When you look around you find some objects in motion and some at rest. In your room, you see your school bag, a table, a chair, a bat, a ball, an almirah, etc. all at rest. In open, you often see birds flying in the air, vehicles moving on the road, squirrels sunning under a tree. How do you decide whether an object is in motion or at rest? An object is said to be in motion if its location or position in space keeps changing with time.

 

We see that squirrel is not at the same place after some time while the tree is at the same place. So we can say that the squirrel is in motion and the tree is at rest. In the case of the wall clock, we find that the wall clock is at rest whereas its hands are in motion. This is because the position of the wall clock does not change whereas the positions of its hands change with time.

 

Types of motion:

 

Recti Linear Motion

When an object changes its position with time we say that the object is in motion. You must have seen a railway train moving on a straight track, march past of soldiers in parade, falling of marble, a car moving on a straight road.

In all these examples we observe that the objects move along a straight line. This type of motion is called rectilinear motion. Sprinters in a 100-meter race also move along a straight track.

 

Periodic Motion

 

If we carefully observe the motion of a child on a swing, the motion of a clock pendulum. We find in both the object of the case repeat its motion after some time. This type of to and fro motion is called periodic motion.

To understand periodic motion we do an activity:

 

• Take a stone tied with a string.
• Hold the string end in your hand and allow the stone to hang from it.
• Pull the stone to one side with the other hand and let it go. What do you observe?
• The stone starts moving to and fro. It keeps repeating its motion after an interval time. This is a simple pendulum.

 

Other examples of periodic motion are strings of a guitar or the surface of drums or �tabla� when being played, ringing bell, the needle of a sewing machine also undergoes periodic motion.

 

Circular Motion

 

You must have heard about artificial satellites orbiting the earth. You see the everyday blades of an electric fan rotating. The hands of a watch also move in the same manner. In all these examples we find that objects move along a circular path.

Let�s do an activity to understand circular motion.

 

• Tie a thread to a small size stone and whirl it with your hand.
• We see that the stone moves along a circular path.
• In motion distance of the stone from your hand always remains the same. This type of motion is called circular motion.
• Make a bright red mark on the blade of an electric fan. Now switch on the fan. What do you see?

 

A red circle is created which shows that the red spot is moving in a circular path. Its distance from the center of the fan always remains the same. Merry go rounds and giant wheels are other examples of circular motion.

 

Combinations of different types of motion

 

Observe carefully the motion of a bicycle. Both of its wheels undergo rotational motion and the bicycle undergoes a rectilinear motion. Similarly, a ball rolling on the ground rotates and moves forward on the ground. These are examples where objects undergo combinations of different types of motion.

 

Standard units of measurements

 

• People in ancient times used sizes of body parts such as the length of a foot, the width of a finger, the length of an arm, and the distance of step as various units of measurements. In ancient India, an �angul� (finger) or a� muthi� (fist) were used to measure small lengths. Like chin is four anguls below the tip of the nose. The center of the eyes is 8 angul above the chin.
• In ancient Egypt, CUBIT was used as a unit of length. A cubit is a length from the elbow to the fingertip. Cubit was also accepted as a unit of length in other parts of the world.
• �Yard� was also accepted as a unit of length. Distance between the end of the outstretched arm and the chin was taken as one yard. People measured the cloth in yards.
• In some parts of the world, the foot was used as a unit of length. Romans used steps to measure the length. 
• During Indus Valley excavations many perfectly geometrical constructions were found, which shows people of Indus Valley Civilization must have used some very good measurement of length.

 

Since sizes of body parts like hand span, length of the foot, arm length, etc. vary from person to person. This must have caused confusion in measurements. Therefore, to maintain uniformity, scientists all over the world accepted a set of standard units of measurement and introduced the International System of Units (SI Units).

 

SI unit of length is a metre. Metre is divided into 100 equal divisions, called centimetre (cm) 1metre = 100 centimetre. Each centimetre has ten equal divisions, called millimetre (mm), 1 centimetre = 10millimetre.Large distances are measured in kilometre (km), 1 km = 1000 metre.

 

Correct measurement of length

 

For measuring lengths different people use different devices. A tailor uses tape whereas a cloth merchant uses a meter rod. For measuring small lengths we use a 15 or 30 cm scale.

Let us measure the length of a small box.
In taking a correct measurement of a length, you are required to take care of the following:

Placing the scale
� Place the scale in contact with the object along its length in such a way that the zero marks of the scale are exactly above one edge of the object.
� Take a reading of the scale where it touches the other edge of the object. The scale reads 12.2cm.
� Therefore, the length of the box is 12.2cm.

Selecting the mark on the scale
In case zero marks are not clearly visible or the end is broken.
You can use any other full mark of the scale. Then you must subtract the reading of this from the reading at the other end. Here the reading at one end is 1.0 cm and at the other end it is 13.2 cm. therefore, the length of the box is (13.2 � 1.0) cm = 12.2 cm.

Position of the eye
Your eye must be exactly in front of the point where the measurement is to be taken.

 

Measuring length and breadth of a floor

Measuring length and breadth of a floor using self-made cricket wicket and bail as measuring tools

1. Take a wicket and start measuring the length of the floor from its corner. Measure the length smaller than the wicket length, if left out at the other corner, using bail.
2. Now measure the breadth of the floor in the same manner.

Record both the observation in the notebook.
Length of the floor = 10 wicket length and one bail length.
The breadth of the floor = 7 wicket length and two bail length

In both the measurements, lengths smaller than the bail length are still left out unmeasured. Now measure the length and the breadth of the same floor using a different set of wickets and bail.
Length of the floor = 9 wicket lengths and two bail length breadth of the floor = 6 wicket length.

We find that the second measurement is different from the first measurement. In both cases measurements are approximate. How can we measure the exact length of a floor or table? We try one more method to measure the length of an object.

Measuring the length of a table using string length as a measuring tool. We take a small length of a string and mark two points on it. This will be a string length. To measure distance smaller than the string length fold the string and mark points for �, �, 1/8 string lengths. We measure the length of the table. This time we find that no part of the length is left out unmeasured.

 

Measuring the length of a curved line

 

You must have seen a tailor using a measuring tape for taking measurements of shoulder, chest, waist, arm, etc. Now think, how can you measure the length of a curved line? We cannot measure the length of a curved line directly by using a meter scale or measuring tape.

 

We can use a thread to measure the length of a curved line.

 

� Take a thread which, you think is longer than the length of the curved line.
� Put a knot on the thread near one of its ends
� Now place a small position of the thread along the line keeping it taut using your fingers and thumb.
� Now hold firmly the threads at this endpoint with fingers of one hand.
� Using the other hand, stretch a little more portion of the thread along the curved line.
� Go on repeating the process till the other end �B� of the curved line is reached.
� Make a mark on the thread where it touches the end �B�
� Now stretch the thread along a meter scale
� Measure the length between the knot and the final mark on the thread.
� This gives us the length of the curved line AB

 

Read More: Uniform Motion and Non-Uniform Motion: Definition and Differences