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#### Page No 199:

#### Question A:

**Solve the following crossword puzzle:**

figure

**Across**

1. The SI unit of time

3. It measures time

4. It is the distance travelled per unit time

5. It measures the speed of a moving vehicle

**Down**

2. It measure the distance moved by a vehicle

#### Answer:

#### Page No 199:

#### Question B.1:

Which of the following is a scalar physical quantity?

(a) velocity

(b) average speed

(c) force

(d) none of these

#### Answer:

(b) Average speed

Average speed is a scalar quantity.

#### Page No 199:

#### Question B.2:

Which of the following is a vector physical quantity?

#### Answer:

(c) Velocity

Velocity is a vector quantity that has both magnitude and direction.

#### Page No 199:

#### Question B.3:

The interval between two events is called

(a) time

(b) distance

(c) speed

(d) velocity

#### Answer:

(a) Time

Time can be defined as the interval between two events.

#### Page No 199:

#### Question B.4:

A body moving along a straight line with a uniform speed is an example of a body having

(a) uniform velocity

(b) variable velocity

(c) variable speed

(d) none of these

#### Answer:

(a) Uniform velocity

A body moving along a straight line with uniform speed is an example of a body having uniform velocity.

#### Page No 199:

#### Question B.5:

36 km/h is same as

(a) 36 m/s

(b) 3.6 m/s

(c) 10 m/s

(d) 0.36 m/s

#### Answer:

(c) 10 m/s

Given: 36 km/h

Converting 36 km to m:

1km = 1000 m****

36 km = 36 $\times $ 1000 = 36000 m

Also, 1h = 60 min

1 min = 60 s

1 h = 60 $\times $ 60 = 3600 s

Hence,

$\frac{36000}{3600}\mathrm{m}/\mathrm{s}=10\mathrm{m}/\mathrm{s}$

#### Page No 199:

#### Question B.6:

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

(a) 100 km

(b) 25 km

(c) 15 km

(d) 10 km

#### Answer:

(a) 100 km

Distance covered by car for first 15 mins = d_{1}_{ }= 40 km/h

Distance covered by car for next 15 mins = d_{2}_{ }= 60 km/h

Total distance travelled by car = d_{1} + d_{2}

= 40 km/h + 60 km/h

= 100 km/h

#### Page No 199:

#### Question C:

**Match the items in Column A with the items in Column B:**

Column A |
Column B |

1. Straight line graph | (a) Second |

2. Pendulum | (b) Speed |

3. Time | (c) Velocity |

4. Scalar | (d) Uniform motion |

5. Vector | (e) Periodic motion |

#### Answer:

Column A |
Column B |

1. Straight line graph | (d) Uniform motion |

2. Pendulum | (e) Periodic motion |

3. Time | (a) Second |

4. Scalar | (b) Speed |

5. Vector | (c) Velocity |

#### Page No 200:

#### Question 1:

Given a second's pendulum having time period 2 sec (time period is the time taken to complete on oscillation). What changes you can make to reduce the time period?

#### Answer:

The time period of a pendulum varies as the square root of the length. Reducing the length of the pendulum will reduce the time period.

#### Page No 200:

#### Question 2:

Is the motion of the body moving along a circular path with uniform sped called uniform motion? Justify your answer.

#### Answer:

No, the motion of the body moving along a circular path with uniform speed is not called uniform motion because its velocity is changing at every instance. It is called uniform circular motion.

#### Page No 200:

#### Question 3:

Given along-side is the distance-time graph of two cars A and B. Which of the car has more speed? Justify your answer.

Figure

#### Answer:

The line representing the distance covered by car A is steeper than that representing the distance covered by car B. Based on this, we can say that car A is travelling faster than car B.

The slope of the distance versus time plot on a graph is equal to the speed of the object. The faster an object travels, the greater the distance covered in a given interval of time, and consequentially, the greater the slope.

#### Page No 200:

#### Question 4:

A stone is dropped from a height 100 m above the ground. Will it have uniform or non-uniform speed as it moves towards the ground?

#### Answer:

A stone dropped from a height of 100 m experiences acceleration due to gravity, which is constant. Under this constant acceleration, the speed of the falling body will increase. Therefore, the motion is non-uniform, assuming that we neglect air resistance.

#### Page No 200:

#### Question 5:

Given along-side is the distance-time graph of the body. With the help of this graph, plot the speed-time graph of the body.

figure

#### Answer:

Given:

Distance covered by body in 20 secs = 40 m

Speed = $\frac{\mathrm{Distance}\mathrm{travelled}}{\mathrm{Time}\mathrm{taken}}$ = $\frac{40\mathrm{m}}{20\mathrm{sec}}$ = $\frac{2\mathrm{m}}{1\mathrm{sec}}$

Therefore, the speed of the body is 2 m/sec.

Hence, the speed-time graph of the body can be plotted as shown:

#### Page No 200:

#### Question D:

**Fill in the blanks:**

1. When a body does not change its position with respect to its surrounding. It is said to be at ..................................... .

2. The standard unit of time is ..................................... .

3. The interval between two events is called ..................................... .

4. ..................................... are used to measure short intervals of time.

#### Answer:

1. When a body does not change its position with respect to its surroundings, it is said to be at __rest__.

2. The standard unit of time is __second__.

3. The interval between two events is called __time__.

4. __Stopwatches__ are used to measure short intervals of time.

#### Page No 200:

#### Question E:

**Write True (T) or False (F) against the following statements in the given brackets:**

1. SI unit of speed is m/s. ( )

2. Velocity is a scalar physical quantity. ( )

3. A body in motion can never have zero average velocity. ( )

4. Every object moves with a constant speed. ( )

5. A pendulum shows oscillatory motion. ( )

6. The speed of a train is expressed in m/h. ( )

#### Answer:

1. True

2. False

Velocity is a vector quantity.

3. True

4. False

Every object need not necessarily move with constant speed.

5. True

6. False

The speed of a train is expressed in km/h.

#### Page No 202:

#### Question A.1:

Distinguish between rest and motion.

#### Answer:

Rest |
Motion |

A body is said to be at rest when it does not change its position with respect to a reference stationary object. | A body is said to be in motion when it changes its position with respect to a reference stationary object. |

#### Page No 202:

#### Question A.2:

What do you mean by a uniform motion?

#### Answer:

The motion of a body is said to be uniform if it covers equal distances in equal intervals of time.

#### Page No 202:

#### Question A.3:

Name two devices used to measure time.

#### Answer:

Time is usually measured by clocks and stopwatches.

#### Page No 202:

#### Question A.4:

What does the slope of the distance-time graph represent?

#### Answer:

The slope of the distance-time plot represents the ratio of the distance travelled by the body to the time taken, i.e., the slope represents the speed of the body.

#### Page No 202:

#### Question A.5:

What do the speedometer and odometer of a car record?

#### Answer:

The speedometer measures the speed of a car, whereas the odometer measures the distance covered by it.

#### Page No 202:

#### Question A.6:

Define average speed.

#### Answer:

Average speed of a body can be defined as the ratio of the total distance travelled by the body to the total time taken.

Average speed = $\frac{\mathrm{Total}\mathrm{distance}\mathrm{travelled}}{\mathrm{Total}\mathrm{time}\mathrm{taken}}$

#### Page No 202:

#### Question B.1:

Name three cities where sundials are found.

#### Answer:

The historical sundials can be found in Jaipur, Delhi and Varanasi.

#### Page No 202:

#### Question B.2:

What is velocity?

#### Answer:

Velocity is a physical property of a body and gives both the speed and direction of motion of the body.

Velocity is a vector quantity and possess both magnitude and direction.

#### Page No 202:

#### Question B.3:

Define speed.

#### Answer:

Speed can be defined as the distance travelled by a body in unit time. The speed (s) of an object is calculated using the following expression:

Speed (s) = $\frac{\mathrm{Distance}\mathrm{travelled}\left(\mathrm{d}\right)}{\mathrm{Time}\mathrm{taken}\left(\mathrm{t}\right)}$

#### Page No 203:

#### Question B.4:

Distinguish between uniform and non-uniform motion.

#### Answer:

Uniform motion |
Non-uniform motion |

The motion of a body is said to be uniform if it covers equal distances in equal intervals of time. | The motion of a body is said to be non uniform if it covers unequal distances in equal intervals of time. |

Uniform motion is represented by a straight line in the distance-time graph. | Non-uniform motion is represented by a zig-zag line in the distance-time graph. |

#### Page No 203:

#### Question B.5:

What are the larger units used to measure time?

#### Answer:

The larger units that are used to measure time are minute, hour, day, month and year.

#### Page No 203:

#### Question B.6:

What is a distance-time graph?

#### Answer:

The distance-time graph is a line graph that represents the distance travelled by a body and the time taken by it to cover that distance.

In the case of uniform motion, the distance-time graph is a straight line.

In the case of non-uniform motion, a zig-zag line is obtained in the distance-time graph.

#### Page No 203:

#### Question B.7:

Define scalar quantity Give two examples.

#### Answer:

Scalar quantities are physical quantities that possess only magnitude but not direction.

Speed and length are examples of scalar quantities.

#### Page No 203:

#### Question B.8:

Define velocity quantity Give two examples.

#### Answer:

Vector quantities are physical quantities that possess both magnitude and direction.

Velocity and force are examples of vector quantities.

#### Page No 203:

#### Question C.1:

The state of rest and motion and relative terms. Explain with the help of an example.

#### Answer:

A body is said to be either at rest or in motion when it does not change its position with respect to a reference stationary object.

Take for example, a person standing inside a moving bus. He is at rest with respect to another person standing inside the bus. However, because the bus is moving, he will be in motion with respect to a person standing outside the bus. Hence, we can say that the state of rest and motion are relative terms.

#### Page No 203:

#### Question C.2:

What is a simple pendulum? Define oscillation and time period of a simple pendulum.

#### Answer:

A simple pendulum consists of a small metallic solid sphere (bob) suspended by a fine thread from a rigid support.

Oscillation is the distance travelled by the pendulum from the mean position to one extreme position, coming back to mean position, going to the other extreme position and then again coming back to mean position.

Time period is the time taken by the pendulum to complete one oscillation.

#### Page No 203:

#### Question C.3:

Express the difference between speed and velocity. Can speed and velocity be same?

#### Answer:

Speed |
Velocity |

Speed measures the distance travelled by a body in unit time. | Velocity not only measures the speed but also indicates the direction of motion of a body. |

Speed is a scalar quantity. | Velocity is a vector quantity. |

Speed and velocity appear to be the same as the magnitude of both are the same. However, they are two different physical quantities as speed has only magnitude but velocity has both magnitude and direction.

#### Page No 203:

#### Question C.4:

Why is speed called a scalar quantity?

#### Answer:

Speed is a scalar quantity as it has only magnitude but not direction. Speed indicates the distance travelled by a body in unit time but not the direction of motion of the body. Hence, it is a scalar quantity.

#### Page No 203:

#### Question C.5:

Why is velocity called a vector quantity?

#### Answer:

Velocity is a physical quantity that has both magnitude and direction. Velocity not only measures the speed of a body but also indicates the direction in which the body is moving. Hence, it is a vector quantity.

#### Page No 203:

#### Question D.1:

The distance covered by a body in motion was recorded after every 1s or 4s. The data recorded is mentioned in the table given below:

Represent the situation graphically and find the speed of the body from the graph.

Time (s) |
0 | 1 | 2 | 3 | 4 |

Distance (m) |
0 | 3 | 6 | 9 | 12 |

#### Answer:

Given data:

Time (s) |
0 | 1 | 2 | 3 | 4 |

Distance (m) |
0 | 3 | 6 | 9 | 12 |

From the graph, it is clear that the speed of the body is 3 m/s.

#### Page No 203:

#### Question D.2:

The distance-time graph in the figure given alongside represents the motion of a body travelling at a uniform speed. Make a table showing the distance covered by the body in 1s, 2s, 3s, 4s and 5s. What is the speed of the body?

Figure

#### Answer:

The data according to the given graph will be:

Distance (m) | 1 | 2 | 3 | 4 | 5 |

Time (s) | 1 | 2 | 3 | 4 | 5 |

From the table, it is clear that the speed of the body is 1 m/s.

#### Page No 203:

#### Question D.3:

Study the graph carefully and write below as to what motion do they indicate:

Figure

#### Answer:

(i) The given graph indicates that the motion of the body is uniform.

(ii) The given graph indicates that the body is at rest.

(iii) The given graph indicates that the motion of the body is non-uniform.

#### Page No 203:

#### Question D.4:

Plot a time-distance graph for the motion of a body with the following data:

Time (s) |
1 | 2 | 3 | 4 | 5 | 6 |

Distance (m) |
10 | 20 | 30 | 40 | 50 | 60 |

#### Answer:

The distance-time graph corresponding to the given data will be as follows:

#### Page No 204:

#### Question D.5:

Which of the following distance-time graphs shows non-uniform motion of a truck?

Figure

#### Answer:

The graph in option (iii) represents the non-uniform motion of the truck because it is not a straight line.

#### Page No 204:

#### Question E.2:

Find the distance between New Delhi and USA, if an aeroplane moving with a speed of 900 km/h takes 12 hours to complete the trip.

#### Answer:

Given:

Speed of aeroplane = 900 km/h

Time taken to complete the journey = 12 hrs

Speed of the body is calculated using the expression:

$\mathrm{Speed}=\frac{\mathrm{Distance}\mathrm{travelled}}{\mathrm{Time}\mathrm{taken}}$

Distance travelled by aeroplane = speed $\times $ time taken

= 900 $\times $12

= 10800 km

Therefore, the distance between New Delhi and USA is 10800 km.

#### Page No 204:

#### Question E.3:

An automobile travelling at an average speed of 75 km/h takes 5 hours to reach town B from town A. Calculate the distance between A and B.

#### Answer:

Given:

Average speed of automobile = 75 km/h

Time taken to complete the journey = 5 h

The speed of the body is calculated using the expression:

$\mathrm{Speed}=\frac{\mathrm{Distance}\mathrm{travelled}}{\mathrm{Time}\mathrm{taken}}$

Distance travelled by automobile = speed $\times $ time taken

= 75 $\times $5

= 375 km

Therefore, the distance between A and B is 375 km.

#### Page No 204:

#### Question E.4:

The distance between two stations is 1995 km. How much time will a car take to cover this distance at an average speed of 19 km/h?

#### Answer:

Given:

Speed of the car = 19 km/h

Distance between the two stations = 1995 km

The speed of the body is calculated using the expression:

$\mathrm{Speed}=\frac{\mathrm{Distance}\mathrm{travelled}}{\mathrm{Time}\mathrm{taken}}$

Time taken by car to cover the distance = $\frac{\mathrm{Distance}\mathrm{travelled}}{\mathrm{Speed}}$

= $\frac{1995\mathrm{km}}{19\mathrm{km}/\mathrm{h}}$

= 105 h

Therefore, the time taken by the car to cover the given distance is 105 hours.

#### Page No 204:

#### Question E.5:

A car takes 20 minutes to cover 15 km. Calculate its speed in km/h.

#### Answer:

Given:

Distance travelled by the car = 15 km

Time taken by the car to cover the given distance = 20 min

Converting the time taken from minutes to hours:

Time taken by the car = 20 min = $\frac{20}{60}$= $\frac{1}{3}$ hours

Speed of the body is calculated using the following relation:

$\mathrm{Speed}=\frac{\mathrm{Distance}\mathrm{travelled}}{\mathrm{Time}\mathrm{taken}}$

Speed = $\frac{15\mathrm{km}}{{\displaystyle \frac{1}{3}}\mathrm{h}}$

= 45 km/h

Therefore, the speed of the car is 45 km/h.

#### Page No 204:

#### Question E.6:

Rohit cycles down from his house to his school at a speed of 18 km/h and reaches there in 30 minutes. How far is the school from his house?

#### Answer:

Time taken by Rohit to reach school from home = 30 min

Speed by which Rohit reaches school = 18 km/h

Converting the time taken from minutes to hours:

Time taken = 30 min = $\frac{30}{60}$= 0.5 h

Distance travelled = speed $\times $ time taken

= 18 km/h $\times $ 0.5 h

= 9 km

Therefore, the distance between Rohit's school and his house is 9 km.

#### Page No 204:

#### Question E.7:

A train is travelling at a speed of 100 km/h. How long will it take to complete a journey of 1000 km, without stopping in-between?

#### Answer:

Given:

Speed of the train = 100 km/h

Distance to be covered = 1000 km

Time taken = $\frac{\mathrm{Distance}\mathrm{travelled}}{\mathrm{Speed}}$

= $\frac{1000\mathrm{km}}{100\mathrm{km}/\mathrm{h}}$ = 10 h

Therefore, the time taken by the train to cover a distance of 1000 km, at a speed of 100 km/h is 10 hours.

#### Page No 204:

#### Question E.8:

Calculate the time period of a simple pendulum if it takes 48 seconds to complete 12 oscillations.

#### Answer:

Number of oscillations made by the simple pendulum = 12

Time taken by the pendulum to make 12 oscillations = 48 s

Time period of the simple pendulum is the time taken by the pendulum to make one oscillation.

Time period of the pendulum = $\frac{48}{12}=4s$

Therefore, time period of the pendulum is 4 seconds.

#### Page No 204:

#### Question E.9:

A car covers 20 km in first hour of its journey, 40 km in next hour and 30 km in third hour. Calculate its average speed.

#### Answer:

Average speed = $\frac{\mathrm{Total}\mathrm{distance}\mathrm{covered}}{\mathrm{Total}\mathrm{time}\mathrm{taken}}=\frac{20\mathrm{km}+30\mathrm{km}+40\mathrm{km}}{3\mathrm{h}}=\frac{90\mathrm{km}}{3\mathrm{h}}=30\mathrm{km}/\mathrm{h}$

The average speed of the car is 30 km/h.

#### Page No 204:

#### Question E.1:

A truck travels 540 kilometres in 4.5 hours. Calculate its speed.

#### Answer:

To calculate the speed of the truck, we use the following relation:

$\mathrm{Speed}=\frac{\mathrm{Distance}}{\mathrm{Time}}=\frac{540\mathrm{km}}{4.5\mathrm{h}}=120\mathrm{km}/\mathrm{h}$

Thus, the speed of the truck is 120 km/h.

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