Rest and Motion

# Motion and Measurement of Distances

## Moving Things Around Us

Think of some objects you have seen recently. List them in Table-1. A school bag, a mosquito, a table, people sitting on desks, people moving about? May be a butterfly, dog, cow, your hand, a small baby, a fish in water, a house, a factory, a stone, a horse, a ball, a bat, a moving train, a sewing machine, a wall clock, hands of a clock? Make your list as large as you can.
Which of these are moving? Which are at rest?

### Table-1.  Objects in rest and motion

 Objects at rest Objects in motion

How did you decide whether an object is in motion or at rest?

You might have noticed that the bird is not at the same place after some time, while the table is at the same place. On this basis you may have decided whether an object is at rest or in motion.

Let us look at the motion of an ant closely.

### Activity-1

Spread a large sheet of white paper on the ground and keep a little sugar on it. Ants are likely to be attracted to the sugar and you will find many ants crawling on the sheet of paper soon. For any one ant, try and make a small mark with a pencil near its position when it has just crawled on to the sheet of paper (Fig -1). Keep marking its position after a few seconds as it moves along on the sheet of paper. After some time, shake the paper free of the sugar and the ants. Connect the different points you have marked, with arrows, to show the direction in which the ant was moving. Each point you have marked shows where the ant moved to, in intervals of a few seconds.

Fig.1 . Motion of an ant

Motion seems to be some kind of a change in the position of an object with time, isn't it?

The blades of the fan or the hands of a clock— how are they moving? Is their movement similar to that of a train?

Let us now look at some types of motion to help us understand these differences.

## Types of Motion

You may have observed the motion of a vehicle on a straight road, march-past of soldiers in a parade or the falling of a stone (Fig -2). What kind of motion is this? Sprinters in a 100-metre race also move along a straight track. Can you think of more such examples from your surroundings?

Fig - 2. Some examples of rectilinear motion

In all these examples we see that the objects move along a straight line. This type of motion is called rectilinear motion.

### Activity-2

Take a stone, tie a thread to it and whirl it with your hand. Observe the motion of the stone. We see that the stone moves along a circular path.

In this motion, the distance of the stone from your hand remains the same. This type of motion is called circular motion.

The motion of a point marked on the blade of an electric fan or the hands of a clock are examples of circular motion.

The electric fan or the clock by themselves are not moving from one place to another. But, the blades of the fan rotate and so do the hands of a clock. If we mark a point anywhere on the blades of a fan or on the hands of a clock, the distance of this point from the centre of the fan or the clock, will remain the same as they rotate.

fig-3.Circular motion

In some cases, an object repeats its motion after some time. This type of motion is called periodic motion. Take the stone tied with a string that you used in Activity-2. Now, hold the string in your hand and let the stone hang from it. Pull the stone to one side with the other hand and let it go. This is a pendulum. Something to have a lot of fun with and something that will help us understand about periodic motion. Motion of a pendulum, a branch of a tree moving to and fro, motion of a child on a swing, strings of a guitar or the surface of drums (tabla) being played, are all examples of periodic motion where an object or a part of it repeats its motion after a fixed interval of time.

fig-4.Examples of periodic motion

Have you observed closely, the motion of a ball on the ground? Here, the ball is rolling on the ground – rotating as well as moving forward on the ground. Thus, the ball undergoes a rectilinear motion as well as rotational motion. Can you think of other examples where objects undergo combinations of different types of motion?

We did many measurement activities and discussed some kinds of motion. We saw that motion is a change in the position of an object with time. The change in this position can be determined through distance measurements. This allows us to know how fast or slow a motion is. The movement of a snail on the ground, a butterfly flitting from flower to flower, a river flowing along on clear rounded pebbles, an aeroplane flying high up in the air — making jet trails, moon going around the Earth, blood flowing inside our bodies, there is motion everywhere around us!

# Summary

1. Motion in a straight line is called rectilinear motion.
2. In circular motion an object moves such that its distance from a fixed point remains the same.
3. Motion that repeats itself after some period of time, is called periodic motion.

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