Solutions of CBSE Science NCERT Physics Sample Paper | Class 9, Motion | Extra Questions

Class 9 Motion (Science)

Ncert Cbse Solutions for Science

Extra Model Questions | Sample Questions

(Important for Examinations) 

Q.1: Describe the terms ‘Rest’ and ‘Motion’.

Ans: Rest - A body is said to be at rest if it does not change its position with respect to its surroundings. For example, a table lying in a room is at rest with respect to the walls of the room.

Motion -A body is said to be in motion if it changes its position with respect to its surroundings. For example, a car running on a road is in motion with respect to trees on roadside.  

Q.2: Describe the various types of motions observed in bodies.

Ans:

1. Translatory motion - When a body moves as a whole along a straight or curved path, it is said to be in translatory motion. Translatory motion is of two types:

(a) Rectilinear motion: Here a body moves as a whole along a straight path. For example, a train moving on straight rails has translatory rectilinear motion.

(b) Curvilinear motion: Here a body moves as a whole along a curved path. For example, a bicycle taking a turn along a curved path.

 2. Rotatory / Rotational motion - When a body rotates about a fixed point or axis, it has a rotatory motion. For example, motion of flywheel about a shaft.

3. Vibratory or Oscillatory motion - When a body moves to and fro about a mean position again and again, it has vibratory or oscillatory motion. For example, the motion of the pendulum of a wall-clock.

4. Complex motion - Sometimes, the motion of a body may be a combination of more than one type of motion. For example, a ball rolling down an inclined plane has both translatory and rotatory motions.  

Q.3: What are scalar quantities? Give examples.

Ans: The physical quantities which require only magnitude and not the direction for their complete description are called scalars or scalar quantities. Distance, speed, time, area, mass, volume, density, work, energy etc are all scalar quantities.

Q.4: What are vector quantities? Give examples.

Ans: The physical quantities which need both magnitude and direction for their complete description are called vectors or vector quantities. Displacement, velocity, force, acceleration, momentum, weight etc. are all vector quantities.

Q.5: What are the differences between the terms ‘distance’ and ‘displacement’?

Ans: The following table shows some differences between Distance and Displacement -

Distance	Displacement
1. Distance is the length of actual path travelled by a body, irrespective of its direction of motion. 2. Distance between two given points may be the same or different for different paths chosen. 3. It is a scalar quantity. 4. Distance covered is always positive or zero.	1. Displacement is the shortest distance between the initial and final positions of a body in a given direction. 2. Displacement between two given points is always same. 3. It is a vector quantity. 4. Displacement covered may be positive, negative or zero.

Q.6: An object has moved through a distance, can it have zero displacement? If yes, support your answer with an example.   

Ans: Yes, an object which has moved through a distance can have zero displacement. Suppose a person throws a ball upwards through a height h and catches back the ball. Then,

distance covered by the ball = h + h = 2h.

displacement of the ball = 0.

Q.7: A farmer moves along the boundary of a square field of side 10 m in 40 s. what will be the magnitude of displacement of the farmer at the end of 2 minutes 20 seconds?

Ans:       

If the farmer starts from point A, then at the end of 2 minutes and 20 seconds i.e. total 140 seconds, he will reach the diagonally opposite corner C. The magnitude of displacement of the farmer is -

  AC = √(AB² + BC²) = √(10² + 10²) = 14.14 m Ans.

Q.8: Which of the following is true for displacement?

(a) It can not be zero.

(b) Its magnitude is greater than the distance travelled by the object.

Ans: Nether of the above are true.

Q.9: What is meant by ‘Uniform Motion’? Give example.

Ans: If an object covers equal distances in equal intervals of time, however small the time interval may be, then the motion of the object is said to be uniform motion. For example, say a car covers 10 km in first 15 min, 10 km in second 15 min, 10 km in third 15 min and so on. Then one can say that the car is in uniform motion.

Q.10: What is non-uniform motion? Give example.

Ans: If an object covers unequal distances in equal intervals of time, then the object is said to be in non-uniform motion. Most of the motions seen in our daily life are non-uniform. For example, if we drop a ball from the roof of a building, we will note that the ball covers 4.9 m in the first second, 14.7 m in the next second, 24.5 m in the third second and so on. That is the ball is covering increasingly larger distances in successive seconds as it falls down. Hence the motion of a freely falling ball or object is non-uniform.   

Q.11: Define the term ‘Speed’. What are its various units?

Ans: Speed is defined as the distance travelled by a body per unit time. Thus,

  Speed = Distance travelled ÷ Time taken

The SI unit of speed is m s^-1, a smaller unit of speed is cm s^-1 and a larger unit is km h^-1.

Q.12: Define the terms ‘Uniform Speed’ and ‘Non-uniform Speed’.

Ans: Uniform speed - when an object covers equal distances in equal intervals of time, however the small these time intervals may be, it is said to be in ‘uniform speed’. For example, if somebody is driving a car at a uniform speed of 40 km/h, then the car will go 20 km every half-hour, 10 km for every quarter of an hour and . . . . 11 m for every second.

Non-uniform speed - when an object covers unequal distances in equal intervals of time then it is said to be in ‘non-uniform speed’. For example, when we start a motorbike, we press its accelerator to increase its speed and at many times we apply brakes to slow down the bike. In such situations, the speed of the bike is non-uniform.        

Q.13: What is ‘average speed’?

Ans: When the speed of a body varies with time, we need to define its average speed. ‘Average speed’ is the total distance travelled by a body, divided by the total time taken to cover the distance.

For example, if a car travels a distance of 20 km in 2 hours, then its -

Average speed = 100 km ÷ 2 hour = 50 km/h    

Q.14: Are rest and motion absolute or relative terms?

Ans: Both these are relative terms.

Q.15: Can an object be at rest as well as in motion at the same time?

Ans: Yes, an object can be at rest as well as in motion at the same time. Because an object may at rest relative one object and at the same time it may be in motion relative to another object.

Q.16: Can the displacement be greater than the distance travelled by an object?

Ans: No, the displacement of an object can be either equal to or less than the distance travelled by an object.  

Q.17: Define the term ‘Velocity’. What is its SI unit?

Ans: Velocity is a physical quantity that gives both the speed and direction of motion of the body. Velocity of a body is defined as the displacement produced per unit time. It is also defined as the speed of a body in a given direction.

Velocity = Displacement ÷ Time

The SI unit of velocity is ms^-1.

Q.18: Distinguish between the terms ‘Speed’ and ‘Velocity’.

Ans:

Speed	Velocity
1. It is the distance traversed by a body per unit time in any direction. 2. It is a scalar quantity. 3. It is always positive or zero but never negative.	1. It is the distance traversed by a body per unit time in a given direction. 2. A vector quantity. 3. It may be positive or negative or zero.

Q.19: Under what conditions is the magnitude of average velocity of an object equal to its average speed?

Ans: When an object moves along a straight line in a given direction, its total distance covered is equal to the magnitude of displacement. Hence only under this condition, its average speed is equal to the magnitude of the average velocity.

Q.20: What does the path of an object look like when it is in uniform motion?

Ans: Straight line path.

Q.21: When is the acceleration of a body positive?

Ans: When the velocity of a body increases with time, its acceleration is positive. For example, acceleration of a bus as it leaves the bus stop. 

Q.22: Give two examples of uniformly accelerated motions.

Ans: Examples of uniformly accelerated motions:

(i) An object moving with a uniform speed along a circular path, has uniform acceleration because the velocity of the object changes continuously due to the change in its direction at every point.

(ii) The motion of a ball rolling down an inclined plane is uniformly accelerated.

Q.23: A bus decreases its speed from 80 km/h to 60km/h in 5 s. Find the acceleration of the bus.

Ans:

u = 80 km/h = {(80 x 1000) ÷ 3600} m/s = 800/36 ms^-1

v = 60 km/h = 600/36 ms^-1

Acceleration, a = (v - u) / t = (600/36 - 800/36) ÷ 5 = -1.11 ms^-2 Ans.

Q.24: A train starting from the railway station and moving with a uniform acceleration attains a speed of 40 km/h in 10 minutes. Find its acceleration.

Ans: Initial speed, u = 0

Final speed, v = 40 km/h = {(40 x 1000) / 3600} ms^-1 = 100/9 ms^-1

Time, t = 10 min = 600 s

Acceleration, a = (v - u) / t = (100/9 - 0) ÷ 600 = 1/54 ms^-2 Ans.  

Q.25: What is distance-time graph of a body? Mention the uses of distance-time graph.

Ans: It is a graph obtained by plotting distance travelled along Y-axis and time along X-axis. The uses of distance -time graph are as follows:

(a) It tells the position of the body at any instant of time.

(b) The distance covered by the body during a particular time interval can be seen from this graph.

(c) The velocity of the body at any instant of time can be determined.

Q.26: Derive the equations of motion for uniformly accelerated motion from velocity-time graph.

Ans: Equations of Motion by Graphical Method

Consider an object moving along a straight line with an initial velocity u and uniform acceleration a. Suppose, it travels distance s in time t. As shown in figure its velocity-time graph is a straight line.

Here OA = ED = u, OC = EB = v, OE = AD = t.  

1. Equation for velocity-time relation:

We know that, Acceleration = Change in velocity ÷ Time

or, a = BD ÷ OE = (BE - ED) ÷ OE

or, a = (v - u) ÷ t

or, v - u = at

or, v = u + at

This proves the first equation of motion.

2. Equation for position-time relation:

Distance travelled by an object in time t is s,

or, s = Area of the trapezium OABE = Area of OADE + Area of ADB

or, s = (OA x OE) + (1/2 x DB x AD) - - - - - - - Eq. (1)

now, DB = BE - DE = v - u = at

putting this value for DB in Eq.(1), we get

s = ut + ½at²

This proves the second equation of motion.

3. Equation for position-velocity relation:

The distance travelled by an object in time t is

s = Area of the trapezium OABE = ½ (EB + OA) x OE = ½ (EB + ED) x OE

or, substituting EB, ED and OE with v, u and t we get,

s = ½ (v + u) t - - - - - - Eq. (2)

But from the first equation of motion we know that

v = u + at

or, t = (v - u) / a

Substituting t in Eq. 2 with this value we get,

s = (v + u) (v - u) ÷ 2a = (v² - u²) ÷ 2a

or, v² - u² = 2as

This proves the third equation of motion.

Q.27: Under what condition will the distance and displacement of a moving object have the same magnitude?

Ans: When the object moves along the same straight line in the same fixed direction.

Q.28: Can the average speed of a moving body ever be zero?

Ans: No, speed being a scalar quantity, is always positive. So the average speed of any moving body can never be zero.

Q.29: Why is the motion of a body in a circular path at a constant speed called accelerated motion?

Ans: The motion is called accelerated motion because the velocity of the rotating body changes continuously due to the change in its direction at every point of its motion.

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