then the same spring but with the mass hanging from it so the spring

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standard we have a mass now of 20 kilograms 20 kilograms and this distance

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is the stretch of the spring and is what we call the X and the force that is

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pulling downwards from the string from the spring sorry it's a spring not a

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string from the spring is the weight and the weight you know that is the second

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equation we have to use here is m times g as we did in the previous exercise so

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we can calculate these 20 kilograms times 9.8 meters per second square and

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And this gives, let me find my calculator, yes, here it is, 20 times 9.8 is 196 newtons, okay? And now this weight is going to be the force in this load.

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So, we have that the weight, in this case, equals k times x.

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As x is the unknown, we are going to solve for x, so the k is going to pass to this side

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dividing, so x equals the weight over the constant.

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then we can calculate that 196 newton over a constant of 250 newton per meter

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ok and this is 0.784 meters that is the same that you have in your book in your

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book is you see that the result is 78.4 centimeters if you convert these two

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centimeters it's okay but anyway it's okay if you give the result in meters

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okay and second part in this exercise says that this is second part how much

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mass would stretch the spring five centimeters so mass to stretch an X of

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five centimeters okay so in this second case the mass is the unknown and the X

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is part of the data okay well summarizing the data we have now is the

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K is the same 250 newtons per meter the stretch is five centimeters and the

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unknown is the mass okay and the equations we have are the same as we did

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before the Hook's law and the definition of the weight okay so we are going to

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solve it our solution in this case what we know that the weight is pulling from

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the from the spring we have an x that is in centimeters so we have to convert it

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to meters so we write centimeters in the denominator to get rid of the

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centimeters and meters up as one meter equals 100 centimeters we have the

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conversion factor we get rid of the centimeters and we can write an x of 0.05

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meters okay as now we have to find the mass but to find the mass we need the

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weight so we are going to calculate the weight this is 250 Newtons per meter

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times 0.05 meters cancel this and this and we have Newtons and then 250 times

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0.05 is 12.5 Newton. Okay and now we know the weight and we know the

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acceleration of the gravity but we don't know the mass so we are going to pass

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this acceleration of the gravity to this side dividing to obtain the mass so the

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mass is the weight over the acceleration of the gravity and this is 12.5

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newtons over 9.8 meters per second square okay and if we solve that we

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obtain a value of zero point with a second I made a mistake with my

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12.5 over 9.8 sorry for that and you get 1.28 if you round up is 2755 so if you

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round up is 1.28 kiloamps which is the result that you should obtain okay so so

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So far we have corrected exercises 4, 6 and 8, and later on I will upload a second video with 2 or 3 more exercises, so that you can check what is the way of doing them, so please write down these notes in your notebook,

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will review your notebook and I will the next task you will have to do is to send

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to me these notes in your notebook with a picture the way you prefer or with a

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scanner the easier and simplest way for you all of you I think have a way to

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take a picture and upload the picture to the to the virtual classroom but I will

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a task for that the task is not added yet so don't get nervous we are slowly

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trying to learn this way to work okay so all the best for you and I hope to see

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you soon see you bye