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## Physics 02, Pulley Based Problems

Where, two blocks are connected with the help of two strings, okay. Now, we need to find
tension force in both the strings, okay, free body diagram. So, make a free body diagram of
5kg block. We have told you about WTN. There is 50 Newton weight, down, now see this,
this string pulls the 5kg block upwards, isn’t it right. String force is always pulled. So I will
show you one more technique. The tension force arrow will be away from the body. The
arrow of tension force that will always be, away from the body. Suppose, if I write the
tension of this string , it will be like that, so there will be tension ‘T’, so, ‘T’ in this string.
Now, we will make FBD of 10kg. We will make FBD very systematically. First of all, W, now
‘T’, tension. Now, in this 10kg block two strings are attached. So, the tension of this string of
this block will be away from 10 kg. To go away from 10kg the arrow will go this way, right
and if it is same string then how will the tension be, same, so it should be ‘T’. So, this string is
also there, and the tension of this string will also exist, away from the body. Will it happen or
not? Done, so these are the force diagrams.
Now, step 2 is acceleration. 5kg and 10kg are at rest, if 5kg and 10kg both are at rest then it
means both of their acceleration will be zero. If acceleration is zero then it means resultant
force is zero. Means upward force is equal to downward force and right force is equal to left
force, done. So, from the first FBD you will see that T is equal to 50 Newton, done and if we
see the other one then T1 is upper force that should be equal to T plus 100. Where the
value of T, we already know. So, is it okay, so that is called 150 N.
I will ask one question. Does this string pull the 10kg block? Where is it pulling? Okay, you
got ‘T’. Who is pulling 10kg block downward? See, this, remove this one. Ground, it is MG,
obviously, ground will pull it. One is MG, then why have they tied the string, has the string
come here for roaming, if there is a string then you have to pull. So, this 10kg is being pulled
down with the help of this 5 kg string. So, this is 5kg pull and this is ground pull and this is
upward string pull, okay. Thank you, I have my own supporters, she is saying that I have told
about this, definitely I had told. This tension will be away from the body because we are
pulling; this way is generally called push. Pull means it goes away. That is why we can
understand that the arrow of tension will be away from the body. Are you all awake?
Find the tension force in each string. There are three blocks connected by three different
strings, we need to find tension in each string. Your method should be systematic. Each and
every student first should draw the free body diagram. 2 kg and then 4 kg and then 6 kg,
make different FBDs of the three blocks. Draw all the forces. We will check FBD, what are
the marks for FBD, out of 10 marks question, 9.5 marks is for FBD and 0.5 is for the answer.
Okay, now let’s make FBD, everyone did it. Everyone will check it. 2kg, 4kg. How many
students have got it correct? From here, one can say T is equal 20, this will be T1 is equal to
T plus 40, that means T1 is equal to 60. If we see from here, what will T2 be, T1 plus 60 that
means 120 Newton. How many students have done it correctly? Okay, done.
Now, listen carefully. This is all about good observation. I will explain to you nicely this full
story, if you will stop making this FBD then you are finished. I will show you a new technique,
but that is only for final situation. You have to make FBD continuously if you want to learn. I
think in a very simple way without applying any logic and equation. Tell me one thing, what
is the work of this string in this world. To hold 2kg block, right. What is the weight of 2kg
block? 20, so 101% the tension should be 20 Newton. Okay, it has to hold, so upward force is
equal to downward force. So, tension should be 101% that is 20N. So, is there any physics in
this, no, there is no physics, okay. Weight is not physics? It is a little, I am joking. Now, what
is the work of this string in this world? To hold both 4kg and 2kg that means how much total
weight it has to hold? 6kg. So now tell what will be the tension of this string? Really, okay.
And now, what is the work of this string in the world? So, what will be the tension of this
string? That is logic oriented. But this comes later, first is this, because what you will do in
school, will you keep paper on top in this way and solve it? There you have to do it in detail
and logically. As I told you that this problem can be solved with common sense, and see you
got the answer very quickly. Done, move, next.
Find the tension in string and acceleration of block. Did you understand what the meaning of
this is? What is the meaning of this thing? That we have pulled this string with 60 Newton.
Yes or no. Listen carefully. I will do this question. Then you do it the rest. This is the tough
one. And you people can do the easy questions that come later. Students, understand its
meaning, it has come many times, all of you listen carefully. This end you are pulling with 60
Newton means you are pulling the tension of this end. Means what will be the tension of
this end, 60 Newton. Means that force indicates that you already know the tension of this
string. How much is it – 60 Newton? So, on this block you are getting 60 Newton tension is
there upwards and the weight 40 is downwards, isn’t it. 60 is greater than 40 that means
where will the block move, upwards. You made FBD and assumed acceleration. Now, what
you will write, resultant force. Big force minus small force is equal to 4 into a. So, f into a is
consequence. What is the forced applied, 60 and 40. So, this 20 is equal to 4a, so a is equal
to 5. My motive behind this example was that you try to understand this information. And
this will be repeated in our topic a thousand times. So, after this you shouldn’t make any
mistakes and you have been given the tension of this string in the exam. Okay, note down,
that’s an important example.
Next, find the acceleration of the blocks and the tension force in each string? We have to
mention tension of both the strings and the acceleration. What do you have to do? Follow
the steps, make the FBD, assume acceleration, write the equation and solve it. See, I will
show you have to save time. Write the FBD here only. But there is one difficulty here, I have
drawn a very neat FBD, you will be able to understand it and if you also draw like this a neat
and clean FBD you will be able to solve, otherwise you will be confused. See, I will do 40, 60,
okay, T, T. Has everyone done it? Now, how much this will be? 120 N, okay, where should
this system work? How did you take that decision? Total force is 120, and total weight is 10.
What is the total weight, 10, means if I see the downward weight, how much will it be? 60
Newton and what is the force used 120. So, the system should go upward. If I put 80 Newton