NCERT Solutions for Class 7 Science - Motion and Time

Q1: Classify the following as motion along a straight line, circular oroscillatory 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 of an electric bell. (vi) Motion of a train on a straight bridge.

Answer: (i) Oscillatory motion
While running, the hands move to and fro and repeat their motion after a given interval of time. Hence, it is an oscillatory motion. (ii)Straight line
The horse is pulling a cart on a straight road. Therefore, it has a motion along a straight line. (iii)Circular motion
Merry-go-round has a circular motion. Therefore, a child sitting inside it will also have a circular motion. (iv) Oscillatory motion
The child on a see-saw goes up and down continuously. It oscillates up-down. Therefore, it is an oscillatory motion. (v)Oscillatory motion
The hammer hits the electric bell and vibrates rapidly. Therefore, it is an oscillatory motion. (vi) Straight line
The train is moving on a straight bridge. Therefore, it has a motion along a straight line.

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) 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 - Second is the SI unit of time.
(ii) Not correct - An object can move with constant or variable speed.
(iii) Correct - The distance between two cities can be very large. Since kilometre is a bigger unit of distance, the distance between two cities is measured in kilometres. 
(iv) Not correct - Time period of a pendulum depends on the length of the thread. Hence, it is constant for a particular pendulum. 
(v) Not correct - The speed of a train is measured either in km/h or in m/s.

Q3: A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?
Answer: 
Numberof oscillations = 20
Total time taken to complete 20 oscillations = 32 s

Q4: The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.
Answer: 
Distance between the two stations = 240 km
Time taken = 4 h

Q5: 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 the car
= Final reading of the odometer of the car - Initial reading of the odometer of the car
= 57336.0 - 57321.0 = 15 km
The given car starts at 8:30 a.m. and stops at 8:50 a.m.
Therefore, time taken by the car to cover the distance is (8:50 - 8:30) min = 20 min
Distance covered by the car = 15 km
Time taken by the car = 20 min
   
Again,
60 min = 1 h
Time taken by the car =

Q6: 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 her school from her home = 15 min = 15 × 60 = 900 s
Speed of her bicycle = 2 m/s
Distance covered = Speed ×Time taken = 2 ×900 = 1800 m 1000 m = 1 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) A car parked on a side road.

Answer: (i) A car moving with a constant speed covers equal distance in equal intervals of time. Such motion of car is represented in the given distance-time graph.
(ii)The distance-time graph of a car parked on a road side is such that with the increase in time, there is no change in distance, as shown in the given figure.

Q8: Which of the following relations is correct?

(i) Speed = Distance × Time
(ii)
(iii)
(iv)
Answer: (ii) Speed of an object is given by the relation

Q9:  The basic unit of speed is: (i) km/min (ii) m/min (iii) km/h (iv) m/s
Answer: (iv)m/s
The basic unit of distance is metre (m).
The basic unit of time is second (s).
Therefore, the basic unit of speed is m/s.
 
Q10: 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
Answer: (ii)25 km
Case I
Speed of the car = 40 km/h
Time taken = 15 min =
Distance covered, d1= Speed ×Time taken = 40 ×0.25 = 10 km
Case II
Speed of the car = 60 km/h
Time taken = 15 min =
Distance covered, d2= Speed ×Time taken = 60 ×0.25 = 15 km
Total distance covered by the car, d = d1+ d2= 10 + 15 = 25 km Therefore, the total distance covered by the car is 25 km.

Q11: Suppose the two photographs, shown in Figure 1 and Figure 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 blue car.
 
  
Figure 1 Vehicles moving in the same direction of on a road
Figure 2 Position of vehicles shown in Figure 1 after some time
Answer: 
Thedistance covered by the blue car (as evident from the photograph) from one white strip to another, which is measured by scale is
1.4 cm.
It is given that 1 cm is equivalent to 100 m.
Therefore, 1.4 cm is equivalent to 140 m.
Distance travelled by the car = 140 m
Time interval between the two photographs = 10 s

Q12: Figure shows the distance-time graph for the motion of two vehicles A and B. Which one of them is moving faster?
 
Distance-time graph for the motion of two cars
Answer: 
Vehicle A is moving faster than vehicle B.
Speed is given by the relation
This relation shows that speed of a vehicle is greater if it covers maximum distance in a given interval of time. To compare the distance, draw a line perpendicular to the time-axis, as shown in the following distance-time graph.
From the graph, it is evident that for a given time t, the distance covered by vehicle A is more than vehicle B. Hence, vehicle A is moving faster than vehicle B.
Q13: Which of the following distance-time graphs shows a truck moving with speed which is not constant?
 
Answer: 
Graph (iii)
In a distance-time graph, the constant speed of a truck will be represented by a straight line.
In a distance-time graph, a straight line parallel to the time axis indicates that the truck is not moving.
A curved line on the distance-time graph indicates that the truck is moving with a speed which is not constant.