Motion and Measurement of Distances – Solutions (Sections A, B & C)
A. Fill in the blanks
1. The standard unit of mass is the ____________________.
Answer: Kilogram (kg)
2. While sitting in a moving bus, we are in ____________________ with respect to the surroundings.
Answer: Motion
3. A moving car, going on a narrow straight road, is having a (nearly) ____________________ motion.
Answer: Rectilinear
4. The motion of the needle of a sewing machine is an example of an ____________________ motion.
Answer: Oscillatory
5. The motion of the earth, around the sun, is ____________________ in nature.
Answer: Periodic
B. Write True or False for the following statements
1. Measurement is needed to make correct decisions.
Answer: True (✓)
2. A metre scale can be used for measuring the length of a playground.
Answer: False (✗)
3. The motion of the moon, around the earth, is a rectilinear motion.
Answer: False (✗)
4. The motion of the pedal of a bicycle is a circular motion.
Answer: True (✓)
5. The motion of a giant wheel is a rotatory motion.
Answer: True (✓)
6. When we draw water from a well, we are pushing at the rope.
Answer: False (✗)
C. Tick (✓) the correct option
1. The standard unit of length is the—
- metre
- kilometre
- millimetre
- centimetre
Answer: ✓ metre
2. The standard unit of time is the—
- second
- year
- day
- minute
Answer: ✓ second
3. The motion of the tip of a pen, while writing, is a/an—
- rectilinear motion
- curvilinear motion
- oscillatory motion
- rotatory motion
Answer: ✓ curvilinear motion
4. Which of the following four ‘motions’ is different from the other three?
- the moving blades of a fan
- a potter’s wheel
- a spinning top
- a fruit falling from a tree
Answer: ✓ a fruit falling from a tree
5. The motion of a swinging pendulum is a/an—
- curvilinear motion
- oscillatory motion
- rotatory motion
- rectilinear motion
Answer: ✓ oscillatory motion
D. Answer the following questions in brief
1. How does measurement help us in our day-to-day life?
Answer:
Measurement helps us to know the correct length, weight, time, and quantity of different things. It is useful in shopping, cooking, travelling, construction, and many other daily activities. It helps us make correct decisions.
2. Which type of watch is used to measure time intervals in a 200 metre race?
Answer:
A stopwatch is used to measure the time taken in a 200 metre race because it measures short time intervals accurately.
3. When do we consider an object to be in motion?
Answer:
An object is said to be in motion when its position changes with time with respect to its surroundings or a fixed point.
4. Name three different types of motions and give one example of each.
Answer:
- Rectilinear motion – A car moving on a straight road.
- Circular motion – The pedal of a bicycle moving in a circle.
- Oscillatory motion – A swinging pendulum.
5. State the difference between a ‘periodic’ and a ‘non-periodic’ motion.
Answer:5. State the difference between a ‘Periodic’ and a ‘Non-periodic’ motion.
| Periodic Motion | Non-periodic Motion |
|---|---|
| It is a motion that repeats itself after equal intervals of time. | It is a motion that does not repeat itself after equal intervals of time. |
| It has a fixed and regular pattern. | It has no fixed or regular pattern. |
| The time taken for each cycle remains the same. | The time taken for each movement is not the same. |
| It is easy to predict when the motion will repeat. | It is difficult to predict when the motion will repeat. |
| Examples include the Earth revolving around the Sun, the hands of a clock, and the swinging of a pendulum. | Examples include a flying bird, a moving car in traffic, a football during a match, and the movement of clouds. |
In simple words:
Non-periodic motion is a motion that does not repeat at regular intervals of time.
Periodic motion is a motion that repeats again and again after the same interval of time.
E. Answer the following questions
1. Why does a measurement need both a ‘number’ and a ‘unit’ for its complete specification?
Answer:
A measurement is complete only when it has both a number and a unit.
- The number tells us how much of something is being measured.
- The unit tells us the standard used for measurement.
Without a unit, the number has no meaning. For example, if we say a table is 2, we do not know whether it is 2 metres, 2 centimetres, or 2 feet long. But if we say the table is 2 metres long, the measurement becomes clear and complete.
Example:
- Correct: The rope is 5 metres long.
- Incorrect: The rope is 5 long.
Therefore, both the number and the unit are necessary for accurate measurement.
2. Why is a foot step, or a hand span, not used as a standard unit of length?
Answer:
A foot step and a hand span are not standard units of length because they are different for different people.
- One person’s hand span may be larger or smaller than another person’s.
- Similarly, the length of a foot step also varies from person to person.
As a result, the same object may have different measurements when measured by different people.
To avoid confusion and ensure accurate measurement, we use standard units like the metre (m) and centimetre (cm), which are the same everywhere.
Example:
Two students measure the length of a classroom using their footsteps. One gets 20 steps, while the other gets 24 steps because their footsteps are of different lengths.
3. Radha was performing an experiment in the Science Laboratory. The metre scale that she was using to measure the length of a string was worn out between its 1 cm and 2 cm marks due to overuse.
(a) She measures the length of a string by keeping its one end at the 3.0 cm mark. If the other end of the string reads 92.8 cm, find the length of the string.
Answer:
Given:
Starting reading = 3.0 cm
Ending reading = 92.8 cm
Formula:
Length of the string = Final reading − Initial reading
= 92.8 cm − 3.0 cm
= 89.8 cm
Answer: The length of the string is 89.8 cm.
(b) Write the precautions to be followed when a metre scale is being used for measuring a given length.
Answer:
The following precautions should be taken while using a metre scale:
- Place the metre scale along the length of the object.
- Keep the zero mark of the scale at one end of the object. If the zero mark is damaged, start from another clear mark and subtract the initial reading from the final reading.
- Keep the scale straight and in contact with the object.
- Read the scale with your eye directly above the mark to avoid parallax error.
- Note the reading carefully in the correct unit (cm or m).
4. Explain the difference between ‘rest’ and ‘motion’. Give two examples of each.
Answer:
An object may either be at rest or in motion, depending on whether its position changes with respect to its surroundings.
| Rest | Motion |
|---|---|
| An object is said to be at rest if its position does not change with time with respect to its surroundings. | An object is said to be in motion if its position changes with time with respect to its surroundings. |
| The object remains at the same place. | The object changes its position continuously. |
| Examples: A book kept on a table, a parked bicycle. | Examples: A moving bus, a flying bird. |
5. Distinguish between:
(a) Rectilinear Motion and Curvilinear Motion
| Rectilinear Motion | Curvilinear Motion |
|---|---|
| Motion along a straight path is called rectilinear motion. | Motion along a curved path is called curvilinear motion. |
| The direction remains along a straight line. | The direction changes continuously. |
| It is the simplest type of motion. | It follows a curved path. |
| Example: A train moving on a straight railway track. | Example: A ball thrown into the air or the tip of a pen while writing. |
(b) Rotation and Revolution
| Rotation | Revolution |
|---|---|
| Rotation is the motion of an object about its own axis. | Revolution is the motion of an object around another object. |
| The object spins in one place. | The object moves in a fixed path around another object. |
| Rotation usually causes day and night (in the case of Earth). | Revolution usually causes seasons (in the case of Earth). |
| Example: A spinning top or the blades of a fan. | Example: The Earth revolving around the Sun. |
6. Define the following types of motion. Give one example of each.
(a) Rectilinear Motion
Definition:
Rectilinear motion is the motion of an object along a straight line.
Example:
A car moving on a straight road.
(b) Circular Motion
Definition:
Circular motion is the motion of an object along a circular path around a fixed point.
Example:
The hands of a clock or the pedal of a bicycle moving in a circular path.
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