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AP 7th Class Science 9th Lesson Motion and Time Questions and Answers
AP Board Solutions Class 7 Science Chapter 9 Motion and Time
Exercises
Question 1.
Classify the following as motion along a straight line, circular or oscillatory motion:
i) Motion of your hands while running.
ii) The motion of a horse pulling a cart on a straight road.
iii) The motion of a child in a merry-go-round.
iv) The motion of a child on a see-saw.
v) The motion of the hammer of an electric bell.
vi) The motion of a train on a straight bridge.
Answer:
i) Oscillatory motion
ii) Straight line
iii) Circular motion
iv) Oscillatory motion
v) Oscillatory motion
vi) Straight line
Question 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 not constant.
v) The speed of a train is expressed in m/h.
Answer:
i) Correct
ii) Not correct
iii) Correct
iv) Not correct
v) Not correct
Question 3.
A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?
Answer:
Time taken to complete 20 oscillations = 32 s
Time taken to complete 1 oscillation = 32/20s 1.6 s
Given, 20 oscillations taking 32 s to complete
Time period of the pendulum i.e. time period for 1 oscillation = time taken to complete 20 oscillations/ total no. of oscillations.
Time period of pendulum =32s/20 = 1.6 seconds.
Question 4.
The distance between two stations is 240 km . A train takes 4 hours to cover this distance. Calculate the speed of the train.
Answer:
Given, the distance between two stations = 240kms
Time taken to cover the distance =4 hours.
Speed = \(\frac{\text { Distance }}{\text { Time }}=\frac{240}{4}\) = 60km/h
Question 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.
Answer:
Initial reading of the odometer of the car = 57321.0 km}
Final reading of the odometer of the car = 57336.0 km}
Distance covered by car = (57336.0-57321.0) km=15 km
Time taken by the car to over the distante = (8: 50 – 8: 30) min = 20 min
Question 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.
Answer:
Time taken by salma to reach school from home = 15 minutes = 15 × 60 = 900 seconds
Speed of bicycle =2 m/s
Distance = time × speed = 900 × 2 = 1800 m
∴ The distance from home to school is 1800m = 1.8 km
Question 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.
Answer:
i) A car moving with a constant speed covers
equal distances in equal intervals of time. It will be a uniform motion. Such motion of car is represented by the given distance time graph.
ii) The distance time graph of a car parked on a roadside is such that with increase in time, there is no change in distance. There is no motion.Therefore the graph so obtained will be parallel to x-axis. Distance- time graph will be as below:
Question 8.
Which of the following relations is correct ?
i) Speed = Distance ×Time
ii) Speed =\(\frac{\text { Distance }}{\text { Time }}\)
iii) Speed =\(\frac{\text { Time }}{\text { Distance }}\)
iv) Speed =\(\frac{1}{\text { Distance } \times \text { Time }}\)
Answer:
ii) Speed = \(\frac{\text { Distance }}{\text { Time }}\)
Question 9.
The basic unit of speed is :
i) km/min
ii) m/min
iii) km/h
iv) m/s
Answer:
iv) m/s
Question 10.
A car moves with a speed of 40km/h for 15 minutes and then with a speed of 60km/h for the next 15 minutes. Total distance covered by the car is:
i) 100 km
ii) 2 5 k m
iii) 15 km
iv) 10 km
Answer:
ii) 25 km
Case I: Given speed of a car =40km/h; Time taken =15min = 15/60 = 0.25h
\(\text { Speed }=\frac{\text { Distance }}{\text { Time }}\)
Distance covered = Speed × time =40 × 0.25=10km
Case II: Given speed of a car =60km/h; Time taken = 15 min=15/60 = 0.25h
\(\text { Speed }=\frac{\text { Distance }}{\text { Time }}\)
Distance covered = Speed × Time =60 × 0.25=15km
Total distance covered =10km+15km=25km.
Question 11.
Suppose the two photographs shown in figure 9.1 and figure 9.2 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 fastest car.
Answer:
Measure the distance moved by the car with the help of a scale and then proceed as given below.
The distance covered by the fastest car, which is measured by scale is 2 cm
It is given that 1 cm is equivalent to 100 m
∴ 2 cm=200m
Distance travelled by car = 200m
Interval of time between the photos taken = 10 seconds.
Speed of blue car = \(\frac{\text { Distance }}{\text { Time }}=\frac{200}{10}\) = 20m/s
Question 12.
Figure shows the distance – time graph for the motion of two vehicles A and B. Which one of them is moving faster?
Answer:
In the distance-time graph, speed is measured by its slope.Vehicle A is moving faster than vehicle B. Because vehicle A has covered more distance than vehicle B in given
Time and Speed = \(\frac{\text { Distance }}{\text { Time }}\)
Question 13.
Which of the following distance – time graphs shows a truck moving with speed which is not constant?
Answer:
The slope of the distance-time graph represents the speed of an object. In graph (iii), the slope is not constant and hence it does not show a uniform motion.
Questions given in the lesson
Page No. 22
Question 1.
Table gives some common examples of motions. Identify the type of motion in each case.
| Example of motion | Type of motion (Along a straight line/ circular/ periodic) |
| Soldiers in a march past | |
| Bullock cart moving on a straight road | |
| Hands of an athlete in a race | |
| Pedal of a bicycle in a motion | |
| Motion of the Earth around the Skin | |
| Motion of a swing | |
| Motion of’a pendulum |
Answer:
| Example of motion | Type of motion (Along a straight line/ circular/ periodic) |
| Soldiers in a march past | Straight line |
| Bullock cart moving on a straight road | Straight line |
| Hands of an athlete in a race | Periodic |
| Pedal of a bicycle in a motion | Circular |
| Motion of the Earth around the Skin | Circular/ periodic |
| Motion of a swing | Periodic |
| Motion of’a pendulum | Periodic |
Page No. 32
Question 2.
Paheli wondered how time was measured when pendulum clocks were not available.
Answer:
Many times measuring devices were used in different parts of the world before the pendulum clocks became popular. Sundials water clocks and sand clocks are some examples of such devices.
Page No. 36
Question 3.
The unit of speed in the given table, in km/h. You can calculate the speeds in m/s yourself.
Fastest speed that some animals can attain
Answer:
Question 4.
Boojho wants to know whether there Is any device that measures the speed.
Answer:
Yes, speed is measured with the help of a device called speedometer.
Extended Learning – Activities and Projects
Question 1.
You can make your own sundial and use it to mark the time of the day at your place. First of all find the latitude of your city with the help of an atlas. Cut out a triangular piece of a cardboard such that its one angle is equal to the latitude of your place and the angle opposite to it is a right angle. Fix this piece, called gnomon, vertically along a diameter of a circular board a shown in Figure. One way to fix the gnomon could be to make a groove along a diameter on the circular board.
Next, select an open space, which receives sunlight for most of the day. Mark a line on the ground along the North-South direction. Place the sundial in the sun as shown in Figure. Mark the position of the tip of the shadow of the gnomon on the circular board as early in the day as possible, say 8:00 AM. Mark the position of the tip of the shadow every hour throughout the day.
Draw lines to connect each point marked by you with the centre of the base of the gnomon as shown in Figure. Extend the lines on the circular board up to its periphery. You can use this sundial to read the time of the day at your place. Remember that the gnomon should always be placed in the North-South direction as shown in Figure.
Answer:
The activity can be performed as follows :
- Find the latitude of the city with the help of Atlas.
- Cut out a triangular piece of a cardboard such that its one angle is equal to the latitude of the location and the angle opposite to it is a right angle.
- Fix this piece, called gnomon, vertically along a diameter of a circular board by making a groove along a diameter on the circular board.
- Choose a space, which receives sunlight for most of the day. Mark a line on the ground along the North-South direction. Place the sundial in the sun.
- Mark the position of the tip of the shadow of the gnomon on the circular board as early in the day as possible. Mark the position of the tip of the shadow every hour throughout the day. Draw lines to connect each point marked by you with the centre of the base of the gnomon.
- Extend the lines on the circular board up to its periphery.
- This Sundial can be used for reading the time at chosen place.
Question 2.
Collect information about time-measuring devices that were used in the ancient times in different parts of the world. Prepare a brief write up on each one of them. The write up may include the name of the device, the place of its origin, the period when it was used, the unit in which the time was measured by it and a drawing or a photograph of the device, if available.
Answer:
The time measuring devices that were used in the ancient times in different parts of the world are :
i) Sundials and Obelisks : In 1500 B.C. simple sundials were used to divide the time interval between sunrise and sunset in 12 different parts. In the ancient Egyptian Obelisk, constructed about 3500 BC was the oldest shadow clock used to measure time. The shadow that move to different marks enabled that to calculate the time which helped to divide the day into two parts.
ii) Water clocks : It was known to have existed in Babylon in Egypt around the 16th century. It was used to measure the time observing steady flow of water from or into a container. Measurements were marked on the containers but there were variations due to the pressure of water level.
iii) Candle clock: The candle was marked with numbers and the burning of wax indicated a specific period of time. It was unknown of the candle clocks were used although it was first referred by a Chinese poet called You Jiangu in 520 A.D.
iv) Sand glass : It is also called an hour glass, made up of two glasses connected vertically by the narrow neck, came into being in the 14th century. It could measure passage of specific time period depending upon the size and width of the class and the quantity of sand in it.
v) Pendulum : A scientific study was done around by an Italian astronomer Galileo Galilei on pendulum where its motion was used to keep a track of time. It was considered to be the world’s most accurate time – keeping invention until 1930. The pendulum clock was invented by Christian Huygens in the year 1658, which was used till 270 years.
Question 3.
Make a model of a sand clock which can measure a time interval of 2 minutes (figure)
Answer:
- Take one plastic cup and place it in upside down on a table. Put the other cup on the top of the first one so that the bottom parts of both cups touch each other.
- Place two lids on the cups and make holes into them. And then invert one cup one another.
- Measure the sand and put on top of one cup timer. Allow the sand to pass through the holes.
- Place the cup timer on top of plate and note down the time taken by the sand to pass through the bottom of the timer.
- Adjust the amount of sand until the time taken by it is 2 minutes for all the sand to pass through the bottom of the timer.
Question 4.
You can perform an interesting activity when you visit a park to ride a swing. You will require a watch. Make the swing oscillate without anyone sitting on it. Find its time period in the same way as you did for the pendulum. Make sure that there are no jerks in the motion of the swing. Ask one of your friends to sit on the swing. Push it once and let it swing naturally. Again measure its time period. Repeat the activity with different persons sitting on the swing. Compare the time period of the swing measured in different cases. What conclusions do you draw from this activity?
Answer:
We can conclude that the time period of a pendulum depends on the acceleration due to gravity and the length of the suspension. So keeping those constant, the time period of the simple pendulum will remain constant. It will not change the changing mass are weight of the Bob.
Activities
Activity 9.1 Page No : 22
Question 1.
Look at Figure (a). It shows the position of some vehicles moving on a road in the same direction at some instant of time. Now look at Figure (b). It shows the position of the same vehicles after some time. From your observation of the two figures, answer the following questions: Which vehicle is moving the fastest of all? Which one of them is moving the slowest of all ?
Answer:
According to the given pictures, the green car is the fastest car as it overtakes the white car and is very close to the blue car (initially green car was far behind the blue car).
Activity 9.2 Page No. 28
Question 2.
Set up a simple pendulum as shown in Fig. (a) with a thread or string of length nearly one metre. Switch off any fans nearby. Let the bob of the pendulum come to rest at its mean position. Mark the mean position of the bob on the floor below it or on the wall behind it. To measure the time period of the pendulum we will need a stopwatch. However, if a stopwatch is not available, a table clock or a wristwatch can be used.
To set the pendulum in motion, gently hold the bob and move it slightly to one side. Make sure that the string attached to the bob is taut while you displace it. Now release the bob from its displaced position. Remember that the bob is not to be pushed when it is released. Note the time on the clock when the bob is at its mean position. Instead of the mean position you may note the time when the bob is at one of its extreme positions. Measure the time the pendulum takes to complete 20 oscillations. Record your observations in Table.
The first observation shown is just a sample. Your observations could be different from this. Repeat this activity a few times and record your observations. By dividing the time taken for 20 oscillations by 20, get the time taken for one oscillation, or the time period of the pendulum. Is the time period of your pendulum nearly the same in all cases ?
| S.No. | Time taken for 20 oscillations (s) | Time period (s) |
| 1) | 42 | 2.1 |
| 2) | ||
| 3) |
Answer:
Aim: To determine the time period of a simple pendulum.
Materials required : Metallic bob, string, stopwatch and a meter scale.
Procedure:
- Set up a pendulum in motion, hold the bob gently and move it slowly to one side.,
- Now release the mob from its displaced position gently.
- Start a stopwatch when the bob is at one of the extreme positions.Keep on counting the number of oscillations made by the pendulum bob.
- Measure the time in which the pendulum bob takes to make a 20 oscillations complete.
- Divide the time taken for 20 oscillations by 20. This will give the time taken by pendulum for making one oscillation. It is the time period for pendulum.
Observations: We observe that the time period of pendulum is nearly the same in all cases.
| S.No. | Time taken for 20 oscillations (s) | Time period (s) |
| 1) | 42 | 2.1 |
| 2) | 41 | 2.05 |
| 3) | 40 | 2.0 |
Conclusion : From the above activity, we can conclude that the time period of the pendulum calculated is seconds. The time period of a given pendulum remains constant in all the cases.
Activity 9.3 Page No : 32
Question 3.
Draw a straight line on the ground with chalk powder or lime and ask one of your friends to stand 1 to 2 m away from it. Let your friend gently roll a ball along the ground in a direction perpendicular to the line. Note the time at the moment the ball crosses the line and also when it comes to rest Figure. How much time does the ball take to come to rest ? Measure the distance between the point at which the ball crosses the line and the point where it comes to rest. You can use a scale or a measuring tape. Let different groups repeat the activity. Record the measurements in Table. In each case calculate the speed of the ball ?
Answer:
Aim: To measure the speed of a ball.
Materials required : Chalk powder or lime, ball, measuring tape.
Procedure:
- Draw a straight line on the ground with chalk powder or lime and ask one of your friends to stand 1 to 2 m away from it,
- Let your friend gently roll a ball along the ground in a direction perpendicular to the line.
- Note the time at the moment the ball crosses the line and also when it comes to rest.
- Measure the distance between the point at which the ball crosses the line and the point where it comes to rest.
- You can use a scale or a measuring tape.
- Let different groups repeat the activity.
- Record the measurements in Table.
Observations:
Distance moved and time taken by a moving ball
| Name of the group | Distance moved by the ball (m) | Time taken (s) | Speed = Distance/ Time taken (m/s) |
| A | 100 | 4 | 100/4 = 25 |
| B | 50 | 5 | 50/5 = 10 |
| C | 80 | 8 | 80/8 = 10 |
| D | 60 | 5 | 60/5 = 12 |
| E | 40 | 8 | 40/8 = 5 |