SCIENCE CHAPTER 12 Motion and Time WORKBOOK ANSWER

 Ch 12 Motion and Time

WORKBOOK ANSWER

. Choose the correct option. (Ap/U/R)
(a) A car covers 12 km in its first hour, then it takes 2 hours to cover the next 28 km and
then takes 3 hours to cover another 50km, then it is said to be moving at ____________
speed.
i. uniform
ii. regular
iii. non-uniform
iv. irregular
Ans: iii
(b) An object ____________ does not change its position with time and its surroundings.
i. in random motion
ii. at rest
iii. in uniform motion

iv. Ans: ii

in non-uniform motion

(c) A train takes 3 minutes to pass through a tunnel at the speed of 72 km/h. Thus, the
tunnel is ____________ long.
i. 3.6 m
ii. 3600 km
iii. 3.6 km
iv. 24 km
Ans: iii
2. Assertion/Reasoning Statement. (An)
In the following questions, two statements are given- one labelled Assertion (A) and the
other labelled Reason (R). Read the statements carefully and choose the correct
alternative (A), (B), (C) and (D) as given below:
(A) Both A and R are true and R is the correct explanation of the assertion.
(B) Both A and R are true but R is not the correct explanation of the assertion.
(C) A is true but R is false.
(D) A is false but R is true.
(a) Assertion: The time interval between one sunrise and the next is called a day.

Reason: The time period from one sunset to sunrise is called night.
Ans: (B) Both A and R are true but R is not the correct explanation of the assertion.
(b) Assertion: Pendulums are used in clocks.
Reason: The pendulum completes one oscillation in different time intervals.
Ans: A is true but R is false.
3. Draw distance-time graphs for(Ap)
a) an object that is not moving
b) an object is moving at a constant speed.

Ans.:

4. Answer the following questions in brief. (4 to 5 lines)(U/R)
(a) Explain the term 'Unit of Measurement' with the help of a real life example
i. A unit of measurement of a quantity tells one thing about the quantity
such as magnitude, scale, size, extent, degree, enormity, level and amount.
ii. If one visits a market in order by any food item, before buying the required
thing she/he has to tell a unit of measurement (like, gram or kilogram)of that
item to the shopkeeper.
(b) What is the significance of SI units?
i. They are commonly used by the people around the world, so that the
people from different region of the world can communicate easily with one
another.
ii. This system has multiple and sub-multiple version of all the units; this can
be utilized by systematic use of prefixes before various units.
5. Solve the following numerical problems.(3 to 4 lines)(Ap)
(a) Roshni drove for 2 hours at a speed of 40 km/h and for 1 hour at 70 km/h. What was her
average speed for the whole journey?
Total distance covered = 40 × 2 +70 ×1 = 80+70 = 150 km

Total time = 2 + 1 = 3 h
Average speed = 150/3 = 50 km/h

(b) A car covers a distance of 600 m in 1 minute whereas another car covers 54 km in 50
minutes. Find the ratio of their speeds.
The speed of the first car = 600 m/60 s = 10 m/s.
The speed of the second car = 54000m/3000 s =18 m/s.
The ratio of the speed = 10/18 = 5:9
6. Answer the following questions in detail. (R/U)
(a) Answer the following question related to speed.
i. Define speed.
ii. Write down the formula of speed.
iii. Derive the unit of measurement of speed with the help of basic SI units.
i. The distance covered by a moving body in unit time is its speed. .
ii. The formula of speed is as follow:

iii.

metre
Speed =
second
-1
= m/s or ms
Therefore, the unit of speed is m/s or ms-1 (meter per second).
(b) What is a distance-time graph? How is it useful?
 Distance time graph is a method to analyze the motion of an object.
 This illustrates how far an object moved and how long it took to reach at a
certain point.
 The Y- axis of the graph is used to plot the distances, whereas X-axis is used to
plot the time.
 The slope of the distance-time graph for an object in uniform motion is straight
line.
 The slope of the distance-time graph for an object in non-uniform motion is
not a straight line.

7. Observe the picture and answer the questions that follow. (Ap)

a) At what time of the day will the shadow be the shortest?
Ans.: At noon when the Sun is just above an object.
(b) At what time of the day will the shadow be the longest?
Ans.: It is the longest during morning and evening when the Sun is low in the sky.
(c) What could be the possible application of observing the shadow of a stick under the
Sun?
Ans.: For time keeping during day light.
(d) In which direction does the shadow of a stick fall?
Ans.: The shadow of the stick falls in the direction opposite to Sun.
8. Observe the experiment and answer the questions that follow. (An)
The length of the string of a pendulum is fixed for the entire experiment. When the bob is
released from position A, it starts moving back and forth as shown in the following diagram

a) Identify the nature of motion exhibited by the bob of the pendulum.
It is oscillation.
(b) With the help of the above diagram, explain the motion of the bob.
i. During oscillation the bob moves from its initial position A to C then from C
to A is counted as one oscillation.

ii. Similarly, when the bob moves from its initial position C to A and then
comes back to C is also counted as one oscillation.
(c) Why should the length of the string be fixed in this experiment?
i. The time period of a simple pendulum depends only upon a factor that is the
length of the string with the help of which the bob is suspended in the air.
ii. If the length of the string of the pendulum was not fixed, then oscillations
would not be equal.