Select Page

## Notes

#### Q1.    Define kinematics?

Ans:   Kinematics:

Kinematics is the study of the motion of an object without discussing the cause of motion.

#### Q2.    Difference between rest and motion?

Ans:   Difference between rest and motion:

Rest:

A body is said to be at rest if it does not change its position with respect to its surroundings.

Motion:

A body is said to be in motion if it changes its position with respect to its surroundings.

The state of rest or motion of a body is relative. For example, a passenger sitting in a moving bus is at rest because he/she is not changing his/her position with respect to other passengers or objects on the bus. But to an observer outside the bus, the passengers and the objects inside the bus are in motion.

#### Q3.    Define the surroundings?

Ans:   Surroundings:

Surroundings are the places in their neighbourhood where various objects are present. Similarly

#### Q4.    List the types of motion?

Ans:   Types of motion:

There are three types of motion.

1. Translatory motion (linear, random and circular)
2. Rotatory motion
3. Vibratory motion (to and fro motion)

#### Q5.    Describe translator motion with the help of examples?

Ans:   Translatory motion:

In translational motion, a body moves along a line without any rotation. The line may be straight or curved.

Examples:

Riders moving in a Ferris wheel are also in translational motion. Their motion is in a circle without rotation.

#### Q6.    Describe the different types of translator motion?

Ans:   Types of translator motion:

Translator motions can be divided into linear motion, circular motion and random motion.

• Linear motion:

The straight-line motion of a body is known as its linear motion.

Examples:

The motion of objects such as a car moving on a straight and level road is linear motion.

Aeroplanes flying straight in air and objects falling vertically down are also the example of linear motion.

• Circular motion:

The motion of an object in a circular path is known as circular motion

Examples:

A stone tied at the end of a string can be made to whirl. The stone moves in a circle and thus has circular motion.

A toy train moving on a circular track. A bicycle or a car moving along a circular track possesses circular motion. The motion of the Earth around the Sun and the motion of the moon around the Earth are also examples of circular motions.

• Random motion:

The disordered or irregular motion of an object is called random motion.

Examples:

The motion of insects and birds are irregular. Thus, the motion of insects and birds is random motion.

The motion of dust or smoke particles in the air is also random motion.

The Brownian motion of a gas or liquid molecules along a zig-zag path is also an example of random motion.

#### Q7.    Describe rotatory motion with the help of examples?

Ans:   Rotatory motion:

The spinning motion of a body about its axis is called its rotatory motion.

Examples:

The top spins about its axis passing through it and thus it possesses rotatory motion. An axis is a line around which a body rotates. In a circular motion, the point about which body moves about which a body goes around is outside the body. In rotatory motion, the line, around which a body moves about, is passing through the body itself.

The motion of a wheel about its axis and that of a steering wheel are the examples of rotatory motion. The motion of the Earth around the Sun is circular and not the spinning motion. However, the motion of Earth about its geographic axis that causes day and night is rotatory.

#### Q8.    Can you point out some differences in circular and rotatory motion?

Ans:   Differences in circular and rotatory motion:

Any turning as if on an axis is rotatory motion.  Any rotatory motion where the radius of gyration length and the axis of rotation are fixed is circular motion. And that’s the difference. Circular motion is just a special case of rotatory motion. That is, there is no fixed axis and radius restriction for rotatory motion, but there is for circular motion.

For example, all planets have rotatory motion around their suns. But most of the orbits are elliptical, so the rotation axis (there are two in an ellipse) and radii of gyration vary as they trek around. So most, if not all, planets do not have circular motion.

Note:

Gyration length:

A length that represents the distance in a rotating system between the point about which it is rotating and the point to or from which a transfer of energy has the maximum effect.

1. #### When a body is said to be at rest?

Ans: A body is said to be at rest if it does not change its position concerning its surroundings.

1. #### Give an example of a body that is at rest and is in motion at the same time.

Ans: Motion and rest are relative concepts. There is no absolute rest. We can define the state of rest or motion only concerning another object or a point in space taken as reference.

Examples:

• A person inside a train considers himself to be at rest concerning the fellow passengers or the walls of the train. But when he looks outside, he finds himself to be in motion concerning the trees outside.
• A passenger sitting in a moving bus is at rest because he/she is not changing his/her position with respect to other passengers or objects inside the bus. But to an observer outside the bus, the passengers and the objects inside the bus are in motion.

1. #### Mention the type of motion in each of the following:

• A ball moving vertically upward.

Ans: Linear motion.

• A child moving down a slide.

Ans: Linear motion.

• Movement of a player in a football ground.

Ans: Random motion.

• The flight of a butterfly.

Ans: Random motion.

• An athlete running in a circular track.

Ans: Circular motion.

• The motion of a wheel.

Ans: Circular motion.

• The motion of a cradle.

Ans: Vibratory motion.

#### Q9.    Describe vibratory motion with the help of examples?

Ans:   Vibratory motion:

To and fro motion of a body about its mean position is known as vibratory motion.

Examples:

Consider the motion if a baby about in a swing. As it is pushed, the swing moves back and forth about its mean position. The motion of the baby repeats from one extreme to the other extreme with the swing. Such type of motion is called vibratory motion.

To and fro motion of the pendulum of a clock about its mean position, it is called vibratory motion.

A baby in a cradle moving to and fro, to and fro motion of the hammer of a ringing electric bell and the motion of the string of a sitar are some of the examples of vibratory motion.

#### Q10.  Differentiate between scalars and vectors?

Ans:   Differentiate between scalars and vectors

 Scalars Vectors A scalar quantity is described completely by its magnitude only. A vector quantity is described completely by magnitude and direction Examples: Examples of scalars are mass, length, time, speed, volume, work, energy, density, power, electric charge, pressure, area, temperature. Examples: Examples of vectors are velocity, displacement, force, momentum, torque, weight, electric potential etc.

#### Q11.  How can vector quantities be represented graphically?

Ans:   Representation of vectors (Symbolic representation of a vector):

To differentiate a vector from a scalar quantity we generally use bold letters to represent vector quantities, such as F, a, d or a bar or arrow over their symbols such as

Vector representation/Graphical representation of a vector:

A straight line is drawn with an arrow head at one end. The length of the line according to some suitable scale, represents the magnitude and the arrow head gives the direction of the vector.

#### Q12.  Define the term position?

Ans:   Position:

The term position describes the location of a place or a point concerning some reference point called the origin.

For example:

You want to describe the position of your school from your home. Let the school be represented by S and home by H. The position of your school from your home will be represented by a straight-line HS in the direction from H to S.

#### Q13.  Explain the difference between distance and displacement?

Ans.   Difference between distance and displacement:

 Distance Displacement Length of a path between two points is called the distance between those points. Displacement is the shortest distance between two points which has magnitude and direction. Distance is a scalar quantity. Displacement is a vector quantity. Distance is denoted by “S” S = vt Its SI unit is metre (m). Displacement is denoted by “d” D = vt Its SI unit is metre (m). Distance S (dotted line) and displacement d(dark line) from points A to B,

#### Q14.  What is the difference between speed and velocity?

Ans.   Difference between speed and velocity:

 Speed Velocity The distance covered an object in unit time is by called its speed. Distance = speed x time Or        S = vt The rate of displacement of a body is called its velocity. V = d/t       or d = vt Speed is a scalar quantity. Velocity is a vector quantity. Sl unit of speed is metre per second. (ms-1) Sl unit of velocity is the same as speed i.e. metre per second. (ms-1)

##### DO YOU KNOW

 Which is the fastest animal on the earth? Falcon can fly at a speed of 200 kmh-1 Cheetah can run at a speed of 70 kmh-1 A LIDAR gun is light detection and ranging speed. It uses the time taken by a laser pulse to make a series of measurements of a vehicle’s distance from the gun. The data is then used to calculate the vehicle’s speed. A paratrooper attains a uniform velocity called terminal velocity with which it comes to the ground.

#### Q15.  Define uniform speed.

Ans.   Uniform speed:

A body has uniform speed if it covers equal distances in equal intervals of time however short the interval may be.

#### Q16.  Define variable speed?

Ans.   variable speed:

If a body covers unequal distances in equal interval of time, however, small the interval may be, the speed of the body is said to be variable.

#### Q17.  Define average speed?

Ans.   Average speed:

The ratio between distance and total time has taken is known as average speed.

#### Q18.  Define uniform velocity?

Ans.   Uniform velocity:

A body has uniform velocity if it covers equal displacement in equal intervals of time however short the interval may be.

#### Q19.  Define the variable velocity?

Ans.   variable velocity:

If speed or direction changes with time then the velocity of such a body is said to be variable.

#### Q20.  Define average velocity?

Ans.   Average velocity:

The ratio between displacement and time is known as the average velocity

#### Q21.  Define acceleration?

Ans.   Acceleration:

Acceleration is defined as the rate of change of velocity of a body.

Unit of acceleration:

SI unit of acceleration is metre per second squared (ms-2)

##### USEFUL INFORMATION

Acceleration of a moving object is in the direction of the velocity of its velocity is increasing. Acceleration of the object is opposite to the direction of velocity if its velocity is decreasing.

##### Q22.  Define uniform acceleration?

Ans.   Uniform acceleration:

A body has uniform acceleration if it has equal changes in velocity in equal intervals of time however short the interval may be.

#### Q23.  Differentiate between positive and negative acceleration?

Ans:   Positive acceleration:

Acceleration of a body is positive if its velocity increases with time. The direction of this acceleration is the same in which the body is moving without a change in its direction.

Negative acceleration/Deceleration or retardation:

Acceleration of a body is negative if the velocity of the body decreases. The direction of negative acceleration is opposite to the direction in which the body is moving. Negative acceleration is also called deceleration or retardation.

##### DO YOU KNOW

A graph may also be used in everyday life such as to show year-wise growth/decline of export, month-wise rainfall, a patient’s temperature record or runs per over scored by a team and so on.

#### Q24.  What do you mean by the graph, variables, independent quantity and dependent quantity?

Ans:   Graph:

The graph is a pictorial way of presenting information about the relationship between various quantities.

Variables:

The quantities between which a graph is plotted are called the variables.

Independent quantity:

One of the quantities is called independent quantity.

Dependent quantity:

The values of which varies with the independent quantity are called dependent quantity.

#### Q25. What is the purpose of the distance-time graph? How it is plotted?

Ans: Distance-time graph:

It is useful to represent the motion of objects using graphs. The terms distance and displacement are used interchangeably when the motion is in a straight line. Similarly, if the motion in a straight line then speed and velocity are also used interchangeably

Note:

In a distance-time graph, time is taken along the horizontal axis while the vertical axis shows the distance covered by the object.

#### Q26. Sketch a distance-time graph for a body at rest. How will you determine the speed of a body from this graph?

Ans: Object at rest:

In the graph shown in the figure, the distance moved by the object with time is zero. That is, the object is at rest. Thus, a horizontal line parallel to the time axis on a distance-time graph shows the speed of the object is zero.

#### Q27. Plot and interpret a distance-time graph for a body moving with constant speed?

Ans: Object moving with constant speed:

The speed of an object is said to be constant if it covers equal distances in equal intervals of time. The distance-time graph as shown in the figure is a straight line.

Its slope gives the speed of the object.

Consider two points A and B on the graph.

Speed of the objects = slope of line AB

The speed found from the graph is 2 ms-1

#### Q28.  Sketch a distance-time graph for a body moving with variable speed?

Ans:   Object moving with variable speed:

When an object does not cover equal distances in equal intervals of time then its speed is not constant. In this case, the distance-time graph is not a straight line as shown in the figure. The slope of the curve at any point can be found from the slope of the tangent at that point. For example.

Thus, the speed of the object at P is 3 ms-1

Note:

The speed is higher at instants when the slope is greater, speed is zero at instants when the slope is horizontal.

#### Q29.  What do you mean by the speed-time graph?

Ans:   speed-time graph:

In a speed-time graph, time is taken along x-axis and speed is taken along the y-axis.

#### What would be the shape of a speed-time graph of a body moving with constant speed?

Ans:   Object moving with constant speed

When the speed of an object is constant (4ms-1) with time, then the speed-time graph will be a horizontal line parallel to time-axis along the x-axis.

A straight line parallel to time-axis represents the constant speed of the object.

#### What would be the shape of a speed-time graph of a body moving with uniformly changing speed?

Ans:   object moving with uniformly changing speed (uniform acceleration):

Let the speed of an object be changing uniformly. In such a case speed is changing at a constant rate. Thus, its speed-time graph would be a straight line such.

A straight line means that the object is moving with uniform acceleration. The slope of the line gives the magnitude of its acceleration.

Speed-time graph gives a negative slope. Thus, the object has deceleration of 0.4ms-2.

#### What would be the shape of a speed-time graph for distance travel by a moving object?

Ans:   Distance travelled by a moving object:

The area under a speed-time graph represents the distance travelled by the object. If the motion is uniform then the area can be calculated using the appropriate formula for geometrical shapes represented by the graph.

#### Q33.  Describe the purpose of different equations of motion?

Ans:   Equations of motion:

There are three basic equations of motion for bodies moving with uniform acceleration. These equations relate initial velocity, final velocity, acceleration, time and distance covered by a moving body.

#### Derive the equation of motion which is independent of distance S.

Ans: Suppose a body is moving with initial velocity vi, and after time t its velocity becomes vf. Then acceleration a is given by

Or            Vf – Vi = at

Vf = Vi + at

Second Method (Graphical method):

The first equation of motion:

Speed-time graph for the motion of a body is shown in the figure. The slope of line AB gives the acceleration of a body.

As

BD = Vf,      CD = Vi and     OD = t

Hence

Or                                                          Vf – Vi = at

Vf = Vi + at

#### Prove that   S = v1 t + ½ at2

Ans: Suppose a body is moving with initial velocity vi and after a certain time t its velocity becomes vf then the total distance S covered in time t, is given by

S = vav x t

) x t        …………………..(1)

From the first equation of motion.                 Vf = Vi + at

Putting the value of Vf in equation (1).

) x t

) x t

)

S = V.t + ½ at2

Second Method (Graphical method):

The second equation of motion:

In the speed-time graph shown in the figure, the total distance S travelled by the body is equal to the total area OABD under the graph. That is

Total distance S = area of (rectangle OACD + triangle ABC)

Area of rectangle OACD = OA x OD

= Vi x t

Area of triangle ABC       = ½ (AC x BC)

= ½ t x at

Since Total area OABD = area of rectangle OACD + area of triangle ABC

Putting values in the above equation, we get

S = Vit + ½ t x at

S = Vit + ½ at2

#### Prove that   2aS = vf2 – vi2

Ans:   Suppose a body is moving with initial velocity vi and after a certain time t its velocity becomes vf then the total distance S covered in time t, is given by

S = vav x t

) x t        …………………..(1)

From the first equation of motion find the value of t.

Vf = Vi + at    or

Putting the value of Vf in equation (1).

2aS = (vf + vi) x (vf – vi)        by using formula (a + b)(a – b) = a2 – b2

2aS = vf2 – vi2

Second Method (Graphical method)

Third equation of motion:

In speed-time graph shown in figure, the total distance S travelled by the body is given by the total area OABD under the graph.

Total area OABD =

Or                               2S = (OA + BD) x OD

Multiply both sides by BC/OD, we get:                         (BC/OD = a)

2S x  = (OA + BD) X OD X

2S x  = (OA + BD) X BC   …………………….(1)

Putting the value in the above equation (1), we get

2S x a = (Vi + Vf) x (Vf – Vi)

2aS = Vf2+ Vi2

##### USEFUL INFORMATION

• To convert ms-1 to kmh-1

1 ms-1 = 0.001km x 3600 = 3.6 kmh-1

Thus, multiply speed in ms-1 by 3.6 to get speed in kmh-1 e.g.,

20 ms-1= 20 x 3.6 kmh-1=72 kmh-1

• To convert kmh-1 to ms-1

1 kmh-1 =  ms-1

Thus, multiply speed in kmh-1 by  to get speed in ms-1 e.g.

50 kmh-1 = 50 x   ms-1 = 13.88 ms-1

• To convert ms-2 to kmh-2

Multiply acceleration in ms-2 by = 12960 to get its value in kmh-2

• To convert kmh-2 to ms-2

Divide acceleration in kmh-2 by 12960 to get its value in ms-2

#### Q37. Drop an object from some height and observe its motion. Does its velocity increase, decrease or remain constant as it approaches the ground?

Ans: Velocity of an object will increase due to earth gravity. That is why for bodies falling freely g is positive.

#### Q38. Explain the motion of freely falling bodies?

Ans: Motion of freely falling bodies:

The acceleration of freely falling bodies is called gravitational acceleration.

It is denoted by g. On the surface of the Earth, its value is approximately 10 ms-2.

For bodies falling down freely, g is positive and is negative for bodies moving up.

Galileo was the first scientist to notice that all the freely falling objects have the same acceleration independent of their masses. He dropped various objects of different masses from the leaning tower of Pisa. He found that all of them reach the ground at the same time.

#### Q39. Write equations of motion for bodies moving under gravity?

Ans: Equations of motion for bodies moving under gravity:

1. Vf = Vi + gt
2. h = Vit + ½ gt2
• 2gh = Vf2 = Vi2

##### SUMMARY

1. Rest: A body is said to be at rest if it does not change its position concerning its surroundings.
2. Motion: A body is said to be in motion if it changes its position with respect to its surroundings.
3. Rest and motion are always relative. There is no such thing as absolute rest or absolute motion.
4. Motion can be divided into the following three types:

Translatory motion:    In which a body moves without any rotation.

Rotatory motion:        In which a body spins about its axis.

Vibratory motion:       In which a body moves to and fro about its mean position.

1. Scalars: Physical quantities which are described by their magnitude only are known as scalars.
2. Vectors: Physical quantities which are described by their magnitude and direction are known as vectors.
3. Position: Position means the location of a certain place or object from a reference point.
4. Speed: The distance travelled in any direction by a boy in unit time is called speed.
5. Uniform speed: if the speed of a body does not change with time then its speed is uniform.
6. Average speed: Average speed of a body is the ratio of the total distance covered to the total time taken
7. Velocity: We define velocity as the rate of change of displacement or speed in a specific direction.
8. Average velocity: Average velocity of a body is defined as the ratio of its net displacement to the total time.
9. Uniform velocity: If a body covers equal displacements in equal intervals of time, however small the interval may be, then its velocity is said to be uniform.
10. Acceleration: The rate of change of velocity of a body is called acceleration.
11. Uniform acceleration: A body has uniform acceleration if it has equal changes in its velocity in equal intervals of time, however small the interval may be.
12. Graph: Graph is a pictorial way of describing information as to how various quantities are related to each other.
13. The slope of the distance-time graph gives the speed of the body.
14. Distance-time graph: Distance-time graphs provide useful information about the motion of an object. The slope of the displacement-time graph gives the velocity of the body.
15. Distance covered by a body is equal to the area under the speed-time graph.
16. Speed-time graph: Speed-time graph is also useful for studying motion along a straight line.
17. Velocity-time graph: The distance travelled by a body can also be found from the area under Velocity-time graph if the motion is along a straight line.
18. Equations of motion for uniformly accelerated motion are:
• Vf = Vi + at
• S = Vit + ½ at2
• 2aS = Vf2 = Vi2
1. Acceleration due to gravity: When a boy is dropped freely it falls with an acceleration towards the earth. This acceleration is called acceleration due to gravity and is denoted by g. the numerical value of g is approximately 10 ms-2 near the surface of the Earth.