Laws of motion/Grade-11/JEE/NEET/Physics

Added: 2026-06-01

[00:02:01]Hi students.

[00:03:09]Hello students, welcome to the student the brand. This is today we are going to discuss about So today we are going to discuss about so loss of motion.

[00:03:25]Loss of motion right so loss of motion grade 11 topic grade 11 topic so let's discuss about it here so first of all what is motion the object moves from one place to another place from one point to another point with respect to time. The body change its position with respect to time from one point to another point. From one position to another position.

[00:04:08]So it is called it is called what is that motion.

[00:04:17]And coming to inertia what is inertia?

[00:04:23]Unless external force acting on it.

[00:04:27]Unless external force acting on it, the body does not move by its own. The body does not move by its own is called inertia.

[00:04:43]So there are three types.

[00:04:46]Inertia of rest, inertia of motion, inertia of direction. So we'll discuss about loss of motion loss of motion understood or not.

[00:05:09]So here what is inertia of rest?

[00:05:13]Inertia of rest. So first one we will discuss about inertia of rest.

[00:05:22]Inertia of rest.

[00:05:25]Second one, inertia of motion.

[00:05:30]Inertia of motion.

[00:05:33]And coming to third one, inertia of direction.

[00:05:40]Inertia of direction.

[00:05:44]Right?

[00:05:46]inertia of rest, inertia of motion, inertia of direction.

[00:05:52]So if you observe here, inertia of rest. Example, if you shake the branches of the tree, the fruits will fall down. Point number one, if you beat the duster with stick, if you beat the duster with stick, the duster comes under motion. Dust particles will fall down.

[00:06:23]Point number two, if you beat the carpet with stick, the carpet comes under motion. Dust particles will fall down.

[00:06:44]And if you hit the carboard coins, if you arrange all coins in vertical position, if you hit the bottom coin, only the bottom coin will move forward.

[00:06:57]Remain coin stays in the same stitch.

[00:07:02]That is inertia of rest.

[00:07:06]And one more example.

[00:07:12]One more example.

[00:07:18]When you when a person is taking long gem and a person is taking long gem right he has to move little bit long distance and move forward in order to gain the motion to continue the motion in air.

[00:07:36]close by that the person can move little longer distance.

[00:07:45]Okay. So little longer jump he will take little longer jump.

[00:07:51]So that is inertia motion. If you on the switch of the fan after uh moving again if you off the switch.

[00:08:04]Okay. Still the fan continues in the state of motion.

[00:08:10]Right. The body continues in the motion.

[00:08:15]The body continuous the motion. The body is moving continuously. Even though you asked the switch, you switched off the fan because of inertia of rest.

[00:08:31]And one more examples I'll tell you take a stone and tie to the thread and wheel along the circular motion and leave the stone will move along tangential direction.

[00:08:45]The stone will move along the tangential direction. And one more example we can say during winter season the person cycling the bicycle back of the back back wheel. If you observe back wheel of the bicycle if you observe the water drop line moves along the tangential direction along the tangential direction. I hope it is very clear. This is about inertia of rest.

[00:09:19]Right. So, and one more example I'll tell because these examples are asking frequently these questions are okay frequently asked in the exams.

[00:09:33]Okay.

[00:09:34]So that one is when you are run when you are standing in the running bus suddenly the bus driver applies brakes we fall forward because our legs are contact with the uh bus ground right so we our upper body is in motion the upper body is in motion suddenly the legs are contact with the legs are contact with the bus surface and trying to comes to rest because see the driver applied the brakes.

[00:10:13]Okay. So when driver applied the brakes our legs are contact with the ground surface of bus trying to trying to come to rest.

[00:10:26]Okay. So but our upper body remains in motion. So it continues that motion.

[00:10:32]That is the reason we will fall forward.

[00:10:37]And one more example I'll give. So when you are standing in the when you're standing in the bus, the bus is at rest.

[00:10:48]Suddenly the bus driver start the bus the bus start moving forward we will fall backward because our legs are contacted with the ground surface of bus are trying to move along the bus are moving along the bus our upper body stays in the same state that's why we are moving backward direction we are falling backward direction so I Hope it is very clear right.

[00:11:20]So this is about inertia motion.

[00:11:26]And one more thing take a glass of water place a paper on it. Place the coin and drag the paper. The coin will fall down due to inertia of rest.

[00:11:38]So I hope it is very clear. Coming to the contact force and non-cont forces.

[00:11:44]contact forces and non-cont forces.

[00:11:48]Contact forces and non-cont forces.

[00:11:57]Non-cont forces.

[00:12:00]So contact forces example.

[00:12:04]So when the bodies are at when the bodies are in contact position the bodies are in contact then the force will act then the force will act. Yes or no when the bodies are at rest or when the bodies are in contact then the force will act.

[00:12:29]So that force is called contact force.

[00:12:34]So example contact example for contact force. Can we give an example for contact force? Sir can you describe can you briefly give idea about contact force? Sir can you tell few examples about contact force?

[00:12:56]Sir, what are the examples for contact force?

[00:12:59]Yes. So examples for contact force, frictional force, tensional force like uh muscular force. Yes or no? Friction force sir. How it is a friction force?

[00:13:16]How it is? How it is a how it comes under contact force? So I'm moving I am moving on the ground. Okay. So, so some force is opposing us. Okay.

[00:13:34]Without moving forward, some force is opposing me to not to moving forward or without moving forward. The force is opposing me. Okay. Force is stopping me.

[00:13:48]Sir, you are moving. No, sir. How can you say the friction is stopping you?

[00:13:53]Okay. So you're blaming the friction how it will stop. Still you are moving right. Still you are moving forward right? How can you say sir friction is stopping you? How can you blame the friction sir?

[00:14:08]Oh that is a point right? Okay I will I'll give an example.

[00:14:14]So if you throw the ball on the ground observe carefully. If you throw the ball on the ground so until certain distance it will move. After remaining distance it's trying automatically it will stop.

[00:14:28]Who is responsible for that?

[00:14:31]Oh sir some force is responsible for that sir.

[00:14:37]Some force is definitely some force is responsible for that. Okay. What is that force? Tell me.

[00:14:44]Um sir we will discuss sir. we will discuss.

[00:14:49]Okay.

[00:14:51]So what is that force sir? I hope you said that that is friction only sir. Yes that is a friction because when you are finally you understood. So listen here. So when you're throwing the ball on the ground so the contact force between the surface of the ball and the ground.

[00:15:15]Okay. So between these two surfaces the contact force will lack that is friction surface. So what is what is the cause of friction? What causes the friction?

[00:15:26]So due to irregularities because of irregularities between the two surfaces there is no perfect smooth surface in the nature. There is always some irregularities.

[00:15:41]Some irregularities will be there is no perfect smooth surface.

[00:15:46]Okay.

[00:15:48]Press some irregularities will be there between one surface to another surface.

[00:15:53]So the interlocking of irregularities causes friction. So I'm taking the body is moving like this. So irregularities.

[00:16:02]So because of this interlocking of irregularities the friction will arise.

[00:16:11]The friction is developed.

[00:16:14]So it opposes the relative motion between the two surfaces.

[00:16:18]So friction will opposes the relative motion between the two surfaces.

[00:16:25]Okay. So it comes under contact force.

[00:16:28]Okay. Done. Yes sir. Sir you said muscular force sir. Can you tell me about it?

[00:16:37]Okay. So muscular force you want. Yes.

[00:16:39]So daily we are carrying the bag from one point to another point. We are lifting the weight from one point on. So we are lifting and moving the object from one point to another point. So we are dragging the bench, arranging the bench, dragging the chair, pulling the chair, opening the door, closing the door. We all do these things, right? By using our modules.

[00:17:05]Okay? So by using our hand we even we are writing on the book we are writing on the book I'm writing on the board by using our hand by using our modules so we are doing all these works muscular force coming to the tension force when the bodies are attached to the ceiling and hang weight will be acting downward direction tension will be acting upward direction so like weights understood or not. So these are the examples for tension force. So tension in the string.

[00:17:44]Okay. So if you suspend the object, we'll be acting downward direction.

[00:17:48]Tension will be upward direction. So I hope it is very clear, right? So that is about contact force. So coming to non-cont force. What are the example for non-cont force?

[00:18:01]So example for non-cont force. Here observe I'm dropping the object.

[00:18:07]I'm dropping the object. Observe or not?

[00:18:10]I'm dropping the object. So without any external force the object is trying to move downward direction.

[00:18:17]Oh the object is trying to move downward direction. Yes or no? So it is moving towards earth center.

[00:18:27]Towards earth center. Understood or not?

[00:18:31]Yes. So if you throw any object upward direction again the body will move towards it at the center.

[00:18:42]So without contact the force will act on the object.

[00:18:46]So that is called gravitational force.

[00:18:48]Due to gravity the bodies are moving towards it center.

[00:18:54]And coming to the one more example magnetic force at certain distance only the object the force will act.

[00:19:05]Okay. If you take north pole north pole ripple north pole south pole attract south pole south pole north pole attract understood or not? So I hope it is very clear.

[00:19:15]Two similar poles will attract two dissimilar poles.

[00:19:21]Two similar poles ripple. So two similar poles north pole north pole ripple.

[00:19:29]North pole south pole attract. South pole north pole attract. South pole south pole ripple. Reple. Reple.

[00:19:37]Attract. Attract.

[00:19:41]Understood or not? Two dissimilar poles will attract. Two similar poles will repel.

[00:19:49]That force is called magnetic force.

[00:19:56]So coming to the electrostatic force.

[00:20:01]Take a comb and comb it. Take near to the piece of papers.

[00:20:08]The papers will getting attracted towards the comb because of electrostatic force. Take a balloon. Drop it and take near to the uncharged balloon.

[00:20:23]The charged bellon will attract the uncharged bellon due to electrostatic force.

[00:20:34]These are all examples for contact force and non-cont force.

[00:20:43]So now I explained about contact force and non-cont force.

[00:20:51]So now we will discuss about Okay. Now we will discuss about momentum.

[00:21:11]Momentum momentum. Momentum P bar equal to MV bar mass into velocity. Mass into velocity.

[00:21:28]So if you are taking the two bodies here, one body is here M1 U1 M2 V2 after collision after collision after collision after collision M1 V1 M2 V2.

[00:21:51]So momentum before collision equal to momentum after collision momentum before collision equal to momentum after collision that is considered as law conversation of linear momentum.

[00:22:12]So one example I'll tell law of conversation of energy. What is law of conversation of energy?

[00:22:21]Energy neither be created nor be destroyed but it changes from one form to another form.

[00:22:28]Energy can be neither energy energy neither be created nor be destroyed but it changes from one form to another form. That is called love converation of energy.

[00:22:49]What about currentation of linear momentum?

[00:22:54]Momentum before collision equal to momentum after collision.

[00:23:00]M1 U1 M2 U2 M1 U1 + M_sub_2 U2 = M1 V1 + M_sub_2 V2.

[00:23:14]This is called L convergation of linear momentum. So based on this one recoil velocity of the gun recoil velocity of the gun right. So before firing gun and bullet both are at rest. After firing bullet will move forward gun will move backward direction. That is the recoil velocity of the gun.

[00:23:42]Okay. So we have to find out that one that's enough. Love conversation of kinetic energy. Energy neither be created nor be destroyed. Kinetic energy. Love conversation of kinetic energy.

[00:23:55]So here kinetic energy before collagen equal to kinetic energy.

[00:24:02]After collagen, kinetic energy before collagen equal to kinetic energy.

[00:24:11]After collagen, kinetic energy before collagen equal to kinetic energy after collagen.

[00:24:24]Okay. So, next one.

[00:24:43]Okay. Done.

[00:24:49]Now impulse.

[00:24:52]Impulse large force acting in a small interval of time.

[00:24:56]I = F into T.

[00:25:01]So moment impulse equal to final momentum minus initial momentum final change in momentum simply we can say simply we can say change in momentum understood or not simply we can say change in momentum.

[00:25:21]So next one so I'll discuss about Newton's first law second law third law first law first law second law third So what is Newton's first law?

[00:25:56]So it first law about inertia.

[00:25:59]First law is about inertia.

[00:26:03]Okay. So mass is a measure of inertia. So inertia is nothing but simple definition in easy terms I'll tell the body does not change its position by its own. That's enough.

[00:26:21]Its position by its own can't be changed.

[00:26:26]Wow.

[00:26:29]Wow.

[00:26:30]What a miracle dialect. Its position by its own cannot be changed.

[00:26:38]W.

[00:26:42]Okay. So now so I already explained about inertia of rest inertia of motion inertia of direction that's okay. Now Newton's second law the net external force acting on the body is equal to mass into acceleration.

[00:27:03]The net force acting on the body is equal to mass into acceleration.

[00:27:09]Okay. Or simply we can say the net force acting on the body is equal to the rate of change in momentum.

[00:27:16]Here you can see. So coming to the second law.

[00:27:22]Second law.

[00:27:24]So F = F is directly proportional to MV minus M U by T. F = K into M into V - U by T. So v - u by t = a v - u by t = a understood or not? Yes. So k = 1 f = m into a k = 1 f = m understood or not? So that is Newton's second law.

[00:28:09]Newton's second law. I hope it is very clear.

[00:28:12]So the rate force acting on the body is is directly proportional to rate of change in momentum. F = K into M into V minus U by T. So V minus U by T is A K = 1 F= M A. Understood or not? So I hope it is very clear. So coming to the next one. So Newton said for every action there is a equal and opposite reaction for every action for every action there is equal and opposite reaction right. Yes. So now lame is theorem.

[00:28:56]Lamb is lamb is lamb is the lamb is lamb is the f_sub_1 f_sub_2 f_sub_3 f_sub_4 f_sub_2 f_sub_1 is alpha f_sub_2 is beta f_sub_3 is gamma F_sub_1 opposite side is alpha. F_sub_2 opposite side is beta. F_sub_3 opposite side is gamma. F_sub_1 by sin alpha.

[00:29:37]F_sub_2 by sin beta and f_sub_3 by sin gamma.

[00:29:49]Okay. f_sub_1 by sin alpha f_sub_2 by sin beta f_sub_3 by sin gamma this is about lame theorem is it clear or not so coming to the next one will wait when the body is in lift when we are in the lift condition. So here you can see when we are in the lift so here I'm taking the normal reaction weight is acting downward direction velocity zero or constant lift is moving constant velocity or so velocity of the lift is zero means it's not moving either it is not moving or moving with the constant velocity T then r = mg r = mg question number no point number one coming to the point number two so the lift is moving downward direction so okay so down or up which one okay so take it up so body lift is moving upward direction with acceleration A lift is moving upward direction with aation A. So R1 is greater than the weight of the object because lift is moving along the R1 direction. So here R1US MG = M A. So R1 = M A + MG.

[00:31:44]Then I'll get R1 = M. I'm taking common a + g.

[00:31:52]Okay.

[00:31:54]So coming to the next one.

[00:31:57]Third one.

[00:31:59]So here you can see lift is moving downward direction.

[00:32:04]Reaction force R2.

[00:32:08]Weight is acting downward direction.

[00:32:11]Aulation will be. So R2 wait lift is moving downward direction understood or not. So in that case mg - r = m a mg - m a = r2.

[00:32:35]So g - a into m = r2.

[00:32:42]Understood or not? I hope it is very clear. Just check once.

[00:32:47]1 2 3 1 2 3 Right.

[00:32:53]Yes. So next one.

[00:33:31]Now we'll we'll discuss about friction.

[00:33:35]What is friction?

[00:33:38]Friction is nothing but friction is also a force. Simply we can say friction is a force which opposes the relative motion between the two bodies. When the body is at rest position friction will be more interlocking will be more. So due to interlocking of irregularities friction will be generated. Okay. So causes of the friction is irregularities between two surfaces when the bl.

[00:34:13]Okay. So because when they placed in contact only interlocking will be there.

[00:34:18]So friction will be generated.

[00:34:20]Understood or not?

[00:34:28]Now, so friction.

[00:34:39]So here one is static friction.

[00:34:44]Static friction.

[00:34:48]Second one is limiting friction and third one kinetic friction.

[00:35:07]under kinetic friction under kinetic friction.

[00:35:21]So we'll discuss about sliding friction sliding friction.

[00:35:33]Second one is rolling friction.

[00:35:40]Rolling friction.

[00:35:42]Understood or not?

[00:35:45]Static friction.

[00:35:47]So when the body is at rest, when the body is at rest, both are interlocking the surfaces. Then at the time friction will be there. That friction is called static friction.

[00:36:03]Limiting friction. So you're applying force on the table. The table is not coming. Again you are applying. Table is not coming. Again you are applying table is not coming. So you're thinking it is not like that. We have to we have to use large force. We have to use maximum force in order to lift the object displace the object move the object from one point another point. So that's why you decided and applying more force.

[00:36:30]Then that time the body start moving.

[00:36:35]When the body is ready to move, then when the body is ready to displace, when the body is at edge, where it is ready to move, start, ready to slide, okay? Anything that time friction is called limiting friction. That time friction is called limiting friction.

[00:37:03]Understood or not? Yes. Coming to the kinetic friction once the body start moving. Once the body start moving here two type sliding and rolling. So you can see the window when you push the window automatically slides over another one.

[00:37:22]Okay. So it slides on another surface.

[00:37:26]So here the interlocking of surfaces will be less because there is no time for contact between the two surfaces because it's continuously sliding when it is sliding.

[00:37:40]Okay. So friction will be there but it will be less compared to static friction. So coming to the rolling friction when the body start moving on the road a best example like a luggage bag wheels are there right when you when you're dragging automatically friction will be less there is no time for contact between the two surfaces so I hope it is very clear understood or not yes so that's enough now I'll draw the graph for friction So let's discuss about it graph.

[00:38:24]I'm drawing the graph.

[00:38:27]So here you can see applied force friction friction applied force applied force.

[00:38:48]So friction is moving.

[00:38:51]Okay. Friction is along yaxis. Applied force along x-axis here.

[00:39:00]So this the ready to start then the body start moving.

[00:39:08]Okay. So here I'm talking about maximum value of static friction and your chinetic friction fs = to mu s into r fk = mu k into r.

[00:39:26]So here maximum value of static friction maximum value of maximum value of static friction.

[00:39:39]So that is called limiting friction.

[00:39:47]Limiting friction.

[00:39:49]And here you can see the bodies start moving. So at this point kinetic friction will be there. Okay.

[00:39:59]Yeah. Everybody also will do it.

[00:40:02]Kinetic friction.

[00:40:09]Kinetic friction. So here is static friction.

[00:40:17]Understood or not?

[00:40:20]So that's enough.

[00:40:22]So now we'll do it.

[00:40:25]Now we'll do free body diagram.

[00:40:30]Okay. So free body diagram. What are the forces are acting on the body? We have to represent understood or not. So that is free body diagram.

[00:40:47]So I'm taking here this the object So this object so this object okay so weight is acting downward direction normal reaction will be acting upward direction. So the body is at rest.

[00:41:21]When the body is at rest, weight is acting downward direction normal reaction. So I'm applying force F. So if it is friction surface, friction will be acting opposite direction. So m into R.

[00:41:37]So aulation will be in this direction.

[00:41:41]Okay. Along horizontal direction motion will be there.

[00:41:47]F - M = M A this is the mass of the object R= mg okay so whatever the forces are acting on it you have to represent understood or not now so I'll take two bodies two bodies of the system So here 1 2.

[00:42:28]So here so m1 m_sub_2 aulation is in this direction.

[00:42:42]force is applying on it. Force is applied.

[00:42:49]So movement of the body along this direction.

[00:42:53]Now if you write F = MA F = M A that is F = M1 + M_sub_2 into A. So, a = m1 + m_sub_2 equation 1 a = f_sub_y.

[00:43:21]So, here you see force is acting on it.

[00:43:23]F_sub_1 m1 m_sub_2 is like this. So, f= ma m f = m1 + m2 a = m1 + m2 equation one.

[00:43:37]So every time when you are writing equation you must follow the Newton's second law.

[00:43:45]Okay. So I hope it is very clear.

[00:43:49]So now contact force f_sub_1 fdash fdash how to find out contact force on m1 m1 a that is equal to m1 f by m1 + m_sub_2 equation two. So m_sub_2 m_sub_2 a that is m_sub_2 into m1 + m_sub_2 in equation three.

[00:44:21]Is it clear or not? Just check once.

[00:44:45]Okay. So, thank you so much for joining and attending and listening. If you like the content, if you like the channel, do like, share and subscribe to student the brand. This is signing off. Thank you.