NCERT Solutions for Class 7 Science Chapter 9
This chapter explores the basic concepts of motion and time, which are fundamental to understanding the physical world around us. Here’s a breakdown of the key ideas:
Motion:
- Motion is anything that changes its position over time. Examples: walking, running, swimming, swinging.
Rest:
- Rest is the opposite of motion. An object is at rest if it remains in the same position over time.
Types of Motion:
- Straight Line Motion: An object moves along a straight line (e.g., a car moving on a highway).
- Circular Motion: An object moves in a circular path (e.g., a merry-go-round).
Measuring Distance:
- We use units like meters (m), centimeters (cm), and kilometers (km) to measure distance traveled by an object.
Measuring Time:
- We use units like seconds (s), minutes (min), and hours (hr) to measure the duration of motion.
Speed:
- Speed tells us how quickly an object changes its position over time.. It is calculated by dividing the distance traveled by the time taken (speed = distance/time).
Uniform Motion:
- An object is in uniform motion if it covers equal distances in equal intervals of time. (This means its speed remains constant.)
Non-Uniform Motion:
- An object is in non-uniform motion if it covers unequal distances in equal intervals of time. (This means its speed keeps changing.)
Distance-Time Graph:
- A distance-time graph is a visual representation of how the distance traveled by an object changes with time. It can help us understand whether the motion is uniform or non-uniform.
Importance of Motion and Time:
- Understanding motion and time is essential for describing various natural phenomena, like the movement of planets, the flow of rivers, and the growth of plants.
- It also plays a crucial role in everyday activities, like calculating travel time or measuring the speed of a car.
Learning about Motion and Time helps us:
- Describe the movement of objects around us.
- Measure and compare speeds.
- Analyze motion using graphs.
- Appreciate the importance of these concepts in science and daily life.
Exercises
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 :
(i) Oscillatory motion.
(ii)Straight line motion.
(iii) Circular motion.
(iv) Oscillatory motion.
(v) Oscillatory motion.
(vi) Straight line motion.
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 kilometers.
(iv) The time period of a given pendulum is not constant.
(v) The speed of a train is expressed in m/h
Ans :
(ii) Every object moves with a constant speed.
3.A simple pendulum takes 32s to complete 20 oscillations, what is the time period of the pendulum?
Ans :
- Total Time: We know the simple pendulum takes 32 seconds to complete 20 oscillations.
- Oscillations: There are 20 oscillations in this time.
Finding the Time Period:
- We want to know the time taken for one oscillation.
- Divide the total time (32 seconds) by the number of oscillations (20).
Time period per oscillation = Total Time / Number of Oscillations
Time period = 32 seconds / 20 oscillations
Calculating the Time Period:
- Simplifying the division: 32 seconds / 20 oscillations = 1.6 seconds/oscillation
Therefore, the time period of the pendulum is 1.6 seconds. It takes 1.6 seconds for the pendulum to complete one swing back and forth.
4.The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.
Ans :
Given Information:
- Distance (d) = 240 km
- Time (t) = 4 hours
Calculating Speed:
Speed (s) = distance (d) / time (t)
s = 240 km / 4 hours
Speed Calculation:
- If we keep time in hours: s = 240 km / 4 h = 60 km/h
- If we convert time to minutes: s = 240 km / 240 minutes = 1 km/minute (which is also equal to 60 km/h)
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 :
Distance Traveled:
- Initial reading: 57321.0 km
- Final reading: 57336.0 km
- Distance traveled (difference between readings): 57336.0 km – 57321.0 km = 15.0 km
Time Taken:
The time difference between the readings is from 8:30 AM to 8:50 AM.
There are two ways to calculate the time taken:
- Minutes: We can directly calculate the difference in minutes: 8:50 AM – 8:30 AM = 20 minutes.
- Hours: Convert the time difference to hours:
- Time taken = (8:50 AM – 8:30 AM) = 1 hour (since there’s a difference of 1 hour)
- However, within that 1 hour, the car only traveled for 20 minutes. So, we can express the time taken as 1 hour * (20 minutes / 60 minutes/hour) = 1/3 hours.
Speed in km/min:
We can use either method for time taken to calculate the speed.
- Using minutes: Speed (km/min) = Distance (km) / Time (minutes) Speed = 15.0 km / 20 minutes = 0.75 km/min
Speed in km/h:
- Using hours: Speed (km/h) = Distance (km) / Time (hours) Speed = 15.0 km / (1/3 hours) = 15.0 km * (3 hours/1) (multiplying by the reciprocal to convert hours to minutes) Speed = 45.0 km/h
6.Salma takes 15 minutes from her house to reach her school on a bicycle. If the bicycle has a speed of 2 m/min, calculate the distance between her house and the school.
Ans :
Time taken =15 min
Speed = 2 m/min .
Distance = speed x time = 2 x 15 = 30 m
Distance between Salma’s school and her house is 30 m.
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 :
8. Which of the following relations is correct?
(i) Speed = Distance Time
(ii) Speed = Distance/Time
(iii)Speed – Time/Distance
(iv)Speed = 1/Distance Time
Ans : (ii) Speed = Distance/Time
9.The basic unit of speed is:
(i) km/min (ii) m/min (iii) km/h (iv) m/s
Ans : (iii) km/h (kilometers per hour)
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 : (ii) 25 km
11.Suppose the two photographs, shown in fig. 13.1 and fig. 13.2 of NCERT 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.0.1 cm/s or 10 m/s
12.Fig 13.10 shows the distance-time graph for the motion of two vehicles A and B. Which one of them is moving faster?
Ans : A is moving
13.Which of the following distance-time graphs shows a truck moving with speed which is not constant?
Ans. (iii)
FAQ’s
What resources are available for studying NCERT Solutions for Class 7 Science Chapter 9 on Motion and Time?
Education85 provides comprehensive NCERT solutions for Class 7 Science Chapter 9, focusing on the concept of Motion and Time. These solutions offer detailed explanations and answers to questions related to motion, its measurement, and the concept of time. Explore our resources to enhance your understanding of this important chapter in your science curriculum.
How can NCERT Solutions for Chapter 9 help me understand the concept of motion and time?
By studying NCERT Solutions for Chapter 9, you can gain a comprehensive understanding of the relationship between motion and time. These solutions provide detailed explanations and examples to clarify concepts such as speed, velocity, and measuring time intervals.
How can I use NCERT Solutions for Chapter 9 to improve my understanding of motion and time?
You can use NCERT Solutions for Chapter 9 as a comprehensive study guide to review key concepts, solve practice questions, and clarify doubts related to motion and time. Additionally, you can engage in practical activities and experiments to further reinforce your understanding.
Are NCERT Solutions for Class 7 Science Chapter 9 suitable for exam preparation?
Yes, NCERT Solutions for Chapter 9 serve as excellent study materials for exam preparation. They cover all essential topics related to motion and time and include practice questions to help you test your understanding and prepare effectively for exams.
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