How to draw Free Body Diagrams? - Introduction

[@madvlogssv4514]

U r great. I could learn fbd within minutes which I couldn't make out within hours

[@acousticramanand936]

Sir ..love u..anyone can understand ur explanation easily..thnk u so much 4 dis video sir..

[@amanjain1841]

Puti 11th 12 th nikal gai aj samjh aaya h bhai jab gate ke lie fbd padh rha hu..

[@sweetyghosh]

First of all, great video .nobody explain FBD such an easy manner.....Sir, while drawing FBD of A at 6:02, can I sy that B aplied force m2g on A..why you write N1, not m2g......it confuses me a lot... Could you please please clarify....

[@IITJEEChemistryCourse]

Hi Sweety, Thank you. No, you can not say B applied m2g on A. Let me explain. Q. Who applies mg? Ans. mg is applied by Earth on any object with mass. Mass of A = m1. So, the earth will apply m1g force on A. Mass of B = m2. So, the earth will apply m2g on B. A and B do not apply mg on each other. Q. Why does earth apply mg on any object? Ans. Due to its gravity, it pulls the object towards itself. So, the direction of mg is always towards the earth’s centre. For objects close to the ground, we always draw it in the downward direction as centre of the earth is below us. Q. Okay, so where is the reaction force for this mg? Ans. It is acting on the earth. So, B applies a reaction equal to m2g. But not on A. It applies this force on earth. Similarly, A applies m1g on earth. Q. Why don’t we see/draw that reaction force? Ans. Because the earth is not a part of the system ( A + B ) we’re solving for. Q. What force do A and B apply on each other then? Ans. Contact force. It is different from mg. It has two parts - Normal reaction and friction. Here friction is not shown as objects are not sliding on each other. Q. Why do I write N1? Ans. N is the normal reaction force. It only acts when: 1. Two objects are in contact. A and B are touching each other. 2. They need to push each other. 3. This force is always perpendicular to the surface of contact. (Normal = perpendicular) As B is being pulled by the earth, it tries to go down. Like when you drop a ball, it goes down. But A is in the way of B. So, they push each other. This force is the normal reaction N1. It is different from mg. I hope this clears some of your confusion. Let me know if you have more questions.

[@sweetyghosh]

@@IITJEEChemistryCourse first of all accept my PRANAM... Thank you so mich for explaining this by managing time from your busy schedule.... It's ow crystal clear to me.... Only one doubt haunting me.... Please sir, tell me..... At 5:35 , as body B sits on A, isn't is that A feels force m2g by B... Likewise isn't ground feels force due to combined mass of A and B multiplied by force due to gravity...of course this force is because of force due to gravity.......

[@IITJEEChemistryCourse]

@@sweetyghosh You are right in saying that A feels some force from B as B sits on A and is pulled down by earth. So, it pushes A. But this force is not m2g. This is the contact force. Now, was B able to move? No. Who stopped it? A did, by applying a contact force(N1) on B. The reaction of this is felt by A, and it is also N1. This is confusing because we keep writing N = mg in our equations. We write that because in this case, the net force on B is 0. This means, the forces acting on B must cancel each other. So, the value of N = mg. But this is not true always. By same logic, both A and B are pulled down by earth. So they push the ground, and the ground pushes them back. This push is the force that ground feels. And since A+B were not able to move, this push is numerically equal to the gravitational force on A+B combined. Hence N' = (m1+m2)g

[@sweetyghosh]

@@IITJEEChemistryCourse thanks a lot sir..... Now it's clear to me.....you are great...Now onwards I will follow your channel....

[@bhaskarjyotideka6905]

Sir kya 'F' force ko represent krna zaroori hai?? Ya sirf reaction(N2) force due fo F dikhane se hoga??

[@sitanshudas3000]

thankyou sir for this vido