# WBJEE: Important Questions and Preparation Tips – Rotational Motion

The chapter **Rotational Motion** is a very important part of Mechanics in the syllabus of **WBJEE** Examination 2018. This article will provide you complete chapter notes of chapter Rotational Motion including all important topics like **angular velocity, angular acceleration, parallel axis theorem, formulae for moment of inertia of different objects like ring, disc, cylinder, thin road** etc. These notes are based on the latest syllabus of WBJEE Examination 2018. These notes will provide you complete summary of the chapter Rotational Motion. Some previous years questions are also included in this article to help students in understanding the difficulty level of the questions asked in previous year examinations of WBJEE.

**About the exam:**

West Bengal Joint Entrance Examination (WBJEE) is a state level common entrance test organized by West Bengal Joint Entrance Examinations Board for admission to the Undergraduate Level Engineering and Medical Courses through a common entrance test in the State of West Bengal.

**Important Concepts:**

**WBJEE 2014 Solved Physics Question Paper**

**Some previous years solved questions are given below:**

**Question 1:**

In which of the following pairs, the two physical quantities have different dimensions?

(A) Planck’s constant and angular momentum

(B) Impulse and linear momentum

(C) Moment of inertia and moment of a force

(D) Energy and torque

**Solution 1:**

Moment of inertia and moment of a force are the two physical quantities which have different dimensions.

Hence, the correct option is (C).

**Important Questions and Preparation Tips – Centre of Mass**

**Question 2:**

A solid uniform sphere resting on a rough horizontal plane is given a horizontal impulse directed through its center so that it starts sliding with an initial velocity *v*_{0}. When it finally starts rolling without slipping the speed of its center is

(A) 2* v*_{0}/7

(B) 3* v*_{0}/7

(C) 5* v*_{0}/7

(D) 6* v*_{0}/7

**Solution 2:**

Angular momentum will remain conserved along point of contact.

Let *v* be the velocity when it starts rolling without slipping the speed of its center.

Hence, the correct option is (C).

**Important Questions and Preparation Tips – Work, Power and Energy**

**Question 3:**

Two particles A and B are moving as shown in the figure. Their total angular momentum about the point O is

(A) 9.8 kg m^{2}/s

(B) Zero

(C) 52.7 kg m^{2}/s

(D) 37.9 kg m^{2}/s

**Solution 3:**

Angular momentum(L) = mv × r

Now, since v and r are perpendicular to each other so,

mv × r = mvr sin90^{0}

L = (6.5) (2.2) (1.5)(-k) + (2.8) (3.1)(3.6)(k) = 9.8k kgm^{2}/s

Hence, the correct option is (A).

**Important Questions and Preparation Tips – Laws of Motion**

**Question 4:**

A tire of radius R rolls on a flat surface with an angular velocity ω and velocity v. If v > Rω, in which direction does friction from the tire act on the road

(A) Towards the left

(B) towards the right

(C) downwards

(D) upwards

**Solution 4:**

At the point of contact of the wheel and the ground, the wheel spins towards the right. Irrespective of the net velocity, the rotational velocity creates a friction towards the left.

Hence, the correct option is (A).

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