NCERT Solutions for Class 7 Science Chapter 13 - Motion and Time
Chapter 13 - Motion and Time Exercise 156
(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.
Concept insight: S.I. unit of time is second.
(ii) Not correct
Concept insight: An object can move with constant or variable speed.
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.
Concept insight: The speed of a train is measured either in km/h or m/s.
Chapter 13 - Motion and Time Exercise 157
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
Vehicle A is moving faster than vehicle B.
Speed is given by the relation:
This relation shows that the speed of a vehicle is greater if it covers greater 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 greater than that covered by vehicle B. Hence, vehicle A is moving faster than vehicle B.
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.
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