# Motion| NCERT Solutions| Class 9

NCERT Solutions of  Chapter: Motion. NCERT Solutions along with worksheets and notes for Class 9.

Page 100

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

Ans- Yes, displacement can be zero if the object returns at the initial point.

Eg:- Let an object starts from Point A and reaches to point B after covering a distance 10 m. Then it moves towards Point A and reaches there. (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?

Ans-  (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) It cannot be zero.

The above statement is true. Because displacement can be zero if object returns to the initial position.

(b) It is not true. Because displacement is the smallest distance in between final position and initial position of the object.

Page 102

(1) Distinguish between speed and velocity.

Ans- Differences between speed and velocity are tabulated below-

 SN Speed Velocity 1 The rate of motion of an object to find out the distance travelled  by the object in unit time. The rate of motion in specific direction of an object to find out the distance travelled  by the object in unit time. 2 It can be calculated with the help of distance travelled by the object. It can be calculated with the help of displacement travelled by the object. 3 Non – uniform Can be uniform and non uniform 4 Eg :- The car travelles 20 m in 10 sec. Eg:- The car travels 20 m in north from the point A in 10 sec.

(2) Under what conditions is the magnitude of average velocity of an object equal to its average speed?

Ans- The magnitude of average velocity of an object is equal to its average speed when we not consider direction of the velocity.

(3) What does the odometer of an automobile measure?

Ans- The odometer of an automobile measures distance covered by automobile.

(4)What does the path of an object looks like when it is in uniform motion?

Ans- The path of an object looks like a straight line when it is in uniform motion.

(5) 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 ms-1.

Ans- Page 103

(1) When will you say a body is in (i) uniform acceleration? (ii) Non – uniform acceleration?

Ans-

(i) When an object changes its velocity by equal amount in every one second is called constant or uniform acceleration.

(ii) When an object changes its velocity by unequal amount in every one second is called non – uniform acceleration.  Page 107

(1) What is the nature of the distance – time graphs for uniform and non – uniform motion of an object?

Ans-

The distance – time graphs for uniform motion of an object will be a straight line as represented below – The distance – time graphs for non -uniform motion of an object will not be a straight line as represented below – (2) What can you say about the motion of an object whose distance – time graph is a straight line parallel to the time axis?

Ans-  If distance – time graph of an object is a straight line parallel to the time axis it shows that the object is stationary.

(3) What can you say about the motion of an object if its speed – time graph is a straight line parallel to the time axis?

Ans- If speed – time graph of an object is a straight line parallel to the time axis it shows that the object is moving with uniform speed.

(4) What is the quantity which is measured by the area occupied below the velocity – time graph?

Ans- The area occupied below the velocity – time graph is the distance travelled by the object.

Page 109  (2) 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 the rest.

Ans- The train is travelling at a speed of 90km/h. When brakes applied, the train was travelling. Hence initial velocity of the train is 90km/h. Then train reaches to rest. Therefore, final velocity of the train is 0 km/h.  (4) A racing car has a uniform acceleration of 4 m/s2. What distance will it cover in 10 s after start?

Ans-

Acceleration, a = 4 m/s2

Time, t= 10s

u = 0 m/s

Let distance travelled by the racing car = s meter           (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 –

The initial velocity of the boat = 0 m/s

Acceleration, a = 3.0 m/s2

Time, t = 8 s

Let distance travel by the boat = s meter      (6) Fig below 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?

Ans-

x axis –

5 small boxes = .4 hour

1 small box = .08 hour

y axis –

7 small boxes = 4 km

1 small box = .57 km     (8) The speed – time graph for a car is shown is fig below- (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? (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 with an acceleration but with uniform speed.

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

Ans-

(a) Yes, an object can move with constant acceleration but zero velocity.

Eg:-  If a ball throw upward and reaches at certain height. Its velocity will be zero but it has acceleration due to gravitational force.

(b) Yes, an object can move with acceleration but with uniform speed.

Eg:- In circular motion the object changed its velocity as direction changes continuously but magnitude of velocity remain same ,ie, speed of the object . The object accelerates continuously.

(c)Yes, an object can move in a certain direction with an acceleration in the perpendicular direction

Eg:- In circular motion, the direction of the object is always at the tangent of the circle but its acceleration towards the radius of the circle.

(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. Helping Topics

Motion

Worksheet Class 9