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Work done by an external non-conservative force is equal to change in the mechanical energy of the system.
Final M. E - Initial M. E = W
If an external force does no work, then mechanical energy will be conserved.
(1) If no external force is acting on the system, then the system's momentum and mechanical energy will be conserved.
(2) If an external force is acting on the system, then :
(a) if work done by an external force is non 0 then both M. E and momentum will change.
(b) if work done by an external force is 0, then M. E will remain same I. e mechanical energy will be conserved, but momentum will change as an external force is acting on the system.

Key points:

1. Friction does not oppose the motion, but it opposes relative movement between the contact surfaces.
2. When there is no relative motion between the contact surfaces, then no friction force will act. Still, if there is no relative motion between the contact surfaces, but there is a tendency to be in relative motion, static friction will work. 
3. Static friction is self-adjusting in nature.
4. Static friction has a maximum limit, and that maximum friction is called as limiting friction.
5. When there is relative motion between the contact surfaces, then kinetic or dynamic friction will act. 
6. Kinetic friction is constant in magnitude.

Hello Dear Students, In this article, we'll talk about a practical and achievable strategy to Score High Marks in IIT JEE PHYSICS.
First of all, let me clear it; the number of books finished doesn't matter how many of them you can retain and are frequently revising. And, be aware that initially, you might think that you will complete and revise every popular book available in the market but, that's not possible. Sorry.
Start with basics, pick up any good book that has most of the topics related to the syllabus, and STICK to that. No Book can cover every type of subject and all variety of questions, and that's why your notes come into play.
For Basics Books I would suggest anyone book like B.M Sharma(Cengage)/Ashish Arora(Physics Galaxy), D.C Pandey or D.C Gupta or any other branch wise written books. I would recommend D.C Gupta because it is covering most of the topics and is priced very low (Spending more money doesn't guarantee a seat in IIT JEE, if you are excited to spend a lot I will request you to sponsor a child's education/books/study material. Their blessings might help you a bit)
And why notes? Because these are the things that are going to be your best book after the end of preparation. From your essential sourcebook, read a topic, and write it down in a precise(no copy-pasting) manner in your notes. Do not write down formulae only, instead try to note the steps involved in derivation because JEE understands matters. And, after that go through your Question Sets that could be from your coaching material, Tutor Marked Assignments(TMA), Test Series, or any other book like H.C.VERMA(HCV), Previous Year Questions(PYQ), Advanced Problems in Physics(Balaji Pub.), Arihant's Books, etc. While solving questions initially, you might be getting stuck in many of the problems. That's Completely normal, and the majority of students suffer from that, the point that you should note here is Improvement is a cyclic process. You can't improve yourself in one go.
Whenever there's a question that is bothering you, try to look for solutions and its concept. If you had studied that concept and noted it down in your notes, go there and highlight that you don't forget that next time, and if you haven't learned that concept, just note it down now and try to solve it again after a few days. When you are doing this, have faith in yourself. Every prodigy you see in your class/coaching/friend circle is 100% backed up by some EXTRA help; he might be taking coaching for coaching, he might be getting help from their IITian parents, Didi or Bhaiya. I could promise you that you would also achieve the same if you had equal access to resources. But, life is not equal and fair for everyone. All you can do is keep learning, improving, and sustaining the momentum. I repeat: do not go behind too many books.
Also, I have seen some students try to finish the same topic from multiple books for that extra safety, that's downright lethal!. Focus on 3-4 books, including NCERT and Exemplars and finish the book by the book. Like first go for your essential Source then NCERT then Exemplar and finally your Question Sets-PYQ-TMA. If there's a test approach, you can cover a single chapter from your 3-4 books. For example, if you finish Newton's Laws of Motion for a test from D.C Gupta then go for HCV then NCERT then Exemplar, and finally to PYQ and assume this chapter is finished now. If you keep adding sources, you can't tell your brain that you've finished that chapter and eventually you'll have low confidence. I hope you're getting my point.
Let me wrap it up, Stick to a few resources, keep writing and updating your notes. Have faith in yourself but don't be careless either. Don't try to build targets that require superhuman capabilities like I will finish NLM, Gravitation, Rotational, and WEP in one week. Sounds funny and stupid, right? But, I've seen those species also.
Finally, if you need help from me, I am always available offline/online and for any suggestions/questions, feel free to ask.
All the Best and luck doesn't matter. The probability of zero doesn't mean an impossible event.
Always,
Sunil Yadav

Hello Dear Students, In this article, we'll talk about an effective and achievable strategy to Score High Marks in IIT JEE PHYSICS.

First of all, let me clear it, the number of books finished doesn't matter what matters is how many of them you are able to retain and are revising frequently. And, be aware that initially, you might think that you will definitely finish and revise every popular book available in the market but, that's not possible. sorry.

Start with basics, pick up any good book which has the majority of the topics related to the syllabus, and STICK to that. No Book can cover every type of topic and all variety of questions and that's why your notes come into play.

For Basics Books I would suggest anyone book like B.M Sharma(Cengage)/Ashish Arora(Physics Galaxy), D.C Pandey or D.C Gupta or any other branch wise written books. Personally, I would recommend D.C Gupta because it is covering most of the topics and is priced very low (Spending more money doesn't guarantee a seat in IIT JEE, if you are really excited to spend a lot I would request you to sponsor a child's education/books/study material. Their blessings might help you a bit)

And, why notes? Because these are the things that are going to be your best book after the end of preparation. From your basic sourcebook, read a topic, and write it down in a precise(no copy-pasting) manner in your notes. Do not write down formulae only, rather try to note the steps involved in derivation because in JEE understanding matters.  And, after that go through your Question Sets that could be from your coaching material, Tutor Marked Assignments(TMA), Test Series, or any other book like H.C.VERMA(HCV), Previous Year Questions(PYQ), Advanced Problems in Physics(Balaji Pub.), Arihant's Books, etc. While solving questions initially you might be getting stuck in many of the questions. That's Completely normal and the majority of students suffer from that, the point that you should note here is Improvement is a cyclic process. You can't improve yourself in one go.

Whenever there's a question that is bothering you, try to look for solutions and its concept. If you had studied that concept and noted it down in your notes, go there and highlight that so that next time you don't forget, and if you haven't studied that concept just note it down now and try to solve it again after a few days. When you are doing this have faith in yourself. Every prodigy that you see in your class/coaching/friend circle is 100% backed up by some EXTRA help, he might be taking coaching for coaching, he might be getting help from their IITian parents, Didi or Bhaiya. I can promise you that if you had similar access to resources you would also achieve the same. But, life is not equal and fair for everyone. All you can do is keep learning, improving, and sustaining the momentum. I repeat do not go behind too many books. 

Also, I have seen some students try to finish the same topic from multiple books for that extra safety, that's completely lethal!. Focus on 3-4 books including NCERT and Exemplars and finish them book by book. Like first go for your basic Source then NCERT then Exemplar and finally your Question Sets-PYQ-TMA. If there's a test approaching then you can cover a single chapter from your 3-4 books. For example, if you finish Newton's Laws of Motion for a test from D.C Gupta then go for HCV then NCERT then Exemplar, and finally to PYQ and assume this chapter is finished now. If you keep on adding sources you can't tell your brain that you've finished that chapter and eventually you'll have low confidence. I hope you're getting my point.

Let me wrap it up, Stick to a few resources, keep writing and updating your notes. Have faith in yourself but don’t be careless either. Don’t try to build targets that require superhuman capabilities like I will finish NLM, Gravitation, Rotational, and WEP in one week. Sounds funny and stupid right? But, I’ve seen those species also. 

Finally, if you need help from me I am always available offline/online and for any suggestions/questions feel free to ask.

All the Best and luck doesn’t matter. The probability of zero doesn’t mean an impossible event.

Always,

Sunil Yadav

It is because when you insert a resistor, or when a conductor (resistors are conductors) is connected to a voltage source such as a battery, the negative terminal will present an opposing electrostatic force the conductor as well as have a ready supply of additional electrons. This field will repel electrons in the conductor and exert a force on them and will attempt to inject fresh electrons into the conductor. At the opposite end, where the conductor is connected to the positive terminal of the voltage source, a force exists to pull electrons from the conductor. 

 Let us solve Irodov problem 1.1 on Kinematics.

The problem states:

A motorboat going downstream overcame a raft at a point A;  τ=60 min later it turned back and after sometime passed the raft at a distance l=6.0 km from point A. Find the flow velocity assuming the duty of the engine to be constant.

To solve this problem most efficiently we will be using the following concepts:

1. Reference Frames in Kinematics

2. Vectors

Let us now see how to approach the problem by looking at a presentation I created :

 (The video is in Bengali)

How to prove if points are Collinear in coordinate geometry? 

 Collinear points definition:

• Three or more points that lie on a same straight line are called collinear points.
• Consider a straight line L in the above Cartesian coordinate plane formed by x axis and y axis.
• This straight line L is passing through three points A, B and C whose coordinates are (2, 4), (4, 6) and (6, 8) respectively.
• {We may also say, alternatively, that the three points A (2, 4), B (4, 6) and C (6, 8)are lying on a same straight line L}
• Three or more points which lie on a same straight line are called collinear points.

How to find if three points are collinear?:

• There are two methods to find if three points are collinear.
• One is slope formula method and the other is area of triangle method.
• Slope formula method to find that points are collinear.
• Three or more points are collinear, if slope of any two pairs of points is same.
• With three points A, B and C, three pairs of points can be formed, they are: AB, BC and AC.
• If Slope of AB = slope of BC = slope of AC, then A, B and C are collinear points.

Example

Show that the three points A (2, 4), B (4, 6) and C (6, 8) are collinear.

Solution:

• If the three points A (2, 4), B (4, 6) and C (6, 8) are collinear, then
• slopes of any two pairs of points will be equal.
• Now, apply slope formula to find the slopes of the respective pairs of points:
• Slope of AB = (6 – 4)/ (4 – 2) = 1,
• Slope of BC = (8 – 6)/ (6 – 4) = 1, and
• Slope of AC = (8 – 4) /(6 – 2) = 1
• Since slopes of any two pairs out of three pairs of points are same, this proves that A, B and C are collinear points.
• Area of triangle to find if three points are collinear.
• Three points are collinear if the value of area of triangle formed by the three points is zero.
• Apply the coordinates of the given three points in the area of triangle formula. If the result for area is zero, then the given points are said to be collinear.
• First of all, recall the formula for area of a triangle formed by three points.

It is

In the formula above, the two vertical bars enclosing the variables represent a determinant.

Let us apply the coordinates of the above three points A, B and C in the determinant formula above for area of a triangle to check if the answer is zero.

Since the result for area of triangle is zero, therefore A (2, 4), B (4, 6) and C (6, 8) are collinear points.