Class VII  Science  Lesson 13. Motion and Time CBSE Adda

Question:1. What are the points that should be kept in mind while choosing scale for drawing a graph?

Answer: The following points should be kept in mind while choosing most suitable scale for drawing a graph:
(a) The difference between the highest and the lowest values of each quantity.
(b) The intermediate values of each quantity to mark the values on the graph.
(c) To utilize the maximum part of the paper on which graph is to be drawn.

Question: 2.What information we get from distancetime graphs?

Answer: Motion of an object can be represented by its distancetime graphs. It gives information about the nature of the motion of an object like uniform or uniform motion

Question:3. What is the difference between Distance and time?

Answer: Distance : The distance covered by a moving object is the actual length of the path followed by the object. Distance is a scalar quantity. SI unit of distance is meter.
Displacement is the shortest distance covered by a moving object from the point of reference (initial position of the body), in a specified direction.

Question: 4.What are the different type of speed?

Answer: Uniform Speed : An object is said to be moving with uniform speed if it covers equal distances in equal intervals of time.
Nonuniform : An object is said to be moving with variable speed or nonuniform speed if it covers equal distances in unequal intervals of time or viceversa.
Average speed : When we travel in a vehicle the speed of the vehicle changes from time to time depending upon the conditions existing on the road. In such a situation, the speed is calculated by taking the ratio of the total distance travelled by the vehicle to the total time taken for the journey. This is called the average speed.
Instantaneous speed : When we say that the car travels at an average speed of 60 km/h it does not mean that the car would be moving with the speed of 60 km/h throughout the journey. The actual speed of the car may be less than or greater than the average speed at a particular instant of time.
The speed of a moving body at any particular instant of time is called instantaneous speed.

Question:5. Explain how motion of a child on a seesaw is an oscillatory motion.

On a seesaw child goes up and comes down from mean position and repeats itself .So it is an example of oscillatory motion.

Question: 6.What is the nature of distancetime graph for the motion of an object moving with (i)a constant speed (ii)Changing speed?

Answer: (i) straight line and (ii) curved line

Question: 7. Explain, how in ancient times, a day, a month and a year was measured.

Answer: The time between one sunrise and the next was called a day.
A month was measured from one new moon to the next .
A year was fixed as the time taken by the earth to complete one revolution of the sun.

Question:8. Classify the following as motion along a straight line, circular or oscillatory motion:

Answer: (i) Motion of your hands while running. :  oscillatory motion
(ii) Motion of a horse pulling a cart on a straight road : straight line motion (iii) Motion of a child in a merrygoround. :  straight line motion (iv) Motion of a child on a seesaw. :  oscillatory motion (v) Motion of the hammer of an electric bell. oscillatory motion (vi) Motion of a train on a straight bridge :  straight line motion 
Question:9. What is velocity?

Answer: Velocity can be defined as the rate of change of displacement . SI unit of velocity is m/s (meter/second Velocity is a vector quantity.

Question:10.Which devices are used to measure time intervals?

Answer: Clocks and watches are used to measure time intervals.

Question: 11. What is a bob?

Answer: The metallic ball of the simple pendulum is called the bob of the pendulum.

Question:12. What are quartz clocks?

Answer: Clocks or watches which have an electric circuit with one or more cells are called quartz clocks.

Question:13. Define time period of a simple pendulum

Answer: The time taken by the simple pendulum to complete one oscillation is called its time period.

Question:14. What is the use of odometer in a vehicle?

Answer: Odometer is used in a vehicle to measure distance covered by the vehicle.

Question: 15. What is the use of speedometer in a vehicle?

Answer: The speedometer of a vehicle measures the speed of the vehicle in km/h.

Question:16. How do we know an object is moving faster compared to another object?

By finding the distance travel by a moving body in unit time 1 hr or 1 sec we know the speed of an object that help us to know which one is moving faster i.e having grater speed.

Question: 17. A simple pendulum takes 15 seconds to complete 5 oscillations. What is the time period of the pendulum?

Answer: The time taken by pendulum to complete 1 oscillation is called time period
The time taken by pendulum to complete 5 oscillation = 15sec
The time taken by pendulum to complete 1 oscillation = 15/5= 3 sec
So time period = 3 sec

Question:18. If a car is moving with a speed of 5 km/h on a highway find the distance travelled by the car in 3 hours?

Answer: We know distance travelled by a body = speed X time
So, distance travelled by the car =5x3=15 km 
Question:.19. The distance between two stations is 240 km. A train takes 6 hours to cover this distance. Calculate the speed of the train.

Answer: We know distance travelled = speed X time
So, speed = distance / time Hence, speed of the car = 240 / 6 = 40 km / h 
Question:20. Sunil covers a distance of 2.4 km from her house to reach her school on a bicycle. If the bicycle has a speed of 2 m/sec, calculate the time taken by her to reach the school

Answer: Total distance traveled =2.4km=2.4x1000m=2400m. Speed is 2m/sec
As time = (Distance/speed) time=2400/2=1200sec. 
Question:21. A car is moving with speed 72 km/hr. Convert this speed into metre/sec

Answer: Given speed is 72 km/hr
Speed into metre /sec = [72 x 1000m]÷ [1x60x60sec]=20m/sec

Question:22. A simple pendulum takes 30 seconds to complete 6 oscillations. What is the time period of the pendulum?

Time period= The time taken by a pendulum to complete 1 oscillation = 30/6= 5 secQuestion:23. A bus covers a distance from A to B at 40 km/h and while returning it travels at 50 km/h. calculate the average speed.

Total distance covered = (40 + 50) Km = 90 Km Total time taken = 2 hr
Average speed = (total distance travelled)/(total time taken) = (90 km)/2hr = 45 km/hr = 45 [1000/3600]m/sec = 12.5 m/s

Question: 24. A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?

Time taken to complete 20 oscillations = 32 s
Time period of the pendulum = Time taken to complete 1 oscillations = 32/20= 1.6s

Question: 25. The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.

Answer: The distance between two stations = 240 km
Time taken by the train to cover that distance = 4 hours Speed of the train = 240/4 Km/h =60 km/h 
Question:26. 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: Distance traveled by the car = 57336.0 km  57321.0 km = 15 Km
Time taken by the car to cover distance = 08:50 AM 08:30 AM = 20 m=20/60hr= 1/3 /hr.
Speed of the car in Km/h = 15 km÷1/3 = 15x3 = 45 Km/h

Question: 27. 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: Distance between her house and the school = Speed of bicycle X Time taken = 2 m/s x 15 X 60 s= 1800 m or 1.8 Km

1. Speed of a body is the distance travelled by the body in  second.

Answer: One

2. if a has different speeds at different times we can calculate the  speed

Answer: Average

3. Change of distance with time may be represented by a distance 

Answer: Time graph

4.  gives the speed of the vehicle every instant in km/h

Answer: Speedometer

5.  measures the total distance covered by the vehicle in meter.

Answer: Odometer

6.  is a device used for measuring wind speed.

Answer: Anemometer

7.  is nothing but speed in a definite direction

Answer: Velocity

8.  is the change of velocity in one second.

Answer: Acceleration

9. The speed of a moving body at any particular instant of time is called
Answer: instantaneous speed.

10. Straight line parallel to time axix of distancetime graph express  the motion of an object.

Answer: Constant

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