Goa Board Class 7 Solutions for Science – Motion and Time (English Medium)
Page No. 156:
Question 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.
Solution :
(i) Oscillatory motion
Concept insight: While running, the hands move to and fro and repeat their motion after regular intervals of time. Hence, their motion is oscillatory.
(ii) Motion along a straight line
Concept insight: The horse is pulling a cart on a straight road. Therefore, it has a motion along a straight line.
(iii) Circular motion
Concept insight: Merry-go-round has a circular motion. Therefore, a child sitting inside it will also have a circular motion.
(iv) Oscillatory motion
Concept insight: The child on a see-saw goes up and down continuously. It oscillates up-down. Therefore, it is an oscillatory motion.
(v) Oscillatory motion
Concept insight: The hammer hits the electric bell and vibrates rapidly. Therefore, it is an oscillatory motion.
(vi) Motion along a straight line
Concept insight: The train is moving on a straight bridge. Therefore, it has a motion along a straight line.
Question 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 not constant.
(v) The speed of a train is expressed in m/h.
Solution :
(i) Correct
Concept insight: S.I. unit of time is second.
(ii) Not correct
Concept insight: An object can move with constant or variable speed.
(iii) Correct
Concept insight: Since the distance between two cities can be very large and kilometre is a big unit of distance, so distance between two cities is measured in kilometres.
(iv) Not correct
Concept insight: Time period of a pendulum depends on the length of the thread. Hence, it is constant for a particular pendulum with a fixed length.
(v) Not correct
Concept insight: The speed of a train is measured either in km/h or m/s.
Question 3:
A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?
Solution :
Question 4:
The distance between two stations is 240 km. A train takes 4 hrs to cover this distance. Calculate the distance of the train.
Solution :
Question 5:
The odometer of a car reads 57321.0 km when clock shows the time 08:30 AM. What is the distance moved by the car, if at 08:50 AM, the odometer of the 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.
Solution :
Question 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.
Solution :
Question 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.
Solution :
Question 8:
Solution :
Page No. 157:
Question 9:
The basic unit of speed is:
(i) km/min
(ii) m/min
(iii) km/h
(iv) m/s
Solution :
Question 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:
(i) 100 km
(ii) 25 km
(iii) 15 km
(iv) 10 km
Solution :
Question 11:
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.
Solution :
The distance covered by the blue car (as evident from the photograph) from one horizontal white strip to another, which is measured by scale is 1.2 cm.
It is given that 1 cm is equivalent to 100 m.
Therefore, 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 s
Question 12:
Solution :
Question 13:
Which of the following distance-time graph shows a truck moving with speed which is not constant?
Solution :
Graph (iii)
In a distance-time graph, the constant speed of a body is be represented by a straight line.
In a distance-time graph, a straight line parallel to the time axis indicates that the body is not moving.
A curved line on the distance-time graph indicates that the body is moving with a speed which is not constant.