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Physics For Class IX (Science Group)
UNIT 2: KINEMATICS
Self Assessment and Test book Exercise
SELF ASSESSMENT QUESTIONS
Q 1: Define Kinematics.Ans: Kinematics:
The word kinematics is derived from Greek word “Kinema” which means motion.
"Kinematics is the branch of Mechanics which deals with motion of objects without reference of force which causes motion."
Example:
An object changes its position in space in a certain time interval without considering the causes of motion.
Q 2: When is a body said to be in state of rest?
Ans:REST:
A body is said to be in state of rest if it does not change its position with respect to its surroundings.
Example:
- In classroom, various things like, table, chairs, books etc all are in state of rest.
- A parking car is in the state of rest with respect to trees and bushes around it.
- A train is stationed at the platform. A person can notice that the train does not change its position with respect to surroundings; hence the train is in a state of rest.
Q 3: How are rest and motion related to each other?
Ans: Rest And Motion are Relative State:
No body in the universe is in the state of absolute rest or absolute motion. If a body is at rest with respect to some reference point at the same time, it can also be in the state of motion with respect to some other reference point.
For example:
- A Passenger sitting in a moving bus is at rest because passenger are not changing their position with respect to other passengers or objects in the bus. But for another observer outside the bus noticed that the passengers and objects inside the bus are in motion as they are changing their position with respect to observer standing at the road.
- Similarly a passenger flying on aeroplane is in motion when observed from ground but at the same times he is at rest with reference to other passengers on board.
Q 4. Define Translatory Motion?
Ans:Translatory Motion:
"When all points of a moving body move uniformly along the same straight line, such motion is called translatory motion."
Example:
A train is moving along a straight track, we can observe that every part of the train is moving along that straight path.
Q 5. What is vibratory motion?
Ans: Vibratory Motion:
"Back and forth motion of a body about its mean position is called vibratory or oscillatory motion."
Example:
There are many examples of vibratory or oscillatory motion in daily life.
-
Motion of child in swing.
When swing is pulled away from its mean position and then released, the swing start moving back and forth about the mean position. This type of motion is called vibratory or oscillatory motion. - Motion of the clock’s pendulum.
Q 6. Differentiate between translatory motion, rotatory motion and vibratory motion.
Ans: Difference Between Translatory Motion, Rotatory Motion And Vibratory Motion.
| S.NO. | Translatory Motion | Rotatory Motion | Vibratory Motion |
|---|---|---|---|
| 1. | A body moves along a straight line. | The spinning of a body about its axis. | The body move back and forth about mean position. |
| 2. | Movement of an object from one place to another. | The motion of an object about fixed point. | The body moves up and down. |
| 3. | All particles of the rigid body move with the same velocity at every instant of time. | The motion of a rigid body about a fixed axis. Every particle of body move in a circular path | An object repeat its motion itself. |
Q 7. Define Speed.
Ans: See below in section "Types Of Motion" Question (a).
Q 8. What is velocity?
Ans: Ans: See below in section "Types Of Motion" Question (a).
Q 9. Define acceleration.
Ans: See below in section "Types Of Motion" Question (c).
Q 10. Define Vector.
Ans: See below in section "Scalars And Vectors" Question (a)
Q 11. Differentiate with examples between vector and scalar quantities.
Ans: Difference Between Scalar And Vector Quantities.
| S.NO. | Scalars | Vectors |
|---|---|---|
| 1. | Scalar quantities are specified by magnitude only. | Vector quantities are specified by both magnitude and direction. |
| 2. | Scalar quantities change with change in magnitude only. | Vector quantities change either with the change in magnitude or with the change in direction or with the change in both magnitude and direction. |
| 3. | Scalars quantities with the same units can be added or subtracted according to ordinary rules of algebra. | Vectors quantities cannot be added or subtracted by algebra |
| 4. | These are represented by ordinary letters. | These are represented by bold-faced letters or letters have an arrow (⟶) over them. Ā> is read as vector A. |
| 5. | Examples of scalar quantities are speed, temperature, mass, density, time, distance, work and energy etc. | Examples of vector quantities are force ,acceleration , momentum, torque and magnetic field, displacement, velocity and weight etc. |
Test book Exercise
Section (B) Structured Questions
Rest and motion
a) Define rest and motion.Ans:REST:
A body is said to be in rest if it does not change its position with respect to its surroundings.
Example:
- In classroom, various things like, table, chairs, books etc all are in state of rest.
- A parking car is in the state of rest with respect to trees and bushes around it.
- A train is stationed at the platform. A person can notice that the train does not change its position with respect to surroundings; hence the train is in a state of rest.
MOTION:
A body is said to be in motion if it changes its position with respect to its surroundings.
Example:
- A train is stationed at the platform. But as soon as the train starts moving its position continuously changing with respect to its surroundings. Now we can say that the train is in motion.
- A car moving on a road.
b) What is meant by relative motion.
Ans: Rest Ans Motion are Relative State:
No body in the universe is in the state of absolute rest or absolute motion. If a body is at rest with respect to some reference point at the same time, it can also be in the state of motion with respect to some other reference point.
For example:
- A Passenger sitting in a moving bus is at rest because passenger are not changing their position with respect to other passengers or objects in the bus. But for another observer outside the bus noticed that the passengers and objects inside the bus are in motion as they are changing their position with respect to observer standing at the road.
- Similarly a passenger flying on aeroplane is in motion when observed from ground but at the same times he is at rest with reference to other passengers on board.
Types of motion
a) Define speed and velocityAns: SPEED:
The speed of an object determines that how fast an object is moving?
It is rate of change of position of an object.
OR
"Distance covered by the body in unit time is called speed."
It is denoted by V. There are many ways to determine speed of an object. These methods depend on measurement of two quantities.- The distance traveled
- The time taken to travel that distance
Thus the average speed of an object can be calculated as:
Speed = distance traveled / time taken
V = S /t
Unit:Speed is an scalar quantity and its S.I unit is meter per second (m/s or ms-1)
VELOCITY:
Velocity means speed of an object in a certain direction. Thus velocity of an object can be defined as:
"Rate of change of displacement with respect to time is called velocity."
OR
"Distance covered by the body in unit time in a particular direction is called velocity."
It is denoted by V.Formula:
Velocity = Change in displacement / time taken
v = Δd/t
Here,"d" is displacement of the moving object
"t" is time taken by object and
"v" is velocity.
Nature
Velocity is a vector quantity.
Unit:
S.I. unit of velocity is meter per second (m/s or ms-1).
b) What is difference between distance and displacement. OR Differentiate with examples between distance and displacement?
Ans: Differentiated Between Distance and Displacement
| S.NO. | Distance | Displacement |
|---|---|---|
| 1. | The total length covered by moving body without mentioning direction of motion. | The distance measured in straight line in a particular line or direction. |
| 2. | It is an scalar quantity. | It is a vector quantity. |
| 3. | The S.I unit is metre (m). | The S.I unit is metre (m). |
| 4. | Distance covered can never be negative. It is always positive or zero. | Dispalcement may be positive, negative or zero. |
| 5. | Distance between two given points may be the same or different path chosen. | The displacement between two given points is always the same. |
| 6. | It is denoted by 'S'. | It is denoted by 'd'. |
| 6. | It only need magnitude for specification. | It needs not only magnitude but also direction for specification. |
c) Define acceleration?
Ans: ACCELERATION:
An object accelerates when its velocity changes. Since velocity is a vector quantity so it has both magnitude and direction.
Thus acceleration is produced when ever:- Velocity of an object changes
- Direction of motion of the object changes,
- Speed and direction of motion of the object change.
Rate of change of velocity of an object with respect to time is called acceleration.
It is denoted by a.Formula:
Unit:
Acceleration is a vector quantity. Its SI unit is metre per second per second (ms-2).
Positive Acceleration:
When velocity of an object increases or decreases with passage of time, it causes acceleration. The increase in velocity gives rise to positive acceleration. It means the acceleration is in the direction of velocity.
Deceleration:
Acceleration due to decrease in velocity is negative and is called deceleration or retardation. The direction of deceleration is opposite to that of change velocity.
Scalars and vectors
a) Define scalar and vector quantities?Ans: SCALARS:
Definition:
The physical quantities that have magnitude and a suitable unit are called scalar quantities.
OR
Physical quantities, which are completely specified by their magnitude only, are called scalar quantities.
It is denoted by ordinary letter.For example:
- The mass of a watermelon is 3 kg, where 3 is the magnitude and kg is a suitable unit such quantities are called scalar quantities.
- The other examples of scalar quantities are speed, temperature, mass, density, time, distance, work and energy etc.
VECTORS:
Definition:
The physical quantities which are completely specified by magnitude with suitable unit and particular direction are called as “Vector” quantities.
It is denoted by letters with arrow (→) over them.For example:
- A bus traveling with a velocity of 50 ms-1 in the direction of North.
- The other examples of vector quantities are force ,acceleration , momentum, torque and magnetic field, displacement, velocity and weight etc.
b) How represent vector quantities are represented graphically? OR Represent vector quantities by drawing.
Ans: Representation of vector:
Vector diagram is an easy way to represent a vector quantity.
The directed line segment can be used to represent a vector.
- Magnitude: The length of the line segment gives the magnitude of the vector
- Direction: Arrow head gives its direction.
A car travailing at 50ms-1 in the direction of 30° North of East. Its velocity can be represent as:
Motion Due To Gravity
a) Define motion under gravity?Ans: MOTION DUE TO GRAVITY OR FREE FALL MOTION OR ACCELERATION DUE TO GRAVITY:
“When a body falls in such a way that no other force accepts the weight acting on it, then such motion is called free fall motion or motion under gravity. Its velocity increases continuously till it strikes the ground and then a body get some acceleration which is called acceleration due to gravity"
It is define as follow:
“The acceleration produces in a free falling body due to force of gravity is called acceleration due to gravity.”
Acceleration due to gravity is a constant and it is denoted by "g".Its value near the surface of earth is found to be 9.81 ms-2. However for ease of calculation value of ‘g’ is approximated to 10 ms-2.
b) Why gravity is taken negative for an object moving in upward direction?
Ans: Gravitational acceleration or gravity is taken negative for objects moving upward direction. Because if a body moving upwards, the acceleration due to gravity is downward and hence, it acts in opposite direction of the velocity. So it is considered negative.
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