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

Added: 2026-05-16

[00:02:04]Hello students.

[00:02:43]Hello students, welcome to the student the brand. This is today we are going to discuss about loss of motion.

[00:02:52]Okay, so loss of motion.

[00:02:57]So what do you mean by loss of motion?

[00:03:02]So here when the objects are moving so based on few laws okay so the motion will obey the motion will be there okay so few laws will be applicable so what are those we'll discuss in this class suppose fan is rotating.

[00:03:37]I'll ask one question.

[00:03:40]Uh when you on switch the fan will start rotating even though you off it still it continues.

[00:03:52]Right? That is one question. An athlete want to take like a long game person, he will move from long distance and even bowler also observe fast bowler. So he will move certain distance.

[00:04:14]And one more thing uh the the person who is catching the ball in cricket you observe so the body is moving so he will catch the ball and uh move the hands little back.

[00:04:32]Right?

[00:04:35]So and one more example you can observe if you beat a carpet the dust particles will fall the carpet will comes under motion and you can observe the fruits when you shake the branches of the tree the fruits will fall down even though branches are moving.

[00:05:00]Okay. And one more example you can see the carambo example. If you hit the bottom of the bottom coin only the bottom coin will move forward. Remain coins will fall in the same place.

[00:05:19]And uh so generally we do so many uh we do this experiment right that is glass of water place the paper place the coin or the paper drag the paper coin will fall that experiment.

[00:05:35]So here you can observe all these examples only one thing is common that is inertia.

[00:05:46]Sir what is inertia?

[00:05:48]So early morning if you want to wake up so we are unable to wake up immediately we have to wake up there is no other choice okay so we need to do our daily things right we will get up from the bed but uh initially uh we need we need we will take some time to get up from the bat yes or no So until some external force acting either alarm either parents.

[00:06:27]Okay.

[00:06:29]So like this we can say an external force.

[00:06:36]Okay. Now here so unless sectional force acting on the object the object does not move by its own from that state.

[00:06:49]So if I place the book on the table if I place book on the table after some time if I observe the book will be in the same state.

[00:07:02]So if you understand so after even though switched off still the fan continues continuously in motion and one more example best example.

[00:07:20]So generally people will so we have should not do this one but generally when we are coming down we are getting down from the running bus even it is moving little slow also immediately you can't stop yourself we have to go in a particular direction in the same direction of the bus until certain distance if you walk then you have to be can able to stop yourself.

[00:07:52]Okay. And one more thing, when you are standing in the running, when you are standing in the running bus, suddenly the driver applies brakes, what happens?

[00:08:04]We feel falling forward. We will fall forward because our upper body is in motion. Our lower body like legs are contact with the ground. Right?

[00:08:20]So legs are trying to stop our upper body is in motion. So we'll fall forward.

[00:08:26]And one more example. So the bus bus is at rest. So the driver starts suddenly.

[00:08:33]So legs are trying to move forward. Our upper body is at rest. Then we fall forward. Sorry backward. We fall backward. Understood or not? So in this all examples we can think there's a common point that is inertia.

[00:08:56]So unless external force acting on the body the body does not move from its initial state.

[00:09:05]Okay.

[00:09:07]So it opposes the change in motion.

[00:09:13]It does not move by its own. That is called inertia. There are three types are there. Inertia of rest, inertia of motion, inertia of direction. Sir, what do you mean by inertia of rest?

[00:09:29]So until external force acting the body, the body does not move from its place.

[00:09:35]So one example you can take a carpet beat the beat the carpet with a stick then the dust particles will fall forward dust particles will fall down and one more example you can see the fruits of the tree if you shake the branches of the tree the fruits will fall down due to inertia of rest. So if you hit the bottom coin of the carum so that coin will move forward remaining coins all will stay in the same state due to the energy of rest right.

[00:10:18]So the bodies are trying to stay in the same state. It opposes the change in motion.

[00:10:27]Understood or not? So these all example for energy of rest. Coming to inertia of motion unless external force acting the body the body does not move from its state of motion. So example you can say fan even though you switched off the fan still continues its motion.

[00:10:56]Okay.

[00:11:00]And one more example inertia of direction as I said getting from running bus. So we have to move forward in the same direction.

[00:11:12]Okay. So and one more thing when you are in bus suddenly you are taking the Uturn we will fall like this along tangential direction outside out of the set.

[00:11:34]Centripal force. Centrifugal force.

[00:11:37]Centripal force means out of the center.

[00:11:41]Centry center seeking.

[00:11:45]Centripal towards centrifugal out.

[00:11:50]Center seeking means towards the center.

[00:11:53]Centrifugal means away from the center.

[00:11:57]centrip petal force towards centrifugal force away.

[00:12:03]Okay. So even you tie a stone at the end of the thread and will like this. So tie a stone and will like this. Okay. If you leave it moves along the tangential direction along the tangential direction. So I hope it is very clear.

[00:12:25]Understood or not? So these are all example for inertia of rest, inertia of motion, inertia of direction, right? Inertia depends on mass.

[00:12:38]Mass is measure of inertia.

[00:12:41]And coming to the contact force and non-cont force. Contact force example muscular force, tensional force, frictional force.

[00:12:52]So by contacting only the force will act.

[00:12:56]Okay. So those are called contact forces. So friction when you are walking forward so friction will oppose our motion.

[00:13:07]Okay. That's why when you throw the ball after certain time automatically it will stop due to friction. Because of friction we can able to walk. So if you try to walk on the icy floor so like uh on the ice so we can't able to walk even the oil poured on the ground if you put leg on it definitely will fall down because there is no friction.

[00:13:38]Friction will help us at the same time it reduces the efficiency.

[00:13:46]So in general if you are taking wear and tear takes place. So initially if you say observe the shoes uh there are spikes.

[00:13:56]So after one year to if you observe those spikes will reduce the size due to friction.

[00:14:06]Okay. So it will support. So if you if you unable if I am able to write on the board due to frictional because of frictional I can able to write on the board.

[00:14:19]Okay. So these are example for contact forces muscular force by using our majel we can able to lift the object. So example gym we are going to gym and lifting the heavier objects by our maj. We can able to lift the object. Daily we are doing works with the help of hands only right.

[00:14:38]So with the help of our modules.

[00:14:42]So and coming to the next one coming to one more that is non-cont forces. Example aelation due to gravity. If you drop the object so always earth attract the object towards it center.

[00:15:04]So if you throw many times still the body comes down because of aation due to gravity without contact at attract the objects and magnetic force certain distance only the force will force will act on each other like you take two magnets magnet one magnet two. So here you can observe at certain distance only this magnet will attract another magnet this will this magnet will attract the another magnet because uh at certain distance only at certain distance only the force will act. So these are example for non-cont forces non-cont forces. So I hope it is very clear understood or not. Coming to that next one is momentum.

[00:15:57]Linear momentum P bar equal to mass into velocity.

[00:16:03]Okay. So momentum is a vector quantity.

[00:16:07]And coming to impulse large force acting in a small interval of time I equal to force into time that is called impulse. So change in momentum like a final momentum minus initial momentum I equal to force into time.

[00:16:27]Okay. So we didn't start. So loss of motion loss of motion.

[00:16:44]Okay. So I hope it is very clear.

[00:16:47]understood or not? Yes. So coming to the next one impulse completed.

[00:16:59]So lassa motion and law conversation of linear momentum.

[00:17:04]So actually conversation of energy energy neither be created nor be destroyed but it changes from one form to another form.

[00:17:14]Okay. So here you can observe law consideration of linear momentum.

[00:17:17]Momentum before collision equal to momentum after collision. Two bodies collide each other. Momentum mass into velocity.

[00:17:25]Okay. One body is moving another body is moving. Both are collide each other. So momentum before collision equal to momentum after collision. Law of consation of linear moment. Even kindinetic energy also same. Kinetic energy before collision equal to kindinetic energy after collision.

[00:17:43]Right? So these are all examples for linear momentum law conversation of linear momentum law conversation of kinetic energy.

[00:17:52]Okay. So I hope it is very clear. Next one lamis theorem.

[00:17:59]So lamis theorem if you are taking lamis theorem.

[00:18:07]So here force is acting f_sub_1 f f_sub_2 and f_sub_3. f_sub_1 is opposite angle is alpha.

[00:18:23]So f_sub_2 opposite is beta. F_sub_3 opposite is gamma. f_sub_1 by sin alpha.

[00:18:33]f_sub_2 by sin gamma sin beta. The third one is f_sub_3 by sin gamma l theorem.

[00:18:45]Okay. So I hope it is very clear.

[00:19:04]Next one. Wait.

[00:19:09]Wait.

[00:19:11]So here in the lift condition the object is here.

[00:19:20]Normal reaction is upward direction weight is acting downward direction.

[00:19:25]So if the lift is moving with the constant velocity v =0 velocity zero are moving with a constant velocity then r = mg and coming to the next one.

[00:19:44]If lift is moving upward direction so reaction is r1 and mg.

[00:19:52]So this is we can take M and MA.

[00:19:59]Now R1 because upward motion is greater than the weight then automatically MA. So R1 = M G + A.

[00:20:11]So if you observe this formula, if you observe that formula, if you observe this formula, you can see so practically if you observe when you're moving upward direction, when you're moving upward direction in the lift, we feel somewhat weight than actual Okay, here you can see R1 = MG + C. And coming to downward direction, downward direction if you observe. So reaction force is upward and mg is acting downward direction. So aation is moving downward direction. Then mg you can take this is R2.

[00:21:07]R2 = MA.

[00:21:09]Then m g minus a that is equal to r2. So if you observe we feel weightless apparent weight reaction force here we feel weightless when the lift is moving downward direction. If you are in the lift, we feel weightless somewhat and here we feel weight when lift is moving upward direction.

[00:21:41]Is it clear? So I hope it is very clear.

[00:21:44]Next one.

[00:22:00]So if the body is moving on a wedge an inclined plane how it is.

[00:22:10]So I'm taking here like this.

[00:22:18]So this the weight of the object weight is acting downward direction.

[00:22:28]So theta angle here mg cos theta and mg sin theta.

[00:22:36]So reaction force here normal reaction we can take a reaction also simply we can say normally action acting upward perpendicular to perpendicular to the plane. So normally if you are taking the object is on the ground so on the table it is acting downward direction so normally action is acting upward direction or reaction also we can take so here I'm taking a so if the body is moving downward direction Friction will act if it is a friction surface friction will act opposite to the movement of the body.

[00:23:21]Okay. Opposite to the moment of the body, moment of the object, opposite to the moment of the object, the friction will be there. So now we can take n = mg cos theta n = mg cos theta here. So f + mg sin theta mg sin theta = friction. So f - mg sin theta.

[00:23:59]So if it is r mu s into r coicient of friction mus is nothing but quotient of static friction.

[00:24:10]So I hope it is very clear.

[00:24:14]understood or not?

[00:24:16]So like this we can write. So to find out the friction and force normal reaction so everything we can do it.

[00:24:29]Okay. So if bodies are connected bodies are connected how it will be we'll see.

[00:24:42]If two bodies are connected so I'm taking here if two bodies are connected like this so m_sub_1 m_sub_2 force is acting on it so aulation is in this direction when we apply force automatically bodies bodies combined together will Right.

[00:25:09]Connected like a collected bodies bodies are placed like this.

[00:25:23]Okay. Now you can see.

[00:25:29]So f = m.

[00:25:32]Here m = m1 + m_sub_2.

[00:25:37]Then f = m1 + m_sub_2 into a. So a = m1 + m_sub_2. So equation 1.

[00:25:50]So you are applying force on this body m.

[00:25:54]Okay. So this m1 will act force on m_sub_2.

[00:25:59]So at the same time m2 will act force on m1. So both are equal, right? So here this one m1 will act and m_sub_2 also will act.

[00:26:15]Then force on m1 m1 = m1 a that is m1 a what is a a = m by m_sub_1 + m_sub_2 force on m_sub_2 force on m_sub_1 force on m_sub_2 so we can take contact force both are attached S and O both are attached. So that's why we can take contact force force on M1 force on M_sub_2 M_sub_2 J then M_sub_2 into F by M_sub_1 + M_sub_2.

[00:26:59]So if you are taking two three bodies together, three bodies placed like this then in this case.

[00:27:10]So here force we can take different values f_sub_1 f_sub_2 here f_sub_3 oh sorry f_sub_1 both are equal right here f_sub_2 f_sub_2 understood or not is it clear?

[00:27:34]Yes. Now, so here we have to take force between M1 and M_sub_2.

[00:27:44]We have to take force between M1 and M2.

[00:27:48]So here a small change M_sub_2 we will get it.

[00:28:00]Now we can observe.

[00:28:02]So f = m a m = m1 + m_sub_2 + m3 into a right. So aation a = m1 + m2 + m3 aation a = m1 + m_sub_2 into m3. So equation 1.

[00:28:29]So now we have to find out forces between m_sub_1 and m_sub_2 that is f_sub_1.

[00:28:37]Okay. So f_sub_1 = that is m_sub_1 m_sub_2 + m3. So here is the three right? So 3 into a. So f_sub_1 = m_sub_2 + m3 by.

[00:28:57]So a is nothing but f by m_sub_1 + m_sub_2 + m_sub_3. If you are finding f_sub_2 m3 by f into ms1 + m_sub_2 + m3 like this we can write.

[00:29:24]So I'm fine good.

[00:29:28]So I hope it is very clear understood or not like this we can find out. So these are three bodies placed like attached.

[00:29:41]Now if it is connected if bodies are connected with string how it will be same formula no changes so their tensions will come right tension tension will be there in the string.

[00:30:02]So here 1 2 3 F tension T1 T2 and 33 T3 M1 M_sub_2 and M3 reaction force M3G Here R2 here R3 M_sub_2 G R1 M1 G.

[00:30:43]So weight will be acting downward direction reaction will be there on each and every body. So even though it is connected with the string so F = T1 that is F. How to find out F?

[00:30:56]f = m1 + m_sub_2 + m_sub_3 into a then a = m1 + m_sub_2 + m3. So equation 1.

[00:31:14]Okay, tension t2.

[00:31:17]If you want to find out tension ts2 then t2 = m_sub_2 + m_sub_3 into f by m_sub_1 + m_sub_2 + m3 then 33 t3 = m3 by so tension only prebody diagram if you draw so tension t3 will be there on it now.

[00:31:45]So m3 = m3 af into m1 + m_sub_2 into m_sub_3 m1 + m_sub_2 + m3 so that is tension t3 I hope it is very clear once observe once observe the formulas is it clear or not do you have any doubt if you have any doubt you can Ask me.

[00:32:17]So, next one. Pulley mass system.

[00:32:22]We'll do with the pulley mass system.

[00:32:24]Let's do it.

[00:32:39]Hya system.

[00:32:57]So this is m1 m_sub_2.

[00:33:01]So m_sub_2 g m1 g t tension tension t. Body is moving up downward direction. body is moving upward direction. So throughout the strings tension will be same because same string will be there. So we are not changing the string. So without changing the string the tension will be there same.

[00:33:28]So tension will be same throughout the string if you are not changing the string. So if string is different tension will be different.

[00:33:38]Now, so we have to follow the formula f = ma every time you have to use this formula simple only.

[00:33:48]So how to write the equations very very easy if you follow Newton's second f= m.

[00:33:58]Okay. So now m_sub_1 g - t = m1 a then t = t - m_sub_2 g = m_sub_2 if you solve these equations you'll get it understood or not so first law second law third law newton's first law defines inertia newton second law f= ma's third every action there is equal and opposite reaction.

[00:34:36]So I hope it is very clear understood or not.

[00:34:41]Yes.

[00:34:44]So likewise we can take so many cases different different cases we can take. So if you like the content if you like the channel do like share and subscribe to student the brand. Thank you so much. We'll see you in the next class.