Measurement and Motions

Give reasons for the following.

1. A cubit cannot be used as a standard unit for the measurement of length.
 Standard unit of length is a fixed length all over the world, which does not
vary with time, place, and persons and is not affected by any natural
phenomenon. A cubit or a yard cannot be used as a standard unit of length
as it is not fixed in length and varies from person to person.
2. Diameter of a ball cannot be measured using a meter scale.
 Measuring the diameter of a spherical object with a meter scale is not
possible because meter scale is only used to measure straight objects. If
you want to measure a spherical object, then use a measuring tape or a
thread.
3. When sitting inside a moving bus, we are at rest as well as in motion.
 When a body is placed in a moving object it also moves at the same speed
as the moving object. Therefore, when you sit in a moving bus you are also
in motion in the same direction and speed as the bus.
4. Motion of the Earth around the sun is a periodic motion.
 The motion of Earth around the Sun repeats itself after every 365 days or a
year. Hence, it is a periodic motion.
5. It is not recommended to use an elastic tape to measure distance.
 Elastic tapes are stretchable. So, the length of an elastic measuring tape will
change each time due to stretching. So, it will not give correct
measurement of a distance. Therefore, we cannot use an elastic measuring
tape to measure distance
Answer the following questions briefly.
1. Why do we need standard units for measurement?
 All over the world, people use diverse units to measure a specific physical
quantity. This has caused a lot of confusion in measurement. Thus, for
uniformity and convenience, scientists worldwide have accepted a standard
set of units for measurement called the International Systems of Units or SI
units, it consist of a set of standard well-defined units for the measurement
of physical quantities. For example, the unit of length in the SI system is a
meter, the unit of mass is a kilogram, and the unit of time is a second.
2. What is periodic motion? Give an example seen in nature.
 If the motion of an object is repeated along the same path at regular
intervals of time, such motion is said to be periodic motion. Motion of the
moon around the Earth. The rotation of the Earth about its own axis and
 the revolution of the Earth around the sun are examples seen in nature for
periodic motion.
3. What is rotational motion? Give an example from your daily life.
 Rotational motion can be defined as the motion of an object around a
circular path, in a fixed orbit. The fan rotates continuously about an axis.
The distance moved by different points on the fan is not the same, but all
points complete the rotation in the same time. Rotations of the Earth about
to axis, motion of a spinning top, and rotation of a flywheel are examples of
rotational motion.
4. The measurements given below are incomplete. Complete them.
 The length of Ryan's hair is 10.
i. The length of Ryan's hair is 10 cm.
 The bus took a long time, close to 30, to reach school.
i. The bus took a long time, close to 30 minutes, to reach school.
Answer the following questions.
1. Traditional methods of measurement are not reliable compared to standard
methods of measurement. Explain.
 Traditional methods of measurement involve methods like
measurement using the arm, foot, or hand span of a particular person
which can and does vary from person to person and hence can't be
reliable while the standard methods have a fixed standard value which is
universal and same for everybody and every place. It consists of a set of
standard well defined units for measurement of physical quantities, for
example the unit of length in the In the SI system is a meter, the unit of
mass is a kilogram and the unit of time is a second.
2. Give 5 examples of different types of measurements we need in our day-to-
day life.
 Length of a book is measured in centimeters.
 Dimensions of a room are measured in meters
 The distance between 2 cities is measured in kilometers.
 The time taken to travel is measured in seconds or minutes or hours.
 Our body weight is measured in kilogram.
3. What are the different types of motions? Give two examples of each.
 Translational Motion: An object moves on the whole, from one place to
another, so that all its body part moves the same distance in a given
time. It is said to be in translational motion.
 i. Rectilinear motion: If an object moves in a straight line, it is said
to have a rectilinear motion. For example: a car moving in a
straight path.
ii. Curvilinear motion. If an object moves along a curved path, its
motion is called curvilinear motion. For example, a car moving on
a curved path.
 Rotational motion: A body is said to be in a rotatory motion if it moves
or spin about a given axis. For example, rotation of earth on its axis,
motion of a spinning top, rotation of a flywheel.
 Periodic motion: The motion which repeats itself after a period of time is
called a periodic motion. For example, the R and second hand of a watch
shows periodic motion, motion of a bouncing ball under the action
gravity.
 Oscillatory motion: When an object moves to and fro about a fixed
point, it is said to be oscillatory motion. For example, Bob in the
pendulum clock.

4. Can a body have rotational motion and rectilinear motion at the same time?
Justify.
 Yes, a body can have both rotational motion and rectilinear (linear) motion
simultaneously. This is known as general plane motion. General plane
motion occurs when a rigid body undergoes both translational motion
(linear motion) and rotational motion at the same time.
A common example is a wheel rolling along a surface. In this case, the
wheel experiences both translational motion as it moves forward, and
rotational motion as it spins about its axis. The combination of these
motions results in general plane motion.
So, in certain situations, depending on the nature of the motion and the
constraints on the body, both rotational and rectilinear motion can occur
simultaneously.
5. Explain the types of translatory motion with one example for each.
 Translation Motion: If an object moves on the whole, from one place to
another, so that all its body parts move the same distance in a given
time, it is said to be in a translation motion. Consider the motion of a car
moving on a road. The position of the car changes continuously with
time with respect to its surroundings. Moreover, during its journey, all
 parts of the car move equal distances in equal intervals of time. Such
motion is translational motion. It is of two types, rectilinear and
curvilinear motion.
i. Rectilinear motion: If an object moves in a straight line, it is said
to have rectilinear motion. In rectilinear motion. The direction of
the motion of the object remains the same as it is moving along a
straight path. For example: The motion of a car on a straight road.
The motion of a stone falling down and an athlete running on a
straight track.
ii. Curvilinear motion: If an object moves along a curved path, it is
called curvilinear motion. In curvilinear motion, the direction of
the motion of the object changes with time, but the orientation of
the object remains the same. For example, a car moving on a
curved road.
6. All oscillatory motions are periodic, while all periodic motions are not
oscillatory. Is this statement correct? Justify your answer
 Oscillatory motion: The motion of an object in which the object moves
to and fro or back and forth about its mean position (rest position) is
called oscillatory motion. Example: The motion of a pendulum, the
motion of a mass suspended by a string, and the motion of a swing are
examples of oscillatory motion.
 Periodic motion: The motion which repeats itself at a fixed interval of
time is called periodic motion. Example: The motion of a pendulum, the
motion of the Earth about its axis or around the Sun, the Motion of a
swing, the motion of the needle of a sewing machine, and the heartbeat
of a normal person are examples of periodic motion.
 An oscillatory motion is always periodic but a periodic motion is not
always oscillatory because in oscillatory motion the body repeats its
path in equal intervals of time but any body in periodic motion may not
do to and fro motion about their mean position. Some examples are
given below:
i. A pendulum repeats its path in every equal interval of time (time
period) means it is both an oscillatory motion and a periodic
motion.
ii. Earth rotates on its axis in every equal interval of time but it does
not do any kind of to and fro motion, which means it is a periodic
motion but not an oscillatory motion.
7. We know that the Earth is moving around the sun. When we look at the sky,
we see that the sun rises in the east, slowly move towards the West and sets
there. Why?
 We see the sunrise and sunset because of Earth's rotation about its own
axis. Earth is rotating about its own axis from West to East. So, when
seen from the Earth, the Sun appears to rise in the East and set in the
West.