DAV Class 7 Science Chapter 6 Question Answer – Motion and Time

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DAV Class 7 Science Ch 6 Question Answer – Motion and Time

DAV Class 7 Science Ch 6 Solutions – Motion and Time

Something To Know

A. Fill in the blanks.

Question 1.
An object is said to be at rest if it ________ change its position with time.
Answer:
Does not

Question 2.
The SI unit of time is ________
Answer:
Second

Question 3.
A child, sitting in a revolving giant wheel, is an example of ________ motion.
Answer:
Circular

Question 4.
A car, moving on a busy straight road, is an example of ________ motion.
Answer:
Non-uniform

Question 5.
The speedometer of a motorbike measures its speed in ________
Answer:
Kilometre per hour

B. Write True or False for the following statements.

1. The speed of a fast moving train is usually measured in metre per hour.
2. The average speed remains constant for an object having a uniform motion.
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.
4. An object, moving along a straight ling is said to be in uniform motion, if it covers regularly increasing distances in equal intervals of time.
5. The time period of a simple pendulum, that takes 42 seconds to complete 20 oscillations equals 2.1 seconds.
6. The distance-time graph, for a car kept parked on a side road, is a straight line parallel to the time axis.
Answer:
1. False
2. True
3. False
4. False
5. True
6. False

C. Tick (✓) the correct option.

1. Out of the following, the only correct formula is
(a) distance × time = speed
(b) distance = speed × time
(c) \(\text { time }=\frac{\text { speed }}{\text { distance }}\)
(d) \(\text { speed }=\frac{\text { time }}{\text { distance }}\)
Answer:
(b) distance = speed × time

2. A man walks on a straight road from his home to a market 3 km away with a speed of 6 km/h. The time taken by the man to go from his home to market, equals
(a) 18 minutes
(b) 30 minutes
(c) 40 minutes
(d) 50 minutes
Answer:
(b) 30 minutes

3. The odometer of a car reads 57321.0 km when the clock shows the time as 8:30 a.m. The odometer reading changes to 57336.0 km at time 8:50 a.m. The distance, moved by the car, in these 20 minutes, equals
(a) 15 km
(b) 21 km
(c) 26 km
(d) 36 km
Answer:
(a) 15 km

4. Out of the following distance-time graphs, the one that represents a truck at rest, is the graph labelled
Answer:

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

(a) (t₁ + t₂)
(b) 2(t₁ + t₂₎
(c) \(\left[\frac{\left.t_1+t_2\right)}{2}\right]\)
(d) 4(t₁ + t₂)
Answer:
(d) 4(t₁ + t₂)

6. The SI unit of speed is
(a) km/minute
(b) m/ minute
(c) km/hour
(d) m/second
Answer:
(d) m/second

D. Answer the following questions in brief.

Question 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:
Here; speed = 4 km/h and time = 30 minutes = 1/2 h
We know that,
distance = speed × time
So, the distance of school from his house = 2 km

Question 2.
The distance between two stations is 216 km. A bus takes 4 hours to cover this distance. Calculate the average speed of the bus in km/hour.
Answer:
Here, distance = 216 km and time = 4 hours

We know;
\(\text { speed }=\frac{\text { distance }}{\text { time }}\)
or
\(\text { speed }=\frac{216 \mathrm{~km}}{4 \mathrm{~h}}\) = 54 km per hour
So, the average speed of bus = 54 km/hour.

Question 3.
Two Cars, A and B, (starting, at the same time from the same point) are moving with average speeds of 40 km/h and 50 km/h respectively, in the same direction. Find how far will Car B be from Car A after 3 hours.
Answer:
Speed of Car A = 40 km/h and speed of car B = 50 km/h
Distance = speed × time
So, distance covered by Car A = 40 3 = 120 km
and distance covered by Car B = 50 × 3 = 150 km
Difference in distances covered by two cars = 150 km – 120 km = 30 km
So, Car B will be 30 km far from Car A.

Question 4.
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. Find the total distance covered by the car in these 30 minutes.
Answer:
Speed for first 15 minutes = 40 km/h and speed for next 15 minutes = 60 km/h
Distance = speed × time
So, distance covered in 1st 15 minutes = 40 × \(\frac{15}{60}\) = 10 km
and distance covered in next 15 minutes = 60 × \(\frac{15}{60}\) = 15 km
Total distance covered in 30 minutes =10 km +15 km = 25 km

Question 5.
Define the term ‘Periodic Motion’. Give two examples of periodic motions that can be used to measure time.
Answer:
When a motion repeats at certain intervals, it is called periodic motion. Two examples of periodic motion that can be used to measure time are; motion of a pendulum and motion of the hands of a watch.

Question 6.
Distinguish between uniform and non-uniform motion. Give one example of each.
Answer:

Uniform motion	Non-uniform motion
Speed is same at different times.	Speed is different at different times.
Example: motion of a car on a straight road.	Example: Motion of a car on a nonstraight road.

Question 7.
Draw the shape of distance-time graph for
(a) A man, waiting for a bus, standing at one point, on a bus-stand.
Answer:

(b) A man, walking on a level, straight and narrow road, with constant speed.
Answer:

E. Answer the following questions.

Question 1.
A farmer moves along the boundary of a rectangular field ABCD as shown in the figure. He takes 4 minutes to travel across each side.

Is the motion of this farmer uniform or non-uniform? Find his average speed over one complete round of the field.

Answer:
The farmer is covering 40 km in one instance of 4 minutes and 80 m in another instance of 4 minutes. Hence, his motion is non-uniform.
Average speed can be calculated as follows:
Total distance = 80 + 80 + 40 + 40 = 240 m
Total time taken = 16 minute
Average speed = \(\frac{\text { Total distance }}{\text { total time }}\)
= \(\frac{204 \mathrm{~m}}{16 \mathrm{~min}}\) = 15 m/min

Question 2.
During rainy season, Shivam noted that the thundering sound of clouds was heard 6 seconds after the lightning was seen by him. If speed of sound in air is 340 m/s, find the distance of the point where the thundering sound was produced.
Answer:
Here, speed = 340 m/s and time = 6s
Average speed = \(\frac{\text { Total distance }}{\text { total time }}\)
Or, Total distance = Average speed × total time
= 340 m/s × 6s = 2040 m

Question 3.
(a) How can we make a simple pendulum?
Answer:
For making a simple pendulum, you need a plastic ball and a thin but strong thread. Fix one end of the thread on the ball (at a point) with the help of adhesive
tape. Suspend the ball to a rigid support. The simple pendulum is ready.

Value Based Question

In the science class, Drishti observed a video showing the working of a pendulum wall clock. She noted that a slight change in the initial displacement of the (bob) did not affect the time period of the pendulum. When she discussed her observation with her grandfather, he told her that we must also not let the effects of small disturbances to change the daily routine and the values of our life.

Question 1.
State any four values that we should maintain in our life.
Answer:
Discipline, punctuality, hardwork and respect for elders

Question 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:
For this, take a plastic ball and a thin but strong thread. Using these materials, make a simple pendulum.

• Suspend the pendulum to a rigid support. Keep the bob close to (but not touching) a flat surface.
• Draw a straight line of 5 cm on the flat surface. This line should be just below the bob of the pendulum and bob should be near the middle of the line.
• Now, pull the bob to one extreme of this line and release.
• With the help of a stopwatch, measure the time taken for 10 oscillations. Calculate the time period.
• After that, increase the length of the line by 1 cm on either side. Then, repeat the above steps to calculate the time period.
• You will notice that there is no change in time period.
• This shows that a slight displacement of the bob does not affect the time period of a simple pendulum.

Something To Do

Question 1.
The following figure shows the distance- time graph for two students A and B who start from their school and reach their respective homes. Examine it carefully and answer the following questions:

(a) Who starts first from the school?
Answer:
B is the first to start from school.

(b) Who reaches his home first?
Answer:
A is the first to reach home.

(c) Who covers more distance?
Answer:
A covers more distance.

(d) Whose average speed is more?
The average speed of A is more.

Question 2.
Assume that the distance between Bengaluru and Delhi is 1200 km by air and 1500 km by train. A person takes 2 hours to reach there by air but needs 25 hours to reach there by train. Find the average speed in both the cases. Repeat this exercise by collecting data for different cities.
Answer:
For plane: distance = 1200 km and time = 2 hour
For train: distance = 1500 km and time = 25 hour
\(\text { speed }=\frac{\text { distance }}{\text { time }}\)

So, speed of plane = \(\frac{1200}{2}\) = 600 km per h
and speed of train = \(\frac{1500}{25}\) = 60 km per h

Question 3.
Draw a long straight line on the ground. Mark points at distances 10 m, 20 m, 30 m and so on. Compare your speed of walking with that of your friends. Measure the time taken to cover a known distance using a stopwatch. Find out who amongst you is the fastest. Represent your results in the form of a table. Draw a bar-graph to show the speeds of different students. Also draw ‘distance-time’ graphs for each student.
Answer:Do it yourself.

DAV Class 7 Science Chapter 6 Solutions – Motion and Time

I. Fill in the blanks.

Question 1.
A pendulum shows the example of ________ motion.
Answer:
Periodic

Question 2.
The SI unit of speed is ________
Answer:
m/s

Question 3.
Distance between two cities is measured in terms of ________
Answer:
Kilometer

Question 4.
Distance between two stars is measured in terms of ________
Answer:
light year

Question 5.
________is the fastest known physical quantity.
Answer:
Light

Question 6.
________ motion is not possible in practical situations.
Answer:
Uniform

II. Tick (✓) the correct option.

1. Which of the following shows a periodic motion?
(a) A flying bird
(b) Moon revolving around the earth
(c) A car on a circular path
(d) A train on a straight track
Answer:
(b) Moon revolving around the earth

2. A pendulum takes 12 second to complete one oscillation. What is the time period of this pendulum?
(a) 12 s
(b) 6 s
(c) 24 s
(d) 36 s
Answer:
(a) 12 s

3. A car travels at a speed of 45 km per h. What is the speed of this car in m/sec?
(a) 5 m/s
(b) 10 m/s
(c) 12.5 m/s
(d) 25 m/s
Answer:
(c) 12.5 m/s

4. A student draws a distance-time graph in which he shows a straight line parallel to the time axis. Which of the following is depicted by the graph?
(a) A car with increasing speed
(b) A car with decreasing speed
(c) A car at constant speed
(d) A static car
Answer:
(d) A static car

5. A planet is moving around its axis. Which type of motion is being shown by the planet?
(a) Circular motion
(b) Rotaation
(c) Curvilinear motion
(d) Rectilinear motion
Answer:
(b) Rotaation

III. Answer the following questions.

Question 1.
Harley covers a distance of 280 km in 4 hours. Find his average speed during this trip.
Answer:
Here, distance = 280 km and time = 4 hours
We know; speed = \(\frac{\text { distance }}{\text { time }}\) = \(\frac{280}{4}\) = 70 km per h

Question 2.
Rajdhani Express takes 20 hours to travel from Mumbai to Delhi. If the distance between Mumbai and Delhi is 1800 km; what is the average speed of Rajdhani Express?
Answer:
Here, distance = 1800 km and time = 20 hours
We know; \(\text { speed }=\frac{\text { distance }}{\text { time }}=\frac{1800}{20}\) = 90 km per h

Question 3.
The average speed of a motorbike is 60 km/h. How much time does it take to cover 210 km?
Here, distance = 210 km and speed = 60 km/h
We Know; time = \(\frac{\text { distance }}{\text { speed }}=\frac{210}{60}=3 \frac{1}{2} \mathrm{~h}\)

Question 4.
What do you understand by uniform and non-uniform motion?
Answer:
Uniform Motion: When an object moves along a straight path and covers equal distance in equal time, its motion is said to be uniform motion.
Non-uniform Motion: When an object covers different distances in equal times, its motion is said to be non-uniform motion.

Question 5.
What do you understand by speed? Explain.
Answer:
Speed: Speed gives the measure of rate of motion.
Speed = Distance/Time
The SI unit of speed is metre per second (m/s). It is also expressed in other units such as m/minute or km/h.

IV. Projects

Question 1.
Take a railway timetable and look for timings of a train at major stations. Using this data, plot a distance-time graph for the selected train.
Answer:
Do it yourself.

Question 2.
Take a plastic ball and a string. Make a pendulum from them. Bring the pendulum in periodic motion and with the help of a stopwatch find its time period.
Answer:
Do it yourself.