NCERT Class 9 Science Exploration Chapter 04 Describing Motion Around Us PDF Download

Chapter 4: Describing Motion Around Us

Everything in nature is in motion, from massive astronomical objects to subatomic particles. And what a variety in motion we have in nature - flitting butterflies, slithering snakes, hopping hares, galloping horses, tendrils of climbers twinning around a support, closing of flytraps, dancing dust particles in a sunbeam, smoke particles moving in air, rising and falling of ocean tides, and gathering clouds!

Isn't motion in nature wonderful? But how do we study the wide variety of complex motions around us? As you have read in the first chapter, to explore a complex phenomenon, scientists first study it in its idealised simplified forms. Such types of motion are linear, circular, and oscillatory about which you learnt in earlier grades. In this chapter, you will learn more about linear motion (motion in a straight line) and uniform circular motion.

Earlier, you learnt about some physical quantities, such as distance, time and speed. Now, you will learn about some more physical quantities, such as displacement, average velocity and average acceleration. You will also learn to describe motion not only in words, but also with numbers, equations and graphs.

Think It Over

• How much distance should we maintain from the truck ahead to avoid a collision if it suddenly applies the brakes?
• Does this distance depend upon the speed with which we are moving?

4.1 Motion in a Straight Line

You have learnt that when an object moves in a straight line, its motion is called linear motion. It can also be called motion in a straight line. It is the simplest kind of motion. Have you noticed it around you, such as children in a swimming race, a vertically falling ball, a car moving along a straight stretch of a highway or a train moving on a straight track (Fig. 4.1)?

[Figure 4.1: Objects in a straight line motion, See in your textbook]

To discuss about the motion of an object, you first need to describe its position at various instants of time.

4.1.1 Describing position

How do we describe the position of an object? For that, as you learnt earlier, we first need to specify a fixed point as the reference point. The distance and direction of the object with respect to the reference point, at any instant of time, describes the position of the object at that instant of time. Note that apart from the distance, we also specify the direction from the reference point in which the object is located to describe its position. And when do we say that an object is in motion? If the position of the object with respect to the reference point changes with time, the object is said to be in motion. On the other hand, the object is said to be at rest if its position with respect to the reference point does not change with time.

Teacher's Note

Position, motion, and rest all depend on the reference point you choose. If you pick the ground as your reference point, a person in a moving car is in motion. But if you pick the car itself as your reference point, that person is at rest. Always be clear about which reference point you are using when describing motion.

Let us take the example of an athlete running on a straight track (Fig. 4.2).

[Figure 4.2: An athlete running on a straight track, See in your textbook]

To describe the position of the athlete, let us take her starting point as the reference point. As shown in Fig. 4.3, let us make a straight line with distances marked on it and mark the reference point on it as the origin 'O'. The athlete starts running from O, and her positions at two instants of time are marked by points B and A.

[Figure 4.3: Reference point and positions of the athlete at different instants of time on a straight line, See in your textbook]

To describe the position of an object, we also need to specify its direction. For the object moving in a straight line, the object can move only in one of the two directions - forward and backward. Thus, the direction is represented by plus (+) and minus (\( - \)) signs as shown in Fig. 4.3. Positions to the right of the reference point O are generally taken as positive, and to the left of O as negative (Fig. 4.3).

4.1.2 Distance travelled and displacement

Suppose an athlete starts running from point O at time \( t = 0 \) s, reaches point B at \( t = 4 \) s, then reaches point A at \( t = 10 \) s, then runs back along the same path till point B reaching there at \( t = 16 \) s (Fig. 4.4). How much is the total distance travelled by the athlete between the starting and stopping positions? The total distance travelled is OA + AB = 100 m + 60 m = 160 m.

Let us now think about the distance between the starting and the stopping positions of the athlete. It is OB = 40 m, which is different from the total distance travelled by the athlete. So, let us now define another quantity - displacement.

Displacement is the net change in the position of an object between the two given instants of time. A complete description of physical quantities like displacement requires specifying both a direction and its numerical value (with units). The numerical value (with units) of such a physical quantity is called its magnitude. The magnitude of displacement is the distance between the object's positions at the two instants. The direction of displacement is specified from the position at the first instant towards the position at the second instant. To describe the total distance travelled,

[Figure 4.4: Reference point and positions of athlete at different instants of time, See in your textbook]

Note

An instant of time and a time interval are not the same thing. An instant of time is a single reading of a clock at a given point of time. Whereas, a time interval is the time duration between two instants of time, i.e., between two readings of a clock.

Teacher's Note

Distance and displacement are different. Distance is always positive and is the total path length covered. Displacement can be zero, positive, or negative, and it only cares about where you start and where you end. In the example, the athlete travels 160 m but has a displacement of only 40 m because she ends up at a position 40 m from the start, not where she began.

Key Points

• Linear motion occurs when an object moves in a straight line, and describing it requires choosing a reference point and specifying both distance and direction.
• Distance is the total length of the path travelled by an object, while displacement is the net change in position, which requires both magnitude and direction to describe completely.
• Motion and rest are relative concepts that depend on the reference point chosen; an object at rest with respect to one reference point may be in motion with respect to another.
• An instant of time is a single moment, whereas a time interval is the duration between two moments.
• Displacement can be less than, equal to, or even zero compared to distance, depending on the path taken and the starting and ending positions.

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