NCERT Solutions Class 7 Science Chapter 13 Motion and Time
1. 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 of an electric bell.
(vi) Motion of a train on a straight bridge.
Answer:
(i) Oscillatory
(ii) Motion along a straight line
(iii) Circular motion
(iv) Oscillatory motion
(v) Oscillatory motion
(vi) Motion along a straight line.
2. 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 constant.
(v) The speed of a train is expressed in m/h.
Answer:
Incorrect statements are:
(ii) Every object moves with a constant speed.
(iv) The time period of a given pendulum is constant.
(v) The speed of a train is expressed in m/h.
3. A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?
Answer:
Number of oscillations = 20
Total time taken to complete 20 oscillations = 32 s
Time period = \(\frac{\text{Total time taken }}{\text{Number of oscillations}}\)
\(= \frac{32}{20}\)
\(= 1.6s\)
4. 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 two stations = 240 kms
Total time take = 4 hrs/240 minutes
Speed = \(\frac{\text{Distance}}{\text{Time}}\)
\(= \frac{240}{4}\)
= 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.
Answer:
Initial reading of the odometer = 57321.0
Final reading of the odometer = 57336.0
Distance covered by the car = Final reading of the odometer – Initial reading of the odometer
= 57336.0 – 57321.0 = 15 kms
Starting time of car is 8:30 and it stops at 8:50
Hence, time taken by car = 20 mins
Speed = \(\frac{\text{Distance}}{\text{Time}}\)
\(= \frac{15}{20}\)
\(= 0. 75 km/ min\)
\(20min = \frac 1{60} \times 20\)
\(= \frac 13 h\)
Speed = \(\frac{\text{Distance covered}}{\text{Time take}}\)
\(= \frac{15}{1/3}\)
= 45 km/h
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.
Answer:
Time taken by Salma to reach her school by bicycle = 15 mins = 15 x 60 = 90 s
Speed of Salma’s bicycle = 2m/s
Speed = \(\frac{\text{Distance covered}}{\text{Time take}}\)
Distance covered = speed x time taken
= 2 x 900 = 1800 m
1000m = 1 km
\(1800m = \frac 1{1000} \times 1800\)
= 1.8 kms
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.
Answer:
8. Which of the following relations is correct?
(i) Speed = Distance × Time
(ii) Speed = Distance/Time
(iii) Speed = Time/Distance
(iv) Speed = 1/Distance x Time
Answer:
Correct option is (ii) Speed = Distance/Time
9. The basic unit of speed is:
(i) km/min
(ii) m/min
(iii) km/h
(iv) m/s
Answer:
Correct option is (iv) m/s
