Class 8 BASIC SCIENCE

Chapter 1

2

MOTION

3

PREFACE Learning

science

is

crucial

to

understand

the

physical reality of the universe. Science learning can be made an effortless activity if we utilize the right learning materials for getting thorough grasp of the subject. This book is based on the topic

‘Motion’.

The

aim

of

this

material

is

to

make the students better understand the concepts of

motions,

different

types

of

motion

and

parameters related to it. An effective summary of the

chapter

is

given

at

the

end.

Besides,

exercises related to the chapter are given at the end to thoroughly master the concepts by the students. Solving problem is an inevitable part of learning science that helps the students to apply the concepts they have learnt so far. The text is 4

written in simple and lucid language to aid in effortless

reading

Extended

activities

experience given

at

for the

the end

students. provide

opportunity for students to continue their learning outside the classroom.

5

Contents Motion………………………………………………………………...…7 Distance and Displacement……………………………………………10 Speed and Velocity…………………………………………………….13 Uniform speed and Non-uniform speed…………..……………………17 Uniform velocity and Non-uniform velocity…………………..………20 Acceleration……………………………………………………………21 Type of motion…………………………………………………………23 Rectilinear motion Circular motions Periodic motions Oscillatory motion Rotational motion Projectile motion

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MOTION Observe the figure given below

Are these objects in motion or at rest? Which are the objects that seem to be in motion? And which is not in motion?

7

You identified that cars parked are not in motion and the car in the road and the birds are moving. But, are you sure that the cars in the parking area not moving? Are they really at rest? let’s examine.

Stationary objects Objects which remains fixed at a place eg tree, house, school, factory are called stationary objects

But how can we sure about these objects being stationary? It can be explained with a reference body. We can state whether an object is in a state of rest or motion only with reference to another object.

Reference body Reference body is the object with respect to which the state of rest or motion of an object is described

Then what do you think about a passenger in a moving train with respect to the train is in motion or in rest? What is the state if it considers with respect to the track? 8

A body is said to be in motion if it moves from one position to another. A common characteristic of all the moving bodies is that they change their position with time. An object is said to be in motion when its position changes with time.

The picture shows that a body is moving from A to B in two different paths. Do they travel same distance in both these ways? What do you think about this?

9

Distance and displacement

Distance Distance is the length of the path travelled

Displacement Displacement is the straight line distance from the initial position to the final position; it has both magnitude and direction. Its unit is meter (m). 10

How can my displacement be zero when I travelled a total distance of 1100m?

What do you think about Anya’s doubt? Can displacement be zero?

Displacement is the distance between final position and initial position in a straight line. If an object returns back to the initial position where from its start its total displacement becomes zero. Why is it necessary to specify the direction while stating the displacement?

11

While stating the displacement, it is necessary to indicate the direction along with the magnitude of the distance travelled. Such physical quantities having both magnitude and direction are referred to as vector quantities. Physical quantities of which the direction is not to be indicated are scalar quantities.

• When a body travels along a straight line in the same direction, the magnitude of its distance and displacement will be equal. Now we know that distance and displacement are not same. Can you list out some differences Distance

Displacement

Distance is a scalar quantity

Displacement is a vector quantity

Distance depends upon the path

Displacement doesn’t depend upon the path. It depends on the starting and end point

It is not necessary that distance will

It is zero if distance is zero, but it can

be zero if displacement is zero

be zero even if distance is not zero

Distance and displacement have the same unit.

12

Speed and velocity Speed is the distance travelled in unit time. Speed = distance/time

What is the unit of speed? Is it a derived unit or a fundamental unit?

Unit of distance = meter Unit of time = second Unit of speed = meter/second The table 1.2 shows the dispalcement covered by Tara, Dev, Neena and Robin

students

displacement

Time taken

Dev Tara Neena Robin

300m 400m 700m 725m

102s 88s 112s 110s

Who covered more displacement in unit time? Can you define velocity based on displacement and time?

13

Velocity is the displacement in unit time Displacement is the straight line distance from the initial position to the final position irrespective of the path taken. While calculating velocity this straight line distance itself is the displacement, even if it is not the actual path taken. But the time taken to cover the actual distance should be used for the calculation

Velocity = displacement/time

What is the unit of velocity? Unit of displacement = meter Unit of time = second Unit of velocity = meter/second

Now do you understand what means velocity and displacement?

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The unit of speed and velocity are same but teacher said that they are different quantities, how?

What would be the reason behind this?

Speed is a scalar quantity whereas velocity is a vector quantity. Speed simply tells us how fast an object is moving. Velocity indicates the speed in a given direction

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speed

Velocity

It is the distance covered by the body per unit time

It is the displacement of the body per unit time

Speed is a scalar quantity

Velocity is a vector quantity

It cannot be zero

It can be zero

Speed of a moving body can never be negative

Velocity of a moving body can be negative, positive or zero

Do you know? Nautical

mile

is

the

unit

for

measuring the distance in the field of aviation and sea transportation. one nautical mile is 1,852 km. the unit for measuring the speed of aero planes and ships is knot. One knot is the speed taken to travel one nautical mile in one hour.

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Uniform speed and non-uniform speed

Don’t you know what is in this picture? This is a speedometer used in vehicles that indicates the speed of the vehicle. What is uniform speed?

If a body in motion covers equal distances in equal interval of time, the body is said to have uniform speed. If it covers unequal distances in equal interval of time, then it is said to have non uniform speed The following figure shows the journey of a car from A to E. complete the table based on the journey of the car 17

A

B

C

D

Distance travelled

E

time

A to B

200m

10s

B to C

20m

10s

C to D

200m

10s

D to E

200m

10s

speed

Is the distance travelled by the car in equal intervals of time the same? Is the speed of the car a constant at all intervals? Therefore, the car has uniform speed. But in most of the cases the speed is not uniform. Didn’t you see the readings in the speedometer of the vehicles? The speedometer of a vehicle displays different speeds at different times how can the speed of vehicles be determined in such situations? Average speed is the only way to determine the speed of vehicles having non uniform speed

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The average speed is the total distance travelled by an object in a particular time interval.

Average speed =total distance travelled Total time taken

Can you help me to find the average speed of my car?

Q: John drove 3 hours at a rate of 50 km/hr and for 2 hours at a rate of 60kmph. What was his average speed for the whole journey?

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Uniform velocity and non-uniform velocity A body has uniform velocity if it covers equal displacement in the same direction in equal intervals of time. A body has non uniform velocity when it covers unequal displacement in equal interval of time. Velocity of an object is changed in following two conditions 1) When speed is changed 2) When direction is changed Thus, non uniform speed of a moving object in same direction or nonuniform speed in different direction or uniform speed in different direction is called non uniform velocity. Example: if a vehicle is moving on a circular path with uniform speed, then its velocity is said to be non-uniform, because on a circular path the direction of moving body changes along with the directions of the curve. If a vehicle moving with uniform speed on a jig-jag path, the velocity of the vehicle will be non-uniform because direction of vehicle s changed with the change of direction of path

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If moving vehicle covers a distance of 10km in the first hour, covers a distance of 20km in the third hour etc. The motion of the vehicle is non uniform.

Acceleration

You have experienced the velocity of a vehicle changing while travelling. Both the speed and direction of vehicle changes during travel. Do you know what is shown in the picture? The device is called an accelerator which is used to bring about change in the velocity of vehicles.

Acceleration is the rate change in velocity Acceleration = change in velocity Time a=

v-u t

where a is acceleration u is initial velocity v is final velocity 21

t is time taken Acceleration is a vector quantity

The unit of acceleration is m/s2

Acceleration due to gravity Earth attracts every object towards its centre. This produces acceleration in an object in free fall. The acceleration produced in an object falling down freely due to the gravitational force of the earth is the acceleration due to gravity at that place. The acceleration due to gravity near the earth surface has a value of 9.8 m/s2. It is denoted by the letter ‘g’

Retardation Retardation means negative acceleration. The velocity of a body may increase or decrease. Acceleration takes place when the velocity of a body changes. So when the velocity of a body increases the acceleration is said to be negative. A body is said to be retarded if its velocity is decreasing. Example: a train is said to be retarded when its slow down on reaching a station, that is the velocity decreases. Retardation is acceleration with negative sign. 22

Types of motion Rectilinear motion Motion along a straight line is called rectilinear motion.

When a bullet is fired from a gun, the bullet moves in a straight line path. So, the movement of a bullet fired from a gun is an example of rectilinear motion.

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• The march past of soldiers in a parade is rectilinear motion • The motion of a cyclist running on a straight road is rectilinear motion • The motion of a sprinter (short distance runner) running on a straight track is also called rectilinear motion. • Motion of a vehicle on a straight road ,motion of a striker on the carom board , motion of a train on a straight track, motion of a falling stone, movement of the drawer of a table , motion of a boy sliding down a slope .

Rectilinear motion takes place in fixed direction

Iam in rectilinear motion

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Circular motion When an objects moves along a circular path, it is called circular motion.

Most of us like to play in an amusement park, don’t we? But did you ever noticed that there are many rides in the amusement park moving in a circular path Don’t you know giant wheel? Giant wheel moves in a circular motion

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Examples • The moon moves around the earth in a circular path, the movement of moon around the earth is an example of circular motion. • The movement of artificial satellites around the earth is also circular motion. • The movement of earth around the sun is also an example of circular motion. • The movement of all the planets around the sun is circular motion. • The movement of tip of the hand of a watch on the dial. • The ceiling fans remains at one place but its blades rotate in circular motion.

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Periodic motion

The motion which repeats itself after regular intervals of time is called periodic motion. Examples: • The motion of second hand of a watch is repeated after regular intervals of time, the motion of second’s hand of a watch is an example of periodic motion. • The revolution of earth around the sun is periodic motion because the earths always take the same amount of time to complete one round around the sun. • The rotation of earth on its axis is a periodic motion because the earth always takes the same time to rotate once on its axis.

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• The revolution of moon around the earth is also a periodic motion because the moon always takes the same time to complete one round around the earth. • Swinging of a pendulum; motion of pendulum of a “pendulum clock”, motion of a child on a swing, motion of the branch of a tree moving to and fro. • The orbiting of a satellite around the earth, the vibration of stretched membrane of a drum (dholak or Tabla), when struck.

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Oscillatory motion Tabla or drums have thin stretched membrane tied over the top of their open ends. If we place some tiny pieces of paper on the stretched membrane of a drum (tabla) and strike the membrane of the drum and strike membrane gently, the pieces of paper starts dancing up and down on the membrane of the drum. This is because when we strike the stretched membrane of the drum, the membrane starts vibrating by small distances. The tiny pieces of paper moves up and down with the vibrating drum membrane and hence appear to be dancing on it. Thus, when a drum is struck, its membrane vibrates and shows periodic motion. The periodic motion shown by swing, pendulum, stretched strings and stretched membrane is actually a special type of periodic motion which is more commonly known as vibrating motion or oscillatory motion

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Rotational motion When an object turns or spins about a fixed axis, it is called rotational motion For example, The motion of a spinning top The spinning of earth on the axis Turning of the blades of a fan Rotation of a windmill Turning of a ball Turning of the hands of watch Turning of potter’s wheel Turning of a bicycle wheel

The difference between circular motion and rotational motion is that in circular motion an object as a whole travels along a circular path but in rotational motion, the objects spins on its axis.

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Observe these pictures

Now you understand about the motion, don’t you? 32

Objects which can have more than one type of motion 1. Example of earth: the earth moves around the sun, so it has circular motion, the earth repeats its motion around the sun after regular intervals of time (of 1year) so it has periodic motion and earth also rotates on its axis, so it has rotational motion. Thus the earth has three type of motion at the same time that is circular motion, periodic motion and rotational motion. 2. Example of merry goes around: if we consider merry go around as a whole, then it shows a rotational motion because it turns on an axle. But the outer part of the merry go around shows circular motion. So, a merry go around has two types of motion at the same time that is rotational motion and circular motion.

I have three types of motion?

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Projectile motion

Projectile motion is the motion of an object thrown (projected) into the air when, after the initial force that launches the object, air resistance is negligible and the only other force that object experiences is the force of gravity. The object is called a projectile, and its path is called its trajectory.

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Examples of projectile motion Firing a canon Throwing a basketball in the basket Archery Javelin throw Water escaping a hose car and bike stunts

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Simple pendulum

The simple pendulum is another mechanical system that moves in an oscillatory motion. It consists of a point mass m suspended by means of a string of length l from a fixed support.

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Physical quantities learnt so far

Physical

Equation or

quantities

definition

Distance

Path length

unit

type

m

Scalar

m

Vector

travelled Displacement

Straight line distance from initial to final position

Speed

Distance/time

m/s

Scalar

velocity

Displacement/time

m/s

Vector

Average speed

Total distance

m/s

scalar

travelled/total time taken

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CHAPTER SUMMARY •

Reference body is the object with respect to which the state of rest or motion of an object is described.

•

Distance is the length of the path traveled. Its unit is meter.

•

Displacement is the straight line distance from the initial position to the final position. It has both magnitude and direction, its unit is meter.

•

Physical quantities have both magnitude and direction refered to as vector quantities. physical quantities of which the direction is not to be indicated are scalar quantities.

•

Displacement is a vector quantity and distance is scalar quantity.

•

When a body travels along a straight line in the same direction, the magnitude of its distance and displacement will be equal.

•

Speed is the distance travelled in unit time.

•

Velocity is the displacement in unit time

•

If a body in motion covers equal distance in equal intervals of time, the body is said to have uniform speed. If it covers unequal distance in equal intervals of time, then it is said to have non uniform speed 38

•

The average speed is the total distance travelled by an object in a particular time interval.

•

A body has uniform velocity if it covers equal displacement in the same direction in equal intervals of time.

•

Acceleration is the rate of change in velocity

•

Negative acceleration is called retardation

•

Different types of motion includes rectilinear motion, circular motion, periodic motion, rotational motion, oscillatory motion and projectile motion.

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Exercises 1. Find the odd one out and give reason Velocity, acceleration, retardation, average speed 2. Why displacement cannot be greater than distance? 3. What is the displacement of a car in 50s if it is moving with a velocity of 500 m/s? 4. A person travelled 5m to the north, the 20m to the east, 5m to the south and finally 10 m to the west. Find the distance and displacement of the person? 5. All objects having uniform speed need not have uniform velocity. Describe with the help of example 6. An object moving along a straight line path. It travels with an a speed 2m/s for 20s and 12m/s for ts. If the total average speed across the whole path is 10 m/s. then find the unknown time t. 7. A car travelling with a velocity of 50 km/hr came to rest in 20s. Find the acceleration of the car and evaluate your answer. 8. Is the motion of an object under free fall uniform or non uniform? Justify your statement.

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Extended activities 1. Write a short note on the significanve of the vector quantities in physics. 2. Does a body undergo retardation whenever the magnitude of velocity decreases. (hint: acceleration is a vector quantity ) 3. Prepare posters based on the topic road safety.

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notes ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………………………………… ……………………………….................................... 42