Class 6 Science Chapter 10 Notes | CBSE

Motion and Measurement of Distances

CBSE Class 6 Science - Chapter 10

Chapter Summary

• Need for Measurement: People in ancient times used body parts (like the length of a foot, the width of a finger, the distance of a step) as units of measurement. However, these varied from person to person, leading to confusion. This led to the need for standard units of measurement.
• Standard Units of Measurement: To ensure uniformity, scientists all over the world have adopted a set of standard units of measurement. The system of units now used is known as the International System of Units (SI units).
• SI Unit of Length: The SI unit of length is the metre (m).
  • 1 metre (m) = 100 centimetres (cm)
  • 1 centimetre (cm) = 10 millimetres (mm)
  • 1 kilometre (km) = 1000 metres (m)
• Correct Measurement of Length:
  • Place the scale in contact with the object along its length.
  • Start measurement from the zero mark of the scale, not from its broken edge.
  • Keep your eye directly above the point where the measurement is to be taken (avoid parallax error).
• Motion: An object is said to be in motion if its position changes with respect to its surroundings in a given time. If its position does not change, it is said to be at rest.
• Types of Motion:
  • Rectilinear Motion: Motion along a straight line.
    Examples: A car moving on a straight road, a falling stone, a march past by soldiers.
  • Circular Motion: Motion along a circular path. The object remains at the same distance from a fixed point.
    Examples: A point on the blade of a rotating fan, the hands of a clock, a stone tied to a string and whirled around.
  • Rotational Motion: The motion of a body about a fixed axis. All parts of the body move in a circle around the axis.
    Examples: A rotating top, a spinning wheel, a ceiling fan. (Note: While points on a fan show circular motion, the fan as a whole exhibits rotational motion).
  • Periodic Motion: Motion that repeats itself after a fixed interval of time.
    Examples: Motion of a child on a swing, the pendulum of a clock, the strings of a guitar, the heartbeat.
  • Some objects can exhibit more than one type of motion simultaneously. For example, a rolling ball on the ground shows both rectilinear (forward) and rotational motion. A bicycle wheel shows circular motion (parts of the wheel) and the bicycle as a whole moves in rectilinear motion.

Questions and Answers

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

Answer:

• Land: Car, Bicycle, Bus, Train
• Water: Boat, Ship
• Air: Aeroplane, Helicopter

2. Fill in the blanks:

(a) One metre is __________ cm.

(b) Five kilometres is __________ m.

(c) Motion of a child on a swing is __________ .

(d) Motion of the Earth around the Sun is __________ .

(e) Motion of a bicycle wheel is __________ .

Answer:

(a) One metre is 100 cm.

(b) Five kilometres is 5000 m.

(c) Motion of a child on a swing is periodic motion.

(d) Motion of the Earth around the Sun is circular motion (or periodic motion).

(e) Motion of a bicycle wheel is circular motion (and rotational motion).

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

Answer: A pace or a footstep cannot be used as a standard unit of length because the length of a pace or a footstep varies from person to person. Different individuals have different foot sizes and stride lengths. For a measurement to be standard, it must be uniform and give the same value regardless of who is measuring it.

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

Answer:

To arrange them, let's convert all to a common unit, say millimetres:

• 1 millimetre (mm) = 1 mm
• 1 centimetre (cm) = 10 mm
• 1 metre (m) = 100 cm = 1000 mm
• 1 kilometre (km) = 1000 m = 1000 x 1000 mm = 1,000,000 mm

Arranging them in increasing magnitude:

1 millimetre, 1 centimetre, 1 metre, 1 kilometre

5. Convert the following lengths into standard units:

(a) 15 cm

(b) 1.5 m

(c) 5.6 km

(d) 5000 mm

Answer: The standard unit of length is the metre (m).

• (a) 15 cm = 15/100 m = 0.15 m
• (b) 1.5 m = 1.5 m (It is already in the standard unit)
• (c) 5.6 km = 5.6 x 1000 m = 5600 m
• (d) 5000 mm = 5000/1000 m = 5 m (Since 1m = 1000mm)

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

Answer:

Given height = 1.65 m

• Converting to cm:
  Since 1 m = 100 cm
  1.65 m = 1.65 x 100 cm = 165 cm
• Converting to mm:
  Since 1 m = 1000 mm (or 1 cm = 10 mm)
  1.65 m = 1.65 x 1000 mm = 1650 mm
  (Alternatively, 165 cm = 165 x 10 mm = 1650 mm)

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

Answer:

Given distance = 3250 m

Since 1 km = 1000 m, to convert metres to kilometres, we divide by 1000.

Distance in km = 3250 / 1000 km = 3.250 km

8. 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 cm

Reading at the other end = 33.1 cm

Length of the needle = (Reading at other end) - (Reading at one end)

Length = 33.1 cm - 3.0 cm = 30.1 cm

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

Answer:

Similarities:

• Both the bicycle wheels and the blades of the ceiling fan show circular motion (points on them move in a circle).
• Both can also exhibit rotational motion (the wheel rotates about its axle, the fan rotates about its central rod).
• If the bicycle is moving at a constant speed, its wheels exhibit periodic motion, similar to the periodic rotation of the fan blades.

Differences:

• A bicycle moving on a straight road exhibits rectilinear motion as a whole (its body moves in a straight line), in addition to the circular/rotational motion of its wheels.
• A ceiling fan, when switched on, remains at a fixed position and only exhibits rotational and circular motion. It does not show rectilinear motion.

10. Why could you not use an elastic measuring tape to measure distance? What would be some of the problems you would meet in telling someone about a distance you measured with an elastic tape?

Answer:

You cannot use an elastic measuring tape to measure distance because an elastic tape can stretch. When you stretch it, the marks on the tape (representing units like cm or inches) would become further apart. This means the length of one unit would change depending on how much the tape is stretched.

Problems in telling someone about a distance measured with an elastic tape:

• Inaccurate measurement: The actual length would vary each time you measure it or if different people measure it with different amounts of stretch.
• Lack of standardisation: The measurement would not be standard, making it impossible to compare or communicate accurately. If you say "it's 5 units long", those 5 units might be longer or shorter than someone else's 5 units, depending on the stretch.
• Communication barrier: Without a consistent, fixed unit of length, describing the measurement to someone else would be meaningless and lead to confusion. They would not know the exact distance you are referring to.

11. Give two examples of periodic motion.

Answer: Two examples of periodic motion are:

• Motion of a child on a swing.
• Motion of the pendulum of a clock.
• Motion of the Earth around the Sun.
• Heartbeat.

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