NCERT Solutions for Class 7th Science Chapter Motion and Time according to the Latest Syllabus Of JKBOSE and CBSE Books
Exercises
1. Classify the following as motion along a straight line, circular or oscillatory motion:
(i) Motion of your hands while running:
Oscillatory motion. The hands move back and forth in a regular pattern.
(ii) Motion of a horse pulling a cart on a straight road:
Motion along a straight line. The horse and cart move in a linear path.
(iii) Motion of a child in a merry-go-round:
Circular motion. The child moves in a circular path around the center of the merry-go-round.
(iv) Motion of a child on a see-saw:
Oscillatory motion. The child moves up and down around the pivot point of the see-saw.
(v) Motion of the hammer of an electric bell:
Oscillatory motion. The hammer moves back and forth to strike the bell.
(vi) Motion of a train on a straight bridge:
Motion along a straight line. The train moves in a linear path over the bridge.
2. Which of the following are not correct?
(i) The basic unit of time is second.
Correct: The basic unit of time is indeed the second (s).
(ii) Every object moves with a constant speed.
Incorrect: Objects can move with varying speeds; not every object moves with a constant speed.
(iii) Distance between two cities are measured in kilometers.
Correct: Distances between cities are commonly measured in kilometers (km).
(iv) The time period of a given pendulum is not constant.
Incorrect: The time period of a simple pendulum is constant for small oscillations, assuming no external influences.
(v) The speed of a train is expressed in m/h.
Incorrect: The speed of a train is typically expressed in kilometers per hour (km/h), not meters per hour (m/h).
3. A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?
Time period (T) = Total timeNumber of oscillations
T = 32 s20 = 1.6 s
Answer: The time period of the pendulum is 1.6 seconds.
4. The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.
Speed (v) = DistanceTime
v = 240 km4 h = 60 km/h
Answer: The speed of the train is 60 km/h.
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.
Distance moved (d) = 57336.0 km – 57321.0 km = 15.0 km
Time taken (t) = 08:50 AM – 08:30 AM = 20 minutes
Speed in km/min = DistanceTime
Speed = 15.0 km20 min = 0.75 km/min
Speed in km/h = 0.75 km/min × 60 min/h = 45 km/h
Answer: The distance moved by the car is 15.0 km, the speed of the car is 0.75 km/min, and the speed in km/h is 45 km/h.
6. Geeta 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.
Time (t) = 15 minutes = 15 × 60 s = 900 s
Distance (d) = Speed × Time
d = 2 m/s × 900 s = 1800 m = 1.8 km
Answer: The distance between her house and the school is 1.8 km.
7. Show the shape of the distance-time graph for the motion in the following cases:
(i) A car moving with a constant speed:
The distance-time graph will be a straight line with a positive slope, indicating uniform motion.
(ii) A car parked on a roadside:
The distance-time graph will be a horizontal line, indicating no change in distance over time.
8. Which of the following relations is correct?
(i) Speed = Distance × Time
(ii) Speed = DistanceTime
(iii) Speed = TimeDistance
(iv) Speed = 1Distance × Time
Correct: Speed = DistanceTime
9. The basic unit of speed is:
(i) km/min
(ii) m/min
(iii) km/h
(iv) m/s
Correct: The basic unit of speed in the SI system is meters per second (m/s).
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:
Distance at 40 km/h: d1 = 40 km/h × 15 min60 min/h = 10 km
Distance at 60 km/h: d2 = 60 km/h × 15 min60 min/h = 15 km
Total distance d = d1 + d2 = 10 km + 15 km = 25 km
Answer: The total distance covered by the car is 25 km.
11. Suppose the two photographs, shown in Fig.9.1 and Fig. 9.2, had been taken at an interval of 10 seconds. If a distance of 100 meters is shown by 1 cm in these photographs, calculate the speed of the blue car.
Distance covered = 100 meters
Time interval = 10 seconds
Speed = DistanceTime = 100 m10 s = 10 m/s
Answer: The speed of the blue car is 10 m/s.
