YASRAJ STUDIES
DAV Class 7 Science Chapter 6 Solutions – Motion and Time
Introduction
Motion is a very common phenomenon in our daily life. Cars, buses, trains, bicycles and people move from one place to another. To describe motion, we need to know the distance travelled and the time taken.
In this chapter, we learn about motion, rest, speed, distance, time, uniform and non-uniform motion, periodic motion, simple pendulum and distance-time graphs.
Something To Know
A. Fill in the Blanks
1. An object is said to be at rest if it __________ change its position with time.
Answer: Does not
2. The SI unit of time is __________.
Answer: Second
3. A child, sitting in a revolving giant wheel, is an example of __________ motion.
Answer: Circular motion
4. A car, moving on a busy straight road, is an example of __________ motion.
Answer: Non-uniform motion
5. The speedometer of a motorbike measures its speed in __________.
Answer: Kilometre per hour (km/h)
B. Write True or False
1. The speed of a fast-moving train is usually measured in metre per hour.
Answer: False
2. The average speed remains constant for an object having a uniform motion.
Answer: True
3. A man walks for 1 minute at a speed of 1 m/s along a straight track. The total distance covered by him is 1 m.
Answer: False
Explanation:
Given:
Speed = 1 m/s
Time = 1 minute = 60 seconds
We know:
Distance = Speed × Time
= 1 × 60
= 60 m
Therefore, the man covers 60 m, not 1 m.
4. An object moving along a straight line is said to be in uniform motion if it covers regularly increasing distances in equal intervals of time.
Answer: False
5. The time period of a simple pendulum that takes 42 seconds to complete 20 oscillations equals 2.1 seconds.
Answer: True
Explanation:
Time period is the time taken for one complete oscillation.
Time period = Total time ÷ Number of oscillations
= 42 ÷ 20
= 2.1 seconds
Therefore, the statement is true.
6. The distance-time graph for a car kept parked on a side road is a straight line parallel to the time axis.
Answer: True
C. Tick (✓) the Correct Option
1. Out of the following, the only correct formula is:
Answer: (b) Distance = speed × time
Speed = Distance ÷ Time
Rearranging this formula gives:
Distance = Speed × Time
Therefore, option (b) is correct.
2. A man walks on a straight road from his home to a market 3 km away with a speed of 6 km/h. Find the time taken.
Answer: (b) 30 minutes
Explanation:
Distance = 3 km
Speed = 6 km/h
We know:
Time = Distance ÷ Speed
= 3 ÷ 6 hour
= 1/2 hour
Since 1 hour = 60 minutes:
1/2 hour = 30 minutes
Therefore, the correct answer is 30 minutes.
3. The odometer of a car reads 57321.0 km at 8:30 a.m. and 57336.0 km at 8:50 a.m. Find the distance moved by the car.
Answer: (a) 15 km
Explanation:
Initial odometer reading = 57321 km
Final odometer reading = 57336 km
Distance travelled:
= Final reading − Initial reading
= 57336 − 57321
= 15 km
Therefore, the car travelled 15 km.
4. Out of the following distance-time graphs, the one that represents a truck at rest is the graph labelled __________.
Answer: The graph having a horizontal straight line parallel to the time axis.
5. In the given diagram of a simple pendulum, the time taken by the bob to move from X to Z is t₁ and from Z to O is t₂. The time period of this simple pendulum is:
Answer: (d) 4(t₁ + t₂)
Time period = 4(t₁ + t₂)
Hence, option (d) is correct.
6. The SI unit of speed is:
Answer: (d) metre/second (m/s)
Therefore:
SI unit of speed = metre/second (m/s)
D. Answer the Following Questions in Brief
1. A boy walks to his school with a constant speed of 4 km/h and reaches there in 30 minutes. Find the distance of the school from his house.
Answer: 2 km
Explanation:
Speed = 4 km/h
Time = 30 minutes
Convert time into hours:
30 minutes = 30/60 = 1/2 hour
We know:
Distance = Speed × Time
= 4 × 1/2
= 2 km
Therefore, the school is 2 km from his house.
2. The distance between two stations is 216 km. A bus takes 4 hours to cover this distance. Calculate its average speed.
Answer: 54 km/h
Explanation:
Distance = 216 km
Time = 4 hours
We know:
Average speed = Total distance ÷ Total time
= 216 ÷ 4
= 54 km/h
Therefore, the average speed of the bus is 54 km/h.
3. Two cars, A and B, start at the same time from the same point. Their speeds are 40 km/h and 50 km/h respectively. Find how far Car B will be from Car A after 3 hours.
Answer: 30 km
Explanation:
Distance covered by Car A:
= 40 × 3
= 120 km
Distance covered by Car B:
= 50 × 3
= 150 km
Difference:
= 150 − 120
= 30 km
Therefore, Car B will be 30 km ahead of Car A.
4. A car moves at 40 km/h for 15 minutes and then at 60 km/h for the next 15 minutes. Find the total distance covered.
Answer: 25 km
Explanation:
First 15 minutes
15 minutes = 15/60 hour = 1/4 hour
Distance:
= 40 × 1/4
= 10 km
Next 15 minutes
Distance:
= 60 × 1/4
= 15 km
Total distance
= 10 + 15
= 25 km
Therefore, the total distance covered is 25 km.
5. Define periodic motion. Give two examples of periodic motion that can be used to measure time.
Answer:
A motion that repeats itself at regular intervals of time is called periodic motion.
Examples:
- Motion of a pendulum.
- Motion of the hands of a clock.
6. Distinguish between uniform and non-uniform motion. Give one example of each.
Answer:
| Uniform Motion | Non-uniform Motion |
|---|---|
| Equal distances are covered in equal intervals of time. | Unequal distances are covered in equal intervals of time. |
| Speed remains constant. | Speed changes with time. |
| Example: A vehicle moving at constant speed on a straight road. | Example: A car moving in heavy traffic. |
7. Draw the shape of the distance-time graph for:
(a) A man waiting for a bus at one point on a bus stand.
Answer: A horizontal straight line.
(b) A man walking on a straight road with constant speed.
Answer: A straight sloping line.
E. Answer the Following Questions
1. A farmer moves along the boundary of a rectangular field ABCD. He takes 4 minutes to travel across each side. Is his motion uniform or non-uniform? Find his average speed over one complete round.
Answer:
The farmer's motion is non-uniform.
Total distance:
= 80 + 80 + 40 + 40
= 240 m
Total time:
= 4 × 4
= 16 minutes
Average speed:
= Total distance ÷ Total time
= 240 ÷ 16
= 15 m/minute
His average speed for the complete round is 15 m/minute.
2. During the rainy season, Shivam heard thunder 6 seconds after seeing lightning. If the speed of sound is 340 m/s, find the distance of the point where the thunder was produced.
Answer: 2040 m or 2.04 km
Explanation:
Speed of sound = 340 m/s
Time = 6 s
We know:
Distance = Speed × Time
= 340 × 6
= 2040 m
Since 1000 m = 1 km:
2040 m = 2.04 km
Therefore, the thunder was produced approximately 2.04 km away.
The lightning is seen before the thunder is heard because light travels much faster than sound.
3. How can we make a simple pendulum?
Answer:
A simple pendulum can be made using a small ball, a strong thread and a rigid support.
Procedure:
- Tie one end of a thread to a small ball.
- Fix the other end of the thread to a rigid support.
- Make sure the ball can swing freely.
- Pull the ball slightly to one side.
- Release it without pushing.
Value-Based Question
1. State any four values that we should maintain in our life.
Answer:
Four important values are:
- Discipline
- Punctuality
- Hard work
- Respect for elders
2. Design an activity to show that a slight change in the initial displacement of the bob does not affect the time period of a simple pendulum.
Answer:
Materials Required
- Small ball
- Strong thread
- Rigid support
- Stopwatch
Procedure
- Make a simple pendulum using the ball and thread.
- Suspend it from a rigid support.
- Pull the bob slightly to one side and release it.
- Measure the time taken for 10 complete oscillations.
- Calculate the time period.
- Now increase the initial displacement slightly.
- Again measure the time taken for 10 oscillations.
- Compare the two time periods.
Observation
The two time periods will be nearly the same when the displacement is small.
Conclusion
A slight change in the initial displacement of the bob does not significantly affect the time period of a simple pendulum.
Something To Do
1. Study the Distance-Time Graph of Two Students
The graph shows the distance-time graphs of students A and B who start from their school and travel towards their respective homes.
(a) Who starts first from the school?
Answer: Student B
(b) Who reaches home first?
Answer: Student A
(c) Who covers more distance?
Answer: Student A
(d) Whose average speed is more?
Answer: Student A
2. Find the Average Speed by Air and Train
The distance between Bengaluru and Delhi is 1200 km by air and 1500 km by train. A person takes 2 hours by air and 25 hours by train.
By Air
Distance = 1200 km
Time = 2 hours
Speed = Distance ÷ Time
= 1200 ÷ 2
= 600 km/h
By Train
Distance = 1500 km
Time = 25 hours
Speed = Distance ÷ Time
= 1500 ÷ 25
= 60 km/h
Answer:
- Average speed by air = 600 km/h
- Average speed by train = 60 km/h
3. Compare Your Walking Speed with Your Friends
Draw a straight line on the ground and mark points at 10 m, 20 m, 30 m and so on. Measure the time taken by different students to cover a known distance.
Answer: Do it yourself.
For each student:
Speed = Distance ÷ Time
Record the results in a table and compare the speeds. A student who covers the same distance in less time has the greater speed.
A bar graph can then be prepared to compare the speeds of the students. Distance-time graphs can also be drawn using the recorded observations.
Important Points to Remember
- Motion means a change in position with time.
- An object that does not change its position is said to be at rest.
- The SI unit of time is second (s).
- The SI unit of speed is metre per second (m/s).
- A speedometer measures the speed of a vehicle.
- An odometer measures the distance travelled by a vehicle.
- Speed = Distance ÷ Time
- Distance = Speed × Time
- Time = Distance ÷ Speed
- Uniform motion means equal distances are covered in equal intervals of time.
- Non-uniform motion means unequal distances are covered in equal intervals of time.
- Periodic motion repeats itself at regular intervals.
- A simple pendulum shows periodic motion.
- A stationary object has a horizontal distance-time graph.
- Uniform motion is represented by a straight sloping distance-time graph.
- Average speed is calculated by dividing total distance by total time.
Conclusion
DAV Class 7 Science Chapter 6 – Motion and Time explains important concepts related to the movement of objects. Students learn how to calculate speed, distance and time, identify uniform and non-uniform motion, understand periodic motion and simple pendulums, and interpret distance-time graphs.
The numerical questions in this chapter are based mainly on the relationship:
Speed = Distance ÷ Time
Understanding this formula and its rearrangements makes it easier to solve most problems related to motion and time.
