*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 ×10 ^{8} 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/s^{2}. 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/s^{2}. What distance will it cover in 10 s after start?

**Ans-**

Acceleration, a = 4 m/s^{2}

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/s^{2} 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/s^{2}

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**