NCERT solution for class 7|| Science || Chapter 13 Motion and Time

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NCERT solution for class 7|| Science || Chapter 13 Motion and Time

Chapter 13 Motion and Time 

Q1. Classify the following as motion along a straight line circular or oscillatory motion .

i) Motion of your hands while running 

ii)Motion of a horse pulling a cart on a straight road . 

iii) Motion of a child in a merry go round .

iv) Motion of a child on a see - saw 

v) Motion of the hammer on an electric bell .

vi ) Motion of a train on a straight bridge  

Answer - 

i) Oscillatory motion .

ii) straight line motion  

iii) circular motion  

iv) oscillatory motion 

v)  oscillatory motion 

vi ) straight line motion .

Q2. Which of the following are 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 kilometres . 

iv) The time period of a given pendulum is constant 

v) The speed of a train is expressed in m /hr  

Answer - i) correct 

ii) Not correct  - correct sentence is - every object moves with a constant and variable speed 

iii) Correct 

iv) Not correct - correct sentence is - the time  period of the pendulum is different on the length of the string and it is constant for the simple pendulum 

v) Not correct  - correct sentence is- the speed of the train is expressed in km/hr or m/ sec 

Q3. A simple pendulum takes 32 seconds to complete 20 oscillations what is the time period of the pendulum? 

Answer -  20 oscillation are complete in 32 seconds 

Number of oscillation = 20 

Time taken for one oscillation =

 total time taken / number of oscillation 

Time taken for one oscillation= 32/20 

           = 1.6 second 

Q4. The distance between two station is 240 km at train takes 4 hours to cover this distance calculate the speed of the train ? 

Answer -  

Given, 

Distance between two station = 240 km 

Time taken = 4hr 

Speed = distance/ time taken 

Speed = 240/4 

Speed= 60km/hr 

Q5. The odometer of a car raids 57321 .0 kilometre when the clock shows the time 8:30 a.m. what is the distance moved by the car if at 8:50 a.m. the odometer reading has changed to 5733 6.0 km calculate the speed of the car in kilometre/ minute during this time express the speed in km /hour also 

Answer - Initial reading of car odometer 5732 1.0 km 

Final reading of car odometer 5733 6.0 km 

Distance  covered by a car =  final reading of odometer - initial reading of odometer 

5733 6.0 - 57321 .0 = 15 km 

Decker starts at 8:30 a.m. and stops at 8 :50 a.m. 

So time taken by a car 8:50 - 8: 30 = 20 min 

Distance covered by car= 15 km 

Time taken by car= 20 minute 

Speed =  distance covered/  time taken 

= 15/20 = 0.75 km/ m 

60 min = 1 hr 

20 min = 1/60x 20 

      = 1/3 hr 

Time taken by the car =  3 hr 

Speed of car = distance covered/ time taken 

Speed = 15/1/3 = 45km/ m .

Q6. Salama goes to  for school on a bicycle is the bicycle has a speed of 2 m/ s calculate the difference between her house and the school.

Answer - Time taken by Salma to reach her school from her home 15 minute = 15 x 60= 900 . 

Speed of bicycle = 2 m/ S 

Speed =  distance covered/  time taken 

Distance covered =  speed X time taken 

Distance covered= 2 x 900 = 1800  m 

1000m = 1 km 

1800 m = 1/1000 x 1800 

      = 1.8 km.

Q7 . Show the shape of the distance time graph for the motion in the following cases 

i) A car moving with a constant speed 

ii) car parked on a side road 

Answer -  a ) car moving with a constant speed shows the distance increases on the increasing the time of interval and shows uniform motion

b) car parked on a roadside shows the straight line distance time graph there is no change in distance to increasing the time of interval

Q8. Which of the following relations is correct 

i) speed = distance x time  

ii) speed  = distance/  time 

iii) speed  =  time / distance 

iv) speed  =  1/ distance X time  

Answer -  speed  = distance/  time 

Q9. The basic unit of the speed is 

i) km/min.       ii ) m/ min.     iii) km/ h.   iv) m/s  

Answer - The basic unit of speed is m/ s 

Distance measured in meter while time is measured in seconds

Q10. A car moves with a speed of 40 km / hour for 15 minutes and then with a speed of 60 km /hour for next 15 minutes the total distance covered by the car is 

i) 100 km.    ii) 25 km.      iii) 15 km .     iv) 10 km  

Answer - 25 km. 

In case 1.  

Speed of car 40 km / hr 

Time 15 min = 15/ 60 = 0.25 hr 

Speed = Distance covered / Time taken  

      40 = distance covered / 0.25 

Distance covered =  40 x 0.25 = 10 km 

In case 2 

 Speed of car  = 60 km/ hr  

 Time 15 min = 15/ 60 = 0.25 hr  

Speed = Distance covered / Time taken  

      60= distance covered / 0.25   

Distance covered = 60 x 0.25 =   15  km  

Total distance covered by car is = 15+   10 = 25 km 

Therefore the total distance covered by a car is 25 km

Q11. Suppose the two photographers shown in figure 13.1 and figure 13.2 and has been taken at an interval of 10 seconds if a distance of 100 m is shown by 1 cm in these photographs calculate the speed of the fastest car   

Answer - Distance covered by a fluke from one horizontal white stripe to another is measured by a scale is 1.2  cm 

It is given that 1cm is equivalent to hundred metre 

So 1.2 cm is equivalent to 120 m 

Distance travelled by the car = 120 m 

Time taken to cover this distance =  time interval between the two photographs 10 second 

Speed =  distance covered  / time taken 

Speed = 120/ 10  

   = 12 m/ s 

Q12. Figure 13.15 shows the distance time graph for the motion of the two vehicles A and B which one of them is moving faster ?   

Answer -  A vehicles is faster than B Because it has covers more distance in less time  

Q 13. Which of the following distance time graph shows a truck moving with speed which is not constant  

 

Answer -Figure (  iii ) shows truck speed which is not constant