# Tips to Solve Physics JEE (Advanced) 2018 Questions – III

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5. Tips to Solve Physics JEE (Advanced) 2018 Questions – III

### Let Us YOU Solve Physics JEE (Advanced 2018) Questions:

Welcome to our next blog. If you are a new visitor, let us tell you a few words about the contents of these blogs. At present, we are selecting a number of questions at random from various Competitive Exams of repute, and guiding the aspiring students to solve them on their own.

In teaching physics, we believe in the principle of Theory First, Problems Next. Therefore, if you visit our website @ www.physicsacademyonline.com, you will be greeted with a vast reservoir of video lectures, accessible against small payment. Every chapter we deal with, we adhere to the above principle. That is, we start with the theories, illustrate them with simple problems, and then pick up more innovative, challenging problems to sharpen the student’s problem-solving skill. What can be better than testing the effectiveness of this approach vis-a-vis the so-called “tough” exam papers? Remember, our task is limited to showing the connection between a question/problem and the relevant theory/theories. No further. The student does most of the problem-solving, and gains in appetite and confidence. A video solution is given at the end just for check.

I have the JEE (Advanced) 2018, Paper-2 with me. This examination was also held on 20 May 2018: Paper-1 in the morning session 9 AM to 12 noon; Paper-2 in the afternoon session 2 PM to 5 PM. Either paper may contain questions from any chapter within Physics syllabus. Just like Paper-1, Paper-2 also contains 18 questions in Physics with maximum marks 60. They are divided into three sections: Section 1 contains six MCQ’s (total marks 4 x 6 = 24), Section 2 contains eight numerical problems (total marks 3 x 8 = 24), and Section 3 contains four MCQ’s based on matching lists (total marks 3 x 4 = 12). Ideally, a student should spend one minute on each mark scored.

The MCQ’s in Section 1 may have one or more than one options correct. Full marks for choosing all correct options, less marks for choosing part of correct options, negative marking for any wrong choice(s).

Question 1:  A particle of mass m is initially at rest at the origin. It is subjected to a force, and starts moving along x-axis. Its kinetic energy, K, changes with time as dK/dt = γt, where γ is a positive constant of appropriate dimensions. Which of the following statements is/are true?

(A)  The force applied to the particle is constant

(B)  The speed of the particle is proportional to time

(C) The distance of the particle from the origin increases linearly with time

(D) The force is conservative

An alert student notes that some options are inter-dependent. For instance, if option (B) happens to be correct, option (A) will also be correct while option (C) will be incorrect. Therefore, your first target is finding the speed, v, of the particle. What is the formula for kinetic energy, K, of a particle? Subscribe to  www.physicsacademyonline.com, and find this lecture: Chapter Name – Mechanics; Category –  Basic; Topic Name – Work and Energy; Video Name – Concept of Energy and Derivation of Work-Energy Theorem.

Make use of it to get a differential equation in v and t. Integrate between limits (see initial conditions) to find v. Option (B) is decided.

For motion in a straight line, how do you express acceleration and velocity as derivatives with respect to time? Watch this if not seen before: Chapter Name – Mechanics; Category –  Basic; Topic Name – Motion in One and Two Dimensions; Video Name – Graphical Analysis of Rectilinear Motion.

Options (A) and (C) are decided in a jiffy.

A force that is constant in magnitude is not necessarily conservative, such as the force of kinetic friction. What criterion determines whether a force is conservative or not? Brush up with the lecture: Chapter Name – Mechanics; Category –  Basic; Topic Name – Work and Energy; Video Name – Conservative and Non-Conservative Forces.

Apply the criterion, and decide on option (D).

Once you have completed, don’t forget to confirm your answer against the video solution below.

Question 2:   Consider a thin square plate floating on a viscous liquid in a large tank. The height, h, of the liquid in the tank is much less than the width of the tank. The floating plate is pulled horizontally with a constant velocity u0 . Which of the following statements is/are true?

(A)  The resistive force of liquid on the plate is inversely proportional to h

(B)  The resistive force of liquid on the plate is independent of the area of the plate

(C) The shear stress on the floor of the tank increases with u0

(D) The shear stress on the plate varies linearly with the viscosity, η  , of the liquid

This easy question reminds me of a similar question on Surface Tension in Paper-1. I have a feeling that the examiner is well aware of the lack of interest in some students about important fluid properties, such as Viscosity. Now the good thing is, you can study this property well by subscribing to www.physicsacademyonline.com and watching the lecture: Chapter Name – Elasticity and Fluid Mechanics; Category –  Basic; Topic Name – Fluid Dynamics and Viscosity; Video Name – Viscosity.

One single formula – known as Newton’s formula for laminar viscous flow – gives you the viscous force, F, in terms of various quantities. Consulting it, you can pick the correct options in seconds.

One word here. The viscous force is resistive in nature and acts along the tangent to the solid surface. Therefore, tangential or shear stress on the plate and the floor. Sincere students will be advised to look around for other interesting results in the same lecture.

For a check, the video solution below is recommended.