## Motion and Measurement of Distances

**Measurement –**

Measurement means the comparison of an unknown quantity with some known quantity.

This known fixed quantity is called a **unit**.

**Measurement is expressed in two parts :**

- One part is a number and,
- another part is a unit of measurement.

**Example-** The length of a box is 12 m.

here, 12 is number and metre is unit.

### Standard unit of measurement-

In ancient time,

•the length of a foot

•the width of a finger

•the distance of a step were commonly used for different units of measurement.

**#Scientist all over the world have accepted a set of standard units of measurement. The System of units now used is known as the international system of units (SI units).**

**Length-**

The SI unit of length is metre. Each metre is divided into 100 equal divisions called centimetre(cm). Each Centimetre has 10 equal divisions called Millimetre. (mm)

1m=100cm

1cm=10mm

A larger unit of length is called kilometre(km).

**Measuring The Length of a straight line –**

In taking the measurement of a length, we need to take care of the following:

1-Place the scale in contact with the object, along its length.

2- In some scales, the ends may be broken. You may not be able to see the zero mark clearly. In such a case you should avoid taking the measurement from the zero mark of the scale. You can use any other full marks of scale as 1.0 cm then you must subtract the reading of this mark from the reading of the other

**For example-** The reading at one end is 1.0 cm and the other end is 13.0 cm

Therefore, the length of the object is

13.0-1.0 =12.0m

3-correct position of the eye is also important for taking the measurement. Your eye must be exactly in front of the point where the measurement is to be taken.

**Measuring the length of a curved line – **

We cannot measure the length of a curve line directly by using a metre scale. We can use a thread to measure the length of a curved line.

**Motion-**

Motion is a change in the position of an object with time.

**Example-**

- The motion of a striker moving on a carrom board.
- The motion of a ball falling freely from the roof of a building.

**Types of motion –**

**Rectilinear motion –**

When an object moves along a straight line.

**Example-**

- Sprinters in 200 m race in a straight track.
- Moving car in a straight road.

**Periodic motion-**

An object repeats its motion after some time, this type of motion is called periodic motion.

**Example- **simple pendulum

#### Circular motion-

The motion of an object in a circular path is called circular motion.

**Example-**

- An electric fan does not move from one place to another but the Blades of fan rotate in the circular path.
- The motion of a merry-go-round.

**EXERCISE**

**1. Give two examples each, of modes of transport used on land, water and air**

**Answer :**

Land- car, bus

Water- Ship, Boat

Air- Helicopter, Airplane

**2. Fill in the blanks:**

**(i) One metre is ______________ cm.**

**(ii) Five kilometer is ______________ m.**

**(iii) Motion of a child on a swing is ______________.**

**(iv) Motion of the needle of a sewing machine is ______________.**

**(v) Motion of wheel of a bicycle is______________.**

**Answer :**

(I) 100 cm.

(ii) 5000 m.

(iii) periodic.

(iv) periodic.

(v) circular.

**3. Why can a pace or a footstep not be used as a standard unit of length?**

**Answer :**

Pace or a footstep cannot be used as a standard unit of length because it varies from one individual to other.

**4. Arrange the following lengths in their increasing magnitude: 1 metre, 1 centimeter, 1 kilometer, 1 millimeter**

**Answer :**

1 millimeter< 1 centimeter< 1 metre< 1 kilometer

**5. The height of a person is 1.65 m. Express it into cm and mm.**

**Answer :**

1.65= 165 cm = 1650 mm

**6. The distance between Radha’s home and her school is 3250 m. Express this distance into km.**

**Answer :**

1km = 1000 m

Therefore , 3250 m = 3.25 km

**7. While measuring the length of a knitting needle, the reading of the scale at one end is 3.0 cm and at the other end is 33.1 cm. What is the length of the needle?**

**Answer :**

Reading at one end= 3.0 and

Another end =33.1 cm

Length of needle = 33.1 – 3 = 30.1 cm

**8. Write the similarities and differences between the motion of a bicycle and a ceiling fan that has been switched on.**

**Answer :**

Similarities- Both of them shows circular motion

Differences- Bicycle wheels move in rectilinear motion, but the fan does not move in rectilinear motion.

**9. Why would you not like to use a measuring tape made of an elastic material like rubber to measure distance? What would be some of the problems you would meet in telling someone about a distance you measured with such a tape?**

**Answer :**

Measurement is not accurate in an elastic measuring tape, as it stretches in length and reduces in size when stretched. .It becomes quite difficult for us to tell that to what extend we have:

•stretched a tape

•contracted a tape

As there is no standard measurement of these quantities.

**10. Give two examples of periodic motion.**

**Answer :**

a) The needle of a sewing machine

b) Pendulum