NCERT Solutions for Class 9th Science

Chapter 8 – MOTION

(Complete Downloadable Chapter Solution PDF file is at the bottom of the page)

 

Q.1      An object has moved through a distance. Can it have zero displacement? If yes, support your answer with              an example.

Q.2      A farmer moves along the boundary of a square field of side 10 m in 40 s. What will be the magnitude of                displacement of the farmer at the end of 2 minutes 20 seconds from his initial position?

Q.3      Which of the following is true for displacement?

           (a) It cannot be zero.

           (b) Its magnitude is greater than the distance travelled by the object.

Ans    (a) False, it can be zero if final position is same as initial position.

           (b) False, it can be equal or less than the distance travelled.

Q.4      Distinguish between speed and velocity.

Q.5      Under what condition(s) is the magnitude of average velocity of an object equal to its average speed?

Ans     The magnitude of average velocity of an object will be equal to its average speed if the displacement                       travelled by an object is same as the distance travelled i.e. the object moves in a straight line or                               rectilinear motion.

Q.6      What does the odometer of an automobile measure?

Ans     Odometer of an automobile measures the distance travelled by it.

Q.7      What does the path of an object look like when it is in uniform motion?

Ans     Object in uniform motion will have a straight line path.

Q.8      During an experiment, a signal from a spaceship reached the ground station in five minutes. What was                the distance of the spaceship from the ground station? The signal travels at the speed of light, that is,                    3 × 108 m/s.

Q.9      When will you say a body is in (i) uniform acceleration? (ii) nonuniform acceleration?

Q.10    A bus decreases its speed from 80 km/h to 60 km/h in 5 s. Find the acceleration of the bus.

Q.11     A train starting from a railway station and moving with uniform acceleration attains a speed 40 km/h in            10 minutes. Find its acceleration.

Q.12    What is the nature of the distance-time graphs for uniform and non-uniform motion of an object?

Q.13    What can you say about the motion of an object whose distance-time graph is a straight line parallel to                the time axis?

Q.14    What can you say about the motion of an object if its speed time graph is a straight line parallel to the                  time axis?

Q.15    What is the quantity which is measured by the area occupied below the velocity-time graph?

Ans    Distance is measured by the area occupied below the velocity-time graph.

Q.16    A bus starting from rest moves with a uniform acceleration of 0.1 m/s2 for 2 minutes. Find (a) the speed                acquired, (b) the distance travelled.

Ans    Thought Process:

            v = u + at

            s = ut + ½ at2

            Here, u = 0,

            a = 0.1 ms-2 and t = 2 mins = 120 s

           a) v = u + at

            v = 0 + 0.1 x 120 = 12 m/s

           Hence speed acquired by bus is 12 m/s.

           b) s = ut + ½ at2

           s = 0 + ½ 0.1 x 1202 = 

           Hence distance travelled by bus is 720 m.

Q.17    A train is travelling at a speed of 90 km/h. Brakes are applied so as to produce a uniform acceleration of –              0.5 m/s2. Find how far the train will go before it is brought to rest.

Q.18    A trolley, while going down an inclined plane, has an acceleration of 2 cm/s2. What will be its velocity 3 s              after the start?

Q.19    A racing car has a uniform acceleration of 4 m/s2. What distance will it cover in 10 s after start?

Q.20    A stone is thrown in a vertically upward direction with a velocity of 5 m/s. If the acceleration of the stone            during its motion is 10 m/s2 in the downward direction, what will be the height attained by the stone and            how much time will it take to reach there?

 

EXERCISES

 

Q.1      An athlete completes one round of a circular track of diameter 200 m in 40 s. What will be the distance                  covered and the displacement at the end of 2 minutes 20 s?

Ans     Total time = 2 minutes 20 seconds = 2X60 + 20 = 140 seconds

            Since the athlete covers one loop of the circular track in 40 seconds, he will move 3 loops in 140 seconds.

            Therefore the displacement will be equal to diameter i.e. 200 m.

            Now the distance cover will be equal to 3 times the perimeter of the circular track.

            Therefore, Distance = 3 x (2 π r), where r = radius = dia/2 = 200/2 = 100 m

                                               = x 2 x x 100 = 2200 m

Q.2      Joseph jogs from one end A to the other end B of a straight 300 m road in 2 minutes 30 seconds and then              turns around and jogs 100 m back to point C in another 1 min. What are Joseph’s average speeds and                      velocities in jogging (a) from A to B and (b) from A to C?

Q.3      Abdul, while driving to school, computes the average speed for his trip to be 20 km/h. On his return trip                along the same route, there is less traffic and the average speed is 30 km/h. What is the average speed for            Abdul’s trip?

Q.4      A motorboat starting from rest on a lake accelerates in a straight line at a constant rate of 3.0 m/s2 for                  8.0 s. How far does the boat travel during this time?

Ans    Thought Process:

            s = ut + ½ at2

            Here,    u = 0

            a = 3.0 ms-2

            t = 8.0 s

            So, s = 0 + ½ 3 x 82 = 96 m

            Hence boat will travel 96 m.

Q.5      A driver of a car travelling at 52 km/h applies the brakes and accelerates uniformly in the opposite                        direction. The car stops in 5 s. Another driver going at 3 km/h in another car applies his brakes slowly                  and stops in 10 s. On the same graph paper, plot the speed versus time graphs for the two cars. Which of              the two cars travelled farther after the brakes were applied?

Q.6      Fig 8.11 shows the distance-time graph of three objects A,B and C. Study the graph and answer the                           following questions:                                   

            (a) Which of the three is travelling the fastest?

            (b) Are all three ever at the same point on the road?

            (c) How far has C travelled when B passes A?

            (d) How far has B travelled by the time it passes C?

Q.7       A ball is gently dropped from a height of 20 m. If its velocity increases uniformly at the rate of 10 m/s2,                 with what velocity will it strike the ground? After what time will it strike the ground?

Q.8      The speed-time graph for a car is shown is Fig. 8.12.

            (a) Find how far does the car travel in the first 4 seconds. Shade the area on the graph that represents the             distance travelled by the car during the period.

            (b) Which part of the graph represents uniform motion of the car?

Q.9      State which of the following situations are possible and give an example for each of these:

            (a) an object with a constant acceleration but with zero velocity

            (b) an object moving in a certain direction with an acceleration in the perpendicular direction.

Ans     (a) It is possible in case of an object throwing upwards. At the highest point the velocity is zero however                 the acceleration is constant and is equal to acceleration due to gravity.

Q.10    An artificial satellite is moving in a circular orbit of radius 42250 km. Calculate its speed if it takes 24                    hours to revolve around the earth.