MOTION AND MEASUREMENT OF DISTANCES

 NOTES

Non-standard measures: The lengths of steps, arms, hands, or fingers of different people are different, therefore the distance measured with their help is not always reliable. These methods are, therefore, called non-standard measures.

Standard measures: Measures that are the same all over the world are known as standard measures.

In October 1960, the 12th general conference on weight and measures adopted the International system of units. “The System International Units” is the set of units to maintain uniformity all over the world.

Metre: It is the standard unit of length. The symbol of metre is m.

Each metre (m) is divided into 100 equal divisions, called centimetre (cm). Each centimetre has ten equal divisions, called millimetre (mm). Thus
1 m = 100 cm
1 cm = 10 mm
For measuring large distances, metre is not a convenient unit. We define a larger unit of length. It is called kilometre (km).
1 km = 1000 m.

Motion: It is a state of objects in which they are moving, that is, they are changing their place with the changing time.

Rest: All the stationary objects which are not in motion, that is, do not change their place with time are said to be at rest.

Rectilinear motion: When the objects change their position with time along a straight line, this type of motion is called rectilinear motion.

Circular motion

• When a body moves in a circular path, its motion is known as circular motion
• Examples:
  • motion of stone tied in a thread and whirled.
  • motion of a blade of an electric fan.
• motion of second’s hand in a clock.
• In circular motion, the object remains at the same distance from a fixed point.

Rotational motion: Motion in which a whole body moves about an axis is called a rotational motion. Example: motion of a top.

Periodic motion: Motion in which an object repeats its motion after a fixed interval of time is called periodic motion.
Examples:

• Oscillations of a pendulum.
• (Motion of a swing.
  

QUESTION ANSWERS AND EXCERCISE

Exercise Questions

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

Solution:

Land- Train, Bus

Water- Ship, Boat

Air- Helicopter, Aeroplane

2. Fill in the blanks:

(i) One metre is ______________ cm.

(ii) Five kilometre 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______________.

Solution:

(i) One metre is 100 cm.

(ii) Five kilometre is 5000 m.

(iii) Motion of a child on a swing is periodic.

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

(v) Motion of the wheel of a bicycle is circular.

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

Solution:

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

4. Arrange the following lengths in their increasing magnitude: 1 metre, 1 centimetre, 1 kilometre, 1 millimetre.

Solution:

1 millimetre, 1 centimetre, 1 metre, 1 kilometre

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

Solution:

1.65= 165 cm = 1650 mm

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

Solution:

1km = 1000 m

Hence 3250 m = 3.25 kms

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?

Solution:

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.

Solution:

Similarities – The blades of a fan and the wheels of a bicycle shows circular motion

Differences- Bicycle 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?

Solution:

An elastic measuring tape will not give accurate measurement as it stretches in length and reduces in size when stretched. When we express measurement taken with elastic tape, we have to tell whether the tape was stretched. If yes, how much. Hence it is very difficult to tell the measurement taken from an elastic tape.

10. Give two examples of periodic motion.

Solution:

a) A needle of a sewing machine

b) Pendulum