Motion chapter of class 9 Science

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Lesson 1.2: 'Rest' & 'Motion'

Video explanation of 'Rest' and 'Motion'


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Section 1 : Lesson 1.3:

Question: People often say that 'rest or Motion is Relative'. What is the meaning of this?

Video Explanation of "Rest or Motion is Relative"

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Section 5: NCERT Exercises

Video solution of Exercise 1:

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Textual Explanation

‘Motion’ is a commonly used word in our everyday life. When an object is moving, we say that it is in motion. We also use another word ‘rest’ along with ‘motion’ in our everyday life. When an object is not moving, we say that it is at rest.

For example, when we stand on the side of a road and watch the cars pass by, we say that the cars are in motion but the trees and houses are at rest. It is very important to understand clearly the concept of motion in class 9 itself. It will help you in upper classes.

Just think, how we decided which objects are in motion and which are at rest? We decided that by considering the position of each object with time. Since the position of the house or the tree did not change with time, we decided that they are at rest. Whereas, the position of the car was changing with time, so we decided that it was in motion. We do it intuitively as we had learnt it while we were growing up.

Now, when we use these two words in physics, their meanings remain the same, but we have to consider a few more points while using them. Because, physics is a part of science, and, in science we have to be very specific.

In the above example when we said that the position of an object was changing or not changing with time, we compared it with respect to the earth’s surface. In other words, we took the earth’s surface as the reference, and looked at the position of an object with respect to that reference. This brings us to the concept of ‘Reference Point’.


REFERENCE POINT: A ‘Reference Point’ is a point (or a location) with respect to which we specify the positions of other objects. It is also called the ‘origin’.


As another example, when we say that the ‘position’ of a school in a village is 2 km north of the railway station, we are using the railway station as the reference point or the origin.

This brings us to the concept of ‘position’ of an object. Whenever we want to specify the positions of objects in physics, we have to do two things:

  • First, we have to fix a reference point.
  • Second, we have to state the distance of each object from that reference point, along with the direction in which it lies.

People often say that “An object may be appearing to be moving for one person and stationary for another person”. Or people say “Rest and Motion are relative”.

When you are learning about rest and motion, the above statements may seem very confusing. And the explanations given in most books and/or by most people do not remove the confusions. Actually they are not very confusing if you learn the concepts the way I am explaining them below. So read on!

Let me explain it with an example. Say, a person is standing on the side of a road watching a car go by inside which you and a friend are sitting. The person will say that you, your friend and the car are in motion. There is no confusion here.

Now, what will you say about your friend - is he ‘at rest’ or ‘in motion’?


Wait, don’t say that he is at rest, only because you have read that in a book and/or other people have said that! This is where my explanation is different from others! It will clear your confusion.


In our everyday life, and, before you had read the book or heard what others say, you would have said that your friend is in motion (i.e. moving). This is because you can see that the trees, the houses, the ground are all moving backwards which means you, your friend and the car are in motion. In fact, you also feel the jerkings of the car on the road, the wind hitting you face etc.

But when you want to answer the above question from the point of view of physics, the answer may be different. As I have mentioned above, first you will have to fix a reference point, and then observe his position with respect to that reference point. Two cases will arise:

Case 1: You fixed the earth’s surface as the reference:

In this case, you will observe that the position of your friend is changing with time. So you will say that he is in motion. You are completely correct!

Case 2: You fixed yourself as the reference point:

In this case, you will observe that the distance of your friend from you (the reference point) is not changing with time and the direction is also the same i.e. the position of your friend is not changing with time with respect to this reference point. Hence, you will say that he is at rest. Again, you are completely correct!

This means that whether an object is in motion or at rest depends on our choice of the reference point. The same object may be ‘in motion’ or ‘at rest’ depending on where we have fixed the reference point. That is why people often say that “An object may be appearing to be moving for one person and stationary for another person, or, Rest and Motion are relative”.

In the book or when people say that ‘you see your friend to be at rest’ in the above example, what they forget to mention is that ‘if you take yourself as the reference point’. This creates the confusion and many students find it difficult to understand the above statements.

I hope the above explanations have made the concepts clear to you.


Activity 8.1: Discuss whether the walls of your classroom are at rest or in motion.

Answer: It will depend on where we take the reference point to be.

If we take the earth’s surface as the reference, then the walls will be at rest.

But if we take the reference point on the sun (or at any other point in outer space), then the walls will be in motion, since they will be moving around the sun together with the earth.

 

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