# Motion and Time NCERT Solutions – Class 7 Science

Exam preparation is incomplete if students do not solve NCERT questions. Solving NCERT questions always gives students an edge in exams. Many of the exam questions follow a pattern very similar to NCERT texbook exercises. Hence, to guide students, Extramarks offers students NCERT Solutions for Class 7 Science Chapter 13. These are clear and concise answers to textbook questions given in this chapter. They will help students to prepare more effectively, write better answers in their exams, and get help in their  revision just before the exams.

## NCERT Solutions for Class 7 Science Chapter 13

### NCERT Solutions for Class 7 Science

You can find all the NCERT answers for Class 7 in-text and end-text questions of this chapter. The NCERT solutions are available to download from the official website or app of Extramarks. Accurate answers and proper presentation of each answer will help you secure good grades in your assignments and exams.

### Introduction

You have already studied different types of motion in Class 6. A motion, as you might know, can be along a straight line, or would be circular or periodic. In this chapter, you will learn in detail about the different types of motion.

13.1 Slow or Fast

You must have noticed different vehicles on the road. You can easily tell which one of them is moving faster than the other one by just looking at them. Read activity 13.1 carefully and look at the diagram to grasp the concept of slow and fast vehicle movement in a better form.

13.2 Speed

The speed of an object is the distance it travels in a given amount of time. You may have heard your father telling you that the car is travelling at 60 kilometres per hour. This indicates that the vehicle travels 60 kilometres in one hour. In this chapter, you will further study the concept of speed and the speed of various items and the time it takes for the objects to perform a task.

13.3 Measurement of Time

Have you ever wondered, how would anyone know the time of the day without a clock? Our ancestors did not have clocks with them at that time. As you may be aware, many natural events repeat themselves at regular intervals of time. This section of this chapter talks about time. You’ll also learn about a pendulum in this segment.

Oscillatory motion, also known as periodic motion, is a basic back and forth motion of the pendulum. The bob is the little metallic ball at the end of a pendulum. The pendulum period is the amount of time it takes to complete one oscillation.

Units of Time and Speed

Since all values are measured in metric units, we also have units for measuring time and speed.  The different units of speed will be discussed in this section.

S.I. unit of time=Second(s)

S.I. unit of speed=Metre per second (m/s)

13.4 Measuring Speed

Now that you have learnt how to calculate distance and time,you can calculate an object’s speed using the time it takes to travel a specific distance. In this section, we will also learn how to determine the speed of a ball through activity 13.3. Thereafter, you can practise calculating the speed of other objects by using the formula. You might have seen a metre-like gadget installed in cars or bikes. Note that one of the metres has km/h written in the corner. This device is known as a speedometer that measures speed in kilometres per hour. Whereas, the odometer is the other metre that measures the distance travelled by the vehicle.

13.5 Distance – Time Graph

In Mathematics, you’ve previously learned about several graphs. The concept of a distance-time graph is discussed in this section. A bar graph, a pie chart, and a line graph, all can show distance and time. This section will tell us about various distance-time graphs and how they’re made. In general, distance-time graphs are used to depict the motion of any object in a visual format. A straight-line graph represents the distance-time graph of an item travelling at a constant speed. You will learn how to read such graphs and how to plot them step by step in this section.

#### Key Features of NCERT Solutions for Class 7 Science

Some of the key features of NCERT Solutions for Class 7 Science by Extramarks include:

• Clear and concise explanations
• Drafted by subject matter experts for accuracy
• Examples and Illustrations that help get a better understanding of the solution
• Great for last-minute revisions and exam preparations.

Q.1 Classify the following as motion along a straight line, circular or oscillatory motion:
(i) Motion of your hands while running.
(ii) Motion of a horse pulling a cart on a straight road.
(iii) Motion of a child in a merry-go-round.
(iv) Motion of a child on a see-saw.
(v) Motion of the hammer of an electric bell.
(vi) Motion of a train on a straight bridge.

Ans-

 S.No. Straight Line Motion Circular Motion Oscillatory Motion 1. Motion of a horse pulling a cart on a straight road. Motion of a child in a merry-go-round. Motion of your hands while running. 2. Motion of a train on a straight bridge. Motion of a child on a see-saw. 3. Motion of the hammer of an electric bell.

Q.2 Which of the following are not correct?
(i) The basic unit of time is second.
(ii) Every object moves with a constant speed.
(iii) Distances between two cities are measured in kilometres.
(iv) The time period of a given pendulum is constant.
(v) The speed of a train is expressed in m/h.
Ans-

Following statements are not correct:

(ii) Every object moves with a constant speed.

(v) The speed of a train is expressed in m/h.

Q.3 A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?

$\begin{array}{l}\text{Ans-}\end{array}$ $\begin{array}{l}\text{Time period of a simple pendulum is the time taken to complete one oscillation.}\\ \text{The time period is given by,}\\ \text{Time period =}\frac{\text{Time taken}}{\text{Number of oscillations}}=\frac{32\mathrm{s}}{20}=1.6\mathrm{s}\end{array}$

Q.4 The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.

$\begin{array}{l}\text{Distance travelled = 240 km}\\ \text{Time taken = 4 hours}\\ \text{Let the speed of the train be x km/h}\\ \text{Speed =}\frac{\mathrm{distance}}{\text{time taken}}\\ \therefore \text{speed of train =}\frac{240\text{km}}{4\text{h}}=\text{60 km/h}\end{array}$

Q.5 The odometer of a car reads 57321.0 km when the clock shows the time 08:30 AM. What is the distance moved by the car, if at 08:50 AM, the odometer reading has changed to 57336.0 km? Calculate the speed of the car in km/min during this time. Express the speed in km/h also.

Ans-

$\begin{array}{l}\text{Difference in the readings of odometer =}\left(\text{final – initial}\right)\text{readings = distance travelled}\\ \text{= 57336}\text{.0 km = 15 km}\\ \text{Difference in readings of clock = time taken = 08}\text{.50 AM – 08}\text{.30 AM = 20 min}\\ \therefore \text{the speed of car in km/min =}\frac{\text{distance}}{\text{time taken}}\text{=}\frac{\text{15 km}}{\text{20 min}}\text{= 0}\text{.75 km / min}\\ \text{The speed of the car in km/h =}\frac{\text{15 km}}{\text{1/3h}}\text{= 45 km/h}\end{array}$

Q.6 Salma takes 15 minutes from her house to reach her school on a bicycle. If the bicycle has a speed of 2 m/s, calculate the distance between her house and the school.
Ans-

Ans-

Time taken by Salma = 15 min = 15 × 60 s = 900 s

Speed of bicycle = 2 m/s

Let the distance between school and house be x m

Distance travelled = time taken × speed

∴ Distance, x = 900 s × 2 m/s = 1800 m = 1.8 km

Sr9 (1674867)

Q.7 Show the shape of the distance-time graph for the motion in the following cases:
(i) A car moving with a constant speed.
(ii) A car parked on a side road.

Ans-

(i) The shape of the distance-time graph for the motion of car with constant speed will be as follows:

(ii) The shape of the distance-time graph for the motion of car parked on a side road will be as follows:

$\begin{array}{l}\mathrm{Q.8Which}\mathrm{of}\mathrm{the}\mathrm{following}\mathrm{relation}\mathrm{is}\mathrm{correct}?\end{array}$ $\begin{array}{l}\left(\text{(i)}\right)\text{ }\mathrm{speed}=\mathrm{Distance}\text{ }×\text{ }\mathrm{time}\end{array}$ $\begin{array}{l}\left(\mathrm{\left(ii\right)}\right)\text{ }\mathrm{speed}=\frac{\mathrm{Distance}}{\mathrm{time}}\end{array}$ $\begin{array}{l}\left(\mathrm{\left(iii\right)}\right)\text{ }\mathrm{speed}=\frac{\mathrm{time}}{\mathrm{Distance}}\\ \left(\mathrm{iv}\right)\text{ }\mathrm{speed}=\frac{1}{\mathrm{Distance}\text{ }×\text{ }\mathrm{time}}\end{array}$

Ans-

Correct option is (ii).

Q.9 Explanation: Speed is the distance traveled per unit of time.

The basic unit of speed is:
(i) km/min
(ii) m/min
(iii) km/h
(iv) m/s

Ans-

Correct option is (iv).
Explanation: Speed is the distance traveled per unit of time. The standard unit of distance is metre (m) and standard unit of time is second (s).

Q.10 A car moves with a speed of 40 km/h for 15 minutes and then with a speed of 60 km/h for the next 15 minutes. The total distance covered by the car is:
(i) 100 km
(ii) 25 km
(iii) 15 km
(iv) 10 km

Ans-

Average speed of car = (40 km/h + 60 km/h)/2 = 50 km/h
Total distance travelled = speed × total time taken = 50 km/h × (½) h = 25 km/h

Q.11 Suppose the two photographs, shown in Fig. 13.1 and Fig. 13.2, had been taken at an interval of 10 seconds. If a distance of 100 metres is shown by 1 cm in these photographs, calculate the speed of the blue car.

Ans-

The steps to be followed are as follows:
1.
Find the distance travelled by blue car in photographs by measuring distance taking mid-points of car in following manner:

(Here, distance may vary in centimeters due to difference in the size of photographs taken. So, follow only the method for finding right answer, which may vary.)

2. According to the question, 1 cm is equivalent to 100 m.
Therefore, distance travelled = 0.5 cm x 100m = 50m

3. Speed = distance travelled / time taken
Therefore, speed = 50m / 10s = 5 m/s

Q.14 Fig.13.15 shows the distance-time graph for the motion of two vehicles A and B. Which one of them is moving faster?

Ans-

Among the vehicles, vehicle A is moving faster than B. It is so because A has travelled greater distance in shorter period of time in comparison to that of vehicle B. Vehicle B has moved lesser distance in greater period of time as depicted in graph.

Q.13 Which of the following distance-time graphs shows a truck moving with speed which is not constant?

Ans-

Among the given distance-time graphs, following graph shows the movement of a truck with speed which is not constant:

It is so because in this graph vehicle is not covering equal distances in equal intervals of time.

1. According to Chapter 13 of Class 7 Science, what are motion and time?

The term “motion” refers to a change in an object’s position concerning time. You will study about different motions in  Class 7 Motion and Time, including why some motions are rapid, and others are slow, how speed is measured, how time is measured, and the different units of speed and time. You’ll also discover how to draw and comprehend a distance-time graph. Apart from all that,   solving some real-life difficulties is also there in the chapter.

2. What is the unit of time?

The basic unit of time is second. The minute and hour are the larger time units. The second is represented by the letter ‘s’. Though it is the most widely used unit of time, it is not the only one. Other units are also used depending on the situation.

3. What is speed?

Speed is defined as the distance a moving item travels in a given amount of time. Its base unit is the metre per second, but fast-moving vehicles’ speeds are sometimes stated in kilometres per hour. Fast-moving items have a high speed, which means they cover more distance in a given amount of time than slow-moving objects.

4. What is the unit of speed?

Metre per second (m/s) is the unit of speed, defined as distance divided by time. Time is measured in seconds, minutes and hours, and distance is measured in kilometres, centimetres, and millimetres. As a result, speed is measured in various ways, including cm/min, km/hr, and so on. These units are only utilised when necessary.

5. How can I do well in Chapter 13 of Science in Class 7?

You should start studying the concepts of this chapter thoroughly from the NCERT Science book To understand the chapter in a better manner, you can also practice with the NCERT solution books. There you will get comprehensive answers to the chapter based questions which will further help the students to perform well in their exams.

6. How to solve the numerical in Chapter 13 of NCERT Solutions for Class 7 Science?

Getting good grades in this chapter becomes much easier if you understand the numerical part. Students are advised to consult the NCERT solutions provided by Extramarks to grasp the approach of answering problems without difficulty. The questions in the solutions are answered step-wise, with detailed explanations provided for each step. Students can use these solutions to solve textbook problems with ease. To aid students in remembering the formulas during the revision, shortcut techniques and tactics are mentioned in the solutions.

7. What are the topics covered in NCERT for Class 7 Science chapter 13?

The following topics are covered in Chapter 13 of NCERT for  Class 7 Science:

• Slow/Fast – The distance travelled by objects in a given time interval can help us determine which one of them is faster or slower.
• Speed – The speed of an object can be defined as the distance travelled in a unit of time.
• Time measurement
• Speed measurement
• Time-distance graph