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NCERT Class 7 Science Chapter 13 Motion and Time
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Motion and Time
Chapter: 13
NCERT TEXT BOOK EXERCISES
Q. 1. Classify the following as motion along a straight line, circular or oscillatory motion:
(i) Motion of your hands while running.
Ans: Oscillatory.
(ii) Motion of a horse pulling a cart on a straight road.
Ans: Along a straight line.
(iii) Motion of a child in a merry go round.
Ans: Circular.
(iv) Motion of a child on a see-saw.
Ans: Oscillatory.
(v) Motion of the hammer of an electric bell.
Ans: Oscillatory.
(vi) Motion of a train on a straight bridge.
Ans: Along a straight line.
Q. 2. Which of the following is not correct?
(i) The basic unit of time is second.
(ii) Every object moves with a constant speed.
(iii) Distance 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), (iv), (v).
Q. 3. A simple pendulum takes 32s to complete 20 oscillations. What is the time period of the pendulum?
Ans: Time period of a pendulum is time taken to complete 1 oscillation.
Time taken to complete 20 oscillations = 32s
Time taken to complete 1 oscillation
∴ Time period of pendulum is 1.6 seconds.
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.
Ans: Distance = 240 km.
Time taken = 4 hr.
Speed of the train is 60 km/h.
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. Distance = 57336.0 km – 57321 km
= 15 km
Time = 8.50 AM. – 8.30 AM
= 20 min
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. Time taken = 15 min
Speed = 2 m/s.
Distance = speed x time
= 15 x 2 = 30 m.
So, distance between Salma’s school and her house is 30 meters.
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:
Fig. 13. 18.
Q. 8. Which of the following relations is correct?
(i) Speed = Distance x Time
Q. 9. The basic unit of speed is:
(i) km/min
(ii) m/min
(iii) km/h
(iv) m/s.
Ans: (iv) m/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: (ii) 25 km.
Q. 11. Suppose the two photographs shown in Fig. 13.19 and 13.20, had been taken at an interval of 10 seconds. If a distance of 100 meters is shown by 1 em in these photographs, calculate the speed of the blue car.
Fig. 13.19. Vehicles moving in the same direction on a road.
Ans. 0.1 cm/s or 10m/s.
Fig. 13.20. Position of vehicles shown in Fig. 13.19 after some time.
Q. 12. Fig. 13.21 shows the distance-time graph for the motion of two vehicles A and B. Which one of them is moving faster?
Fig. 121. Distance-time graph for the motion of two cars.
Ans: ‘A’ car is moving faster.
Q. 13. Which of the following distance-time graphs shows a truck moving with speed which is not constant?
Fig. 13.22
Ans: (iii)
ADDITIONAL IMPORTANT QUESTIONS
Very Short Answer Questions
Q.1. Name the instrument which is used:
(a) to record the distance covered by a vehicle.
Ans: Odometer.
(b) To indicate the speed of a running vehicle.
Ans: Speedometer.
Q. 2. What name is given to the metallic ball of a simple pendulum?
Ans: Bob.
Q. 3. Name the device whose periodic motion was used for making clocks full recently.
Ans: Pendulum.
Q. 4. Name the material which is used for measuring time in electronic clocks and watches.
Ans: Quartz crystal.
Q. 5. Define speed of a body.
Ans: Distance traveled by a moving body in unit time is called speed.
Q. 6. What do you mean when you say that a car is moving with a speed of 60 kilometers per hour?
Ans: It means that car will cover 60 kilometers distance in one hour.
Q. 7. What is the basic unit of time?
Ans: Second.
Q.8. What is the relation between speed, distance and time?
Ans: The distance covered speed x time.
Q.9. Which of the two is a bigger unit of time: micro second or nano-second?
Ans: Microsecond.
Q. 10. Which of the two measures time more accurately: a quartz clock or a pendulum clock?
Ans: A quartz clock.
Q. 11. What can you say about the motion of an object whose distance-time graph is a straight line parallel to the time axis?
Ans: The object is stationary.
Q. 12. What kind of watch is used to measure time intervals during sport events?
Ans: Stop watch.
Q. 13. What is a simple pendulum?
Ans: A simple pendulum consists of a small metallic bob suspended by a long cotton thread from a rigid support such that the bob is free to swing back and forth.
Q. 14. What is the time period of a simple pendulum?
Ans: The time taken by the pendulum to complete one oscillation is called its time period.
Q. 15. What type of motion is shown by a suspended pendulum?
Ans: Oscillatory motion.
Short Question Answer
Q. 1. What does the path of an object look like when it is in uniform motion?
Ans: In uniform motion, the path of an object can be a straight line, curved line, zig-zag line, or even a circle. It can have any shape. This is because in uniform motion, speed is constant. The direction of motion may change.
Q. 2. What is the nature of the distance- time graphs for uniform and non-uniform motion of an object?
Ans: When the motion is uniform, the distance-time graph is a straight line with some slope. When the motion is non-uniform, the distance-time graph is not a straight line. It can be any curve.
Q. 3. A car travels a distance of 200 km at the speed of 50 km/h. Calculate the time taken to cover the distance.
Ans: Distance = 200 km
Speed = 50 km/h
Car will take 4 h to cover the distance.
Q.4. Find the Speed of a car that travels 300 km in 6 hours.
Ans: Time = 6 hours
Distance = 300 km, Speed =?
Speed = Distance Covered/Time Taken
= 300 km/6 hr
= 50 km/hr.
Q. 5. What is the relation between distance and time
(i) When body is moving with uniform velocity?
Ans: The distance covered by the body is directly proportional to time.
(ii) When body is moving with variable velocity?
Ans: The distance covered by the body is not directly proportional to time.
Q.6. (i) What kind of motion is possessed by a body falling from certain height?
Ans: (i) A body falling freely from certain height above the earth’s surface has non-uniform motion.
(ii) In which units, the speed of fast moving objects is commonly expressed?
Ans: The speeds of fast moving bodies car, scooter, airplane, etc. are generally expressed in units of kilometer per hour (km/h).
Q. 7. Convert a speed of 36 km/h into m/s.
Q. 8. Convert a speed of 15 m/s in km/h.
Q. 9. Find the type of motion in the following case.
(a) A car traveling along a straight road, changes its speed.
Ans: Non-uniform motion.
(b) An athlete running a 100 m race.
Ans: Uniform motion.
(c) Moon revolving around the earth.
Ans: Uniform circular motion.
(d) An ant moving on the floor.
Ans: Non- uniform motion.
Q. 10. It took 2s after lighting for the sound of thunder to reach you, how far did the lightning struck. Speed of sound in air is 346 m/s.
Ans: Speed of sound is 346 m/s in air.
Time = 2s, Distance =?
Or d = s × t = 346 × 2
= 692 m
Therefore, the lightning struck 692 m away.
Short Questions Answer
Q. 1. List three factors in which time period of a simple pendulum depends.
Ans: The time period of a simple pendulum:
(i) depends on the length of the string used.
(ii) does not depend on the extent to which bob of the pendulum is displaced,
(iii) does not depend on the mass of the bob used.
Q. 2. Out of a line graph, a pie-chart and a bar graph which one is the most suitable to show:
(a) runs scored in various overs of a cricket match.
Ans: Bar graph.
(b) variation of distance covered by a car with time?
Ans: Line graph.
(c) percentage composition of air?
Ans: Pie chart.
Q. 3. Draw the shapes of distance time graphs for the motion in the following cases:
(a) a car moving with a constant speed.
(b) a car parked on a side road.
(c) a car moving with a changing speed.
Ans: (a)
Fig. 13. 23.
Fig. 13. 25.
Q. 4. (a) Draw a diagram of simple pendulum at its mean position. Label it.
Ans: A simple pendulum consists of a small metallic ball (or bob) suspended by a light thread from a frictional rigid support.
Fig. 13.26.
(b) What is meant by the amplitude of an oscillation?
Ans: The maximum displacement of the bob from its mean position on either side is called it amplitude.
Q. 5. (a) What are x and y-axis in graph?
Ans: The horizontal line in graph is called x-axis and vertical line is called y-axis.
(b) What is origin in a graph?
Ans: The point where the x-axis and y-axis both intersect each other is called origin of graph.
(c) What is independent variables?
Ans: The variable which increases or decreases freely and does not depend on other variables is called independent variable.
Long Type Questions Answer
Q. 1. Why do we need to measure time?
Ans: We measure time for different reasons:
(i) Doctors need to measure the number of times the heart beats in one minute for a patient.
(ii) Farmers need to measure the time between sowing and harvesting of crop.
(iii) Students need to measure time to complete an examination.
(iv) A traveler needs to measure the time of his journey.
(v) A sportsman needs to measure the time he might take to complete an event.
Q. 2. What is a pendulum? How does it help in determining time?
Ans: Pendulum is a simple device which shows periodic motion. A simple pendulum consists of a non-magnetic metal ball called bob. This bob is suspended with the help of a string. The open end of the string is tied to a support. Bob of the pendulum is held at a side and released. It starts moving in a to and fro motion. This is called an oscillatory motion. The time taken by the pendulum to complete one oscillation is called time period. This time period is always same with a pendulum having same length. Time period changes with the change in the length of the string.
The string of the pendulum is adjusted to the length that it completes an oscillation in a second and it keeps on moving the clock giving us time.
Q. 3. Given below is a table showing time taken by a car to travel various distances. What do you infer from this data?
| Time (S) | Distance (M) |
| 0 | 0 |
| 1 | 10 |
| 2 | 20 |
| 3 | 30 |
| 4 | 40 |
| 5 | 50 |
Ans: We see that the car covers the distance of first 10 m in 1 sec. It again covers the distance of 10 m in another time of 1 sec i.e. 2 – 1 = 1 sec. So, we find that the car covers every 10 m in each 1 sec. It means that the car is covering equal distance in equal interval of time. When a body covers equal distance in equal intervals of time, it is said to be in a uniform motion. So, this data shows that the car is moving with a constant speed.
HOTS QUESTIONS
9.1. The figure given shows the distance time graph for the motion of two vehicles A and I. Which one of them in moving faster?
Fig. 13. 27.
Ans: Vehicle A is moving faster.
Q.2. What does the path of an object look like when it is in uniform motion?
Ans: It is a straight line.
Q.3. A motor-cyclist drives from A to B with a speed of 30 km/h and returns back with a speed of 20km h Find its average speed.
Q. 4. Suggest what kind of biome of a body is repented by the graph given below.
Ans: (a) The body is stationary.
(b) The body is moving with a uniform speed.
(c) The body is moving with a uniform speed for sometimes and then stops.
Q. 5. A sprinter in a 100 m race, covers 4m in the first second, 30 m in the next 4s, 52m in another 4s and finishes the race in 10s. Plot the distance-time graph for the motion of the sprinter.
Ans: Distance-time graph.
SKILL BASED QUESTIONS
Q.1. Observe the following figures and identify them:
Fig. 13. 29.
Ans: (a) Sundial at Jantar-Mantar Delhi.
(b) Sand dial. and
(c) Water clock.
Q. 2. (a) Observe the following figure and identify it.
Fig. 13. 30.
Ans: (a) Sundial at Jantar-Mantar Delhi.
(b) Sand dial. and
(c) Water clock.
Q. 2. (a) Observe the following figure and identify it.
Fig. 13. 31.
(b) Write the functions of them.
Ans: The figure shows the dash board of a car. It consists of two types of meters:
(i) Speedometer.
(ii) Odometer.
(b) The function of speedometer is to measure the speed of the vehicle directly in km/h. The odometer is used to measure the total distance moved by the vehicle.
Q. 3. (a) Observe the following figure and identify it.
(b) Name the devices used by our ancestors of measure the time before pendulum clocks.
Fig. 13. 31.
Ans: (a) A pendulum clock.
(b) Sun dials, water clocks, sound clocks.
FORMATIVE ASSESSMENT
Q. 1. Fill in the following blanks with suitable words:
(i) The distance moved by an object in unit time is called _________.
Ans: Speed.
(ii) If the speed of an object moving along a straight line keeps changing, the motion is said to be ________.
Ans: Non-uniform.
(iii) An object moving along a straight line with constant speed is said to be in _____ motion.
Ans: Uniform.
(iv) The distance-time graph of vehicle moving with constant speed is a ________ line.
Ans: Straight.
(v) The time from one full moon to the next was called a _________.
Ans: Month.
(vi) The S.I unit of speed is __________.
Ans: Meter per second.
(vii) The plot of distance vs time for a body moving with uniform speed is a _________.
Ans: (vii) straight line making an angle with the time- axis.
(viii) The time period of a pendulum is expressed in _________ units.
Ans: Seconds.
(ix) Time interval is generally measured with a _________.
Ans: Stop watch.
(x) The time from one sunrise to the next was called a ___________.
Ans: Day.
Q. 2. State whether the following statements are true or false:
(i) Every object moves with a constant speed.
Ans: False.
(ii) Speedometer indicates the speed in km/s.
Ans: False.
(iii) Odometer measures the distance covered in kilometers.
Ans: True.
(iv) The speed of a train is expressed in m/s.
Ans: False.
(v) Distances between two cities are measured in kilometers.
Ans: True.
(vi) Speed measured over a measurable time interval is called average speed.
Ans: True.
(vii) When a body travels unequal distances in equal intervals of time, it is said to possess non-uniform motion.
Ans: True.
(viii) The SI unit of speed is cm/s.
Ans: False.
(ix) Odometer records the speed of a running vehicle.
Ans: False.
(x) A straight line graph between distance and time shows that the body is moving with a uniform speed.
Ans: True.
Q. 3. Match the following questions:
| Column – A | Column – B |
| 1. Sun dial | (a) Use electronic circuit |
| 2. Sand clock | (b) Periodic motion |
| 3. Simple pendulum | (c) Regulated flow of sand |
| 4. Quartz clocks | (d) Shadow cast by sun |
Ans:
| Column – A | Column – B |
| 1. Sun dial | (d) Shadow cast by sun |
| 2. Sand clock | (c) Regulated flow of sand |
| 3. Simple pendulum | (b) Periodic motion |
| 4. Quartz clocks | (a) Use electronic circuit |
Q. 4. Write one word for the following:
(i) The motion of a body moving along a curved road.
Ans: Curvilinear motion.
(ii) The quantity measured in terms of distance and time.
Ans: Average speed.
(iii) A device used for measuring small time intervals.
Ans: Stop watch.
(iv) The motion of a body traveling equal distances in equal intervals of time.
Ans: Uniform motion.
(v) The geometrical relation shop between distance and time for a moving body.
Ans: Distance-time graph.
(vi) The meter used to record the speed of a moving vehicle.
Ans: Speedometer.
(vii) A device used to measure time.
Ans: Clock.
(viii) The gap or time interval between two events.
Ans: Time.
(ix) The state of a body when it changes its position with respect to its surroundings.
Ans: Motion.
(x) The physical quantity that measures the distance traveled per unit time.
Ans: Speed.
5. Multiple Choice Questions:
(i) Which of the following is based on the study of the shadow cast by the sun?
(a) Simple pendulum
(b) Atomic clock
(c) Sun dial
(d) Quartz clocks
Ans: (c) Sun dial.
(ii) Which of the following is the SI unit of time?
(a) Second
(b) Kilometer
(c) Meter
(d) Gram
Ans: (a) Second.
(iii) The most appropriate unit for expressing the speed of a space rocket is:
(a) m/s
(b) km/h
(c) km/s
(d) km/min
Ans: (b) km/s.
(iv) The instrument installed in a car for measuring the distance traveled by the car is called:
(a) barometer
(b) speedometer
(c) anemometer
(d) odometer
Ans: (d) Odometer.
(v) The slope of a distance time graph of a moving object indicates:
(a) Distance moved by the object
(b) Time taken by the object
(c) Speed of the object
(d) Position of the object.
Ans: (c) Speed of the object.
(vi) Which of the following is the most appropriate to demonstrate the runs scored in the various overs of a cricket match?
(a) time graph
(b) bar graph
(c) pie chart
(d) line graph
Ans: (b) Bar graph.
(vii) The clocks and watches which are used for measuring time are based on:
(a) Rectilinear motion
(b) Circular motion
(c) Periodic motion
(d) Rotational motion
Ans: (c) Periodic motion.
(viii) The distance-time graph of a car which comes to a stop after covering a certain distance will be:
(a) A straight line sloping upwards
(b) A curved line sloping downwards
(c) A straight line parallel to time axis
(d) A straight line parallel to distance axis
Ans: (c) A straight line parallel to time axis.
(ix) The device which is used for measuring time intervals in sports activities is called:
(a) Wrist watch
(b) Stop watch
(c) Stop clock
(d) Quartz watch
Ans: (b) Stop-watch.
(x) The regular vibrations of which of the following electrically driven crystals are used for measuring time very accurately?
(a) Calcite crystals
(b) Quadric crystals
(c) Chrome crystals
(d) Quartz crystals
Ans: (d) Quartz crystals.
