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# NCERT solutions for Class 11 Physics chapter 4 - Motion in a Plane

## Chapter 4 : Motion in a Plane

#### Pages 85 - 88

Q 1 | Page 85

State, for each of the following physical quantities, if it is a scalar or a vector: volume, mass, speed, acceleration, density, number of moles, velocity, angular frequency, displacement, angular velocity.

Q 2 | Page 85

Pick out the two scalar quantities in the following list: force, angular momentum, work, current, linear momentum, electric field, average velocity, magnetic moment, relative velocity.

Q 3 | Page 85

Pick out the only vector quantity in the following list:

Temperature, pressure, impulse, time, power, total path length, energy, gravitational potential, coefficient of friction, charge.

Q 4 | Page 85

State with reasons, whether the following algebraic operations with scalar and vector physical quantities are meaningful:

(a) adding any two scalars, (b) adding a scalar to a vector of the same dimensions, (c) multiplying any vector by any scalar, (d) multiplying any two scalars, (e) adding any two vectors, (f) adding a component of a vector to the same vector.

Q 5.1 | Page 85

Read each statement below carefully and state with reasons, if it is true or false:

The magnitude of a vector is always a scalar

Q 5.2 | Page 85

Read each statement below carefully and state with reasons, if it is true or false:

Each component of a vector is always a scalar

Q 5.3 | Page 85

Read each statement below carefully and state with reasons, if it is true or false:

the total path length is always equal to the magnitude of the displacement vector of a particle.

Q 5.4 | Page 85

Read each statement below carefully and state with reasons, if it is true or false:

the average speed of a particle (defined as total path length divided by the time taken to cover the path) is either greater or equal to the magnitude of average velocity of the particle over the same interval of time

Q 5.5 | Page 85

Read each statement below carefully and state with reasons, if it is true or false:

Three vectors not lying in a plane can never add up to give a null vector.

Q 6 | Page 85

Establish the following vector inequalities geometrically or otherwise:

(a) |b| ≤ |a| + |b|

(b) |b| ≥ ||a| − |b||

(c) |− b| ≤ |a| + |b|

(d) |− b| ≥ ||a| − |b||

When does the equality sign above apply?

Q 7 | Page 86

Given 0, which of the following statements are correct:

(a) abcand must each be a null vector,

(b) The magnitude of (c) equals the magnitude of (bd),

(c) The magnitude of can never be greater than the sum of the magnitudes of bc, and d,

(dmust lie in the plane of a and if a and are not collinear, and in the line of and d, if they are collinear?

Q 8 | Page 86

Three girls skating on a circular ice ground of radius 200 m start from a point Pon the edge of the ground and reach a point Qdiametrically opposite to P following different paths as shown in Figure. What is the magnitude of the displacement vector for each? For which girl is this equal to the actual length of the path skated?

Q 9 | Page 86

A cyclist starts from the centre O of a circular park of radius 1 km, reaches the edge P of the park, then cycles along the circumference, and returns to the centre along QO as shown in Fig. If the round trip takes 10 min, what is the (a) net displacement, (b) average velocity, and (c) average speed of the cyclist?

Q 10 | Page 86

On an open ground, a motorist follows a track that turns to his left by an angle of 60° after every 500 m. Starting from a given turn, specify the displacement of the motorist at the third, sixth and eighth turn. Compare the magnitude of the displacement with the total path length covered by the motorist in each case.

Q 11 | Page 86

A passenger arriving in a new town wishes to go from the station to a hotel located 10 km away on a straight road from the station. A dishonest cabman takes him along a circuitous path 23 km long and reaches the hotel in 28 min. What is (a) the average speed of the taxi, (b) the magnitude of average velocity? Are the two equal?

Q 12 | Page 86

Rain is falling vertically with a speed of 30 m s–1. A woman rides a bicycle with a speed of 10 m s–1in the north to south direction. What is the direction in which she should hold her umbrella?

Q 13 | Page 86

A man can swim with a speed of 4.0 km/h in still water. How long does he take to cross a river 1.0 km wide if the river flows steadily at 3.0 km/h and he makes his strokes normal to the river current? How far down the river does he go when he reaches the other bank?

Q 14 | Page 87

In a harbour, wind is blowing at the speed of 72 km/h and the flag on the mast of a boat anchored in the harbour flutters along the N-E direction. If the boat starts moving at a speed of 51 km/h to the north, what is the direction of the flag on the mast of the boat?

Q 15 | Page 87

The ceiling of a long hall is 25 m high. What is the maximum horizontal distance that a ball thrown with a speed of 40 m s–1 can go without hitting the ceiling of the hall?

Q 16 | Page 87

A cricketer can throw a ball to a maximum horizontal distance of 100 m. How much high above the ground can the cricketer throw the same ball?

Q 17 | Page 87

A stone tied to the end of a string 80 cm long is whirled in a horizontal circle with a constant speed. If the stone makes 14 revolutions in 25 s, what is the magnitude and direction of acceleration of the stone?

Q 18 | Page 87

An aircraft executes a horizontal loop of radius 1.00 km with a steady speed of 900 km/h. Compare its centripetal acceleration with the acceleration due to gravity.

Q 19.1 | Page 87

Read each statement below carefully and state, with reasons, if it is true or false:

The net acceleration of a particle in circular motion is always along the radius of the circle towards the centre

Q 19.2 | Page 87

Read each statement below carefully and state, with reasons, if it is true or false:

The velocity vector of a particle at a point is always along the tangent to the path of the particle at that point

Q 19.3 | Page 87

Read each statement below carefully and state, with reasons, if it is true or false:

The acceleration vector of a particle in uniform circular motion averaged over one cycle is a null vector

Q 20 | Page 87

The position of a particle is given by

r = 3.0t  hati − 2.0t 2  hatj + 4.0  hatk m

Where is in seconds and the coefficients have the proper units for to be in metres.

(a) Find the and of the particle?

(b) What is the magnitude and direction of velocity of the particle at = 2.0 s?

Q 21 | Page 87

A particle starts from the origin at = 0 s with a velocity of 10.0 hatj "m/s" and moves in the x-y plane with a constant acceleration of (8.0 hati + 2.0 hatj) ms^(-2).

(a) At what time is the x-coordinate of the particle 16 m? What is the y-coordinate of the particle at that time?

(b) What is the speed of the particle at the time?

Q 22 | Page 87

hati "and" hatj are unit vectors along x- and y-axis respectively. What is the magnitude and direction of the vectors hati+hatj and hati-hatj ? What are the components of a vector A = 2hati + 3hatj along the directions of hati + hatj and hati - hatj ? [You may use graphical method]

Q 23 | Page 87

For any arbitrary motion in space, which of the following relations are true:

a) V_"average" = (1/2) (v (t1) + v (t2))

b) V_"average" = [r(t2) - r(t1) ] /(t2 – t1)

c) v (t) = v (0) + a t

d) r (t) = r (0) + v (0) t + (1/2) a t2

e) a_"average" = =[ v (t2) - v (t1 )] /( t2 – t1)

(The ‘average’ stands for average of the quantity over the time interval t1 to t2)

Q 24 | Page 87

Read each statement below carefully and state, with reasons and examples, if it is true or false:

A scalar quantity is one that

(a) is conserved in a process

(b) can never take negative values

(c) must be dimensionless

(d) does not vary from one point to another in space

(e) has the same value for observers with different orientations of axes

Q 25 | Page 87

An aircraft is flying at a height of 3400 m above the ground. If the angle subtended at a ground observation point by the aircraft positions 10.0 s apart is 30°, what is the speed of the aircraft?

Q 26 | Page 88

A vector has magnitude and direction. Does it have a location in space? Can it vary with time? Will two equal vectors a and b at different locations in space necessarily have identical physical effects? Give examples in support of your answer.

Q 27 | Page 88

A vector has both magnitude and direction. Does it mean that anything that has magnitude and direction is necessarily a vector? The rotation of a body can be specified by the direction of the axis of rotation, and the angle of rotation about the axis. Does that make any rotation a vector?

Q 28 | Page 88

Can you associate vectors with (a) the length of a wire bent into a loop, (b) a plane area, (c) a sphere? Explain.

Q 29 | Page 88

A bullet fired at an angle of 30° with the horizontal hits the ground 3.0 km away. By adjusting its angle of projection, can one hope to hit a target 5.0 km away? Assume the muzzle speed to the fixed, and neglect air resistance.

Q 30 | Page 88

A fighter plane flying horizontally at an altitude of 1.5 km with speed 720 km/h passes directly overhead an anti-aircraft gun. At what angle from the vertical should the gun be fired for the shell with muzzle speed 600 m s–1 to hit the plane? At what minimum altitude should the pilot fly the plane to avoid being hit? (Take = 10 m s2).

Q 31 | Page 88

A cyclist is riding with a speed of 27 km/h. As he approaches a circular turn on the road of radius 80 m, he applies brakes and reduces his speed at the constant rate of 0.50 m/s every second. What is the magnitude and direction of the net acceleration of the cyclist on the circular turn?

Q 32.1 | Page 88

Show that for a projectile the angle between the velocity and the x-axis as a function of time is given by

theta(t) =tan^(-1) ((v_(0y) - "gt")/v_(o x))

Q 32.2 | Page 88

Show that the projection angle theta_o for a projectile launched from the origin is given by

theta_o =tan^(-1) ((4h_m)/R)

Where the symbols have their usual meaning

## NCERT solutions for Class 11 Physics chapter 4 - Motion in a Plane

NCERT solutions for Class 11 Physics chapter 4 (Motion in a Plane) include all questions with solution and detail explanation. This will clear students doubts about any question and improve application skills while preparing for board exams. The detailed, step-by-step solutions will help you understand the concepts better and clear your confusions, if any. Shaalaa.com has the CBSE Physics Textbook for Class 11 solutions in a manner that help students grasp basic concepts better and faster.

Further, we at shaalaa.com are providing such solutions so that students can prepare for written exams. NCERT textbook solutions can be a core help for self-study and acts as a perfect self-help guidance for students.

Concepts covered in Class 11 Physics chapter 4 Motion in a Plane are Scalars and Vectors, General Vectors and Their Notations, Multiplication of Vectors by a Real Number, Addition and Subtraction of Vectors — Graphical Method, Relative Velocity in Two Dimensions, Resolution of Vectors, Rectangular Components, Scalar and Vector Product of Vectors, Motion in a Plane, Cases of Uniform Velocity, Cases of Uniform Acceleration Projectile Motion, Uniform Circular Motion, Motion in a Plane - Average Velocity and Instantaneous Velocity, Motion in a Plane - Average Acceleration and Instantaneous Acceleration, Angular Velocity, Vector Addition – Analytical Method, Motion in a Plane with Constant Acceleration, Projectile Motion.

Using NCERT Class 11 solutions Motion in a Plane exercise by students are an easy way to prepare for the exams, as they involve solutions arranged chapter-wise also page wise. The questions involved in NCERT Solutions are important questions that can be asked in the final exam. Maximum students of CBSE Class 11 prefer NCERT Textbook Solutions to score more in exam.

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