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#### Page No 157:

#### Question 1:

Which of these is not a standard unit?

(a) second

(b) metre

(c) hand span

(d) gram

#### Answer:

(c) hand span

This unit is not reliable because it varies from person to person. So, it cannot be considered as a standard unit.

#### Page No 157:

#### Question 2:

Which of the following insrtuments measures time most accurately?

(a) sundial

(b) quartz watch

(c) pendulum clock

(d) watch with balance wheel

#### Answer:

(b) quartz watch

These watches have the crystals of a substance called quartz.

These crystals can vibrate very fast and at a very precise rate. These vibrations are used to measure time accurately.

#### Page No 157:

#### Question 3:

Which of these is closest to uniform motion?

(a) car travelling on a busy straight road

(b) car travelling on a busy curved road

(c) car travelling on a straight empty road

(d) car travelling on a curved empty road

#### Answer:

(c) car travelling on a straight empty road

A body is said to be in uniform motion if it travels in a straight line and covers equal distances in equal intervals of time.

#### Page No 157:

#### Question 4:

Which of these can only measure time interval and not the time of day?

(a) sand clock

(b) sundial

(c) pendulum clock

(d) quartz clock

#### Answer:

(a) sand clock

It only measures the fixed time interval and not the time of day.

#### Page No 157:

#### Question 5:

Which of these is ture?

(a) distance = speed / time

(b) distance = speed × time

(c) distance = time / speed

(d) none of these

#### Answer:

(b) distance = speed × time

The distance covered by a moving object in unit time is known as its speed.

Speed = distance / time

So, distance = speed × time

#### Page No 157:

#### Question 6:

The distance-time graph of a car is plotted with on the *X*-axis and distance on the *Y*-axis. The graph is a straight line parallel to the *X*-axis. Which of the following is true about the car?

(a) It is in uniform motion

(b) It is in non-uniform motion

(c) It is not moving

(d) It is moving very fast.

#### Answer:

(c) It is not moving

In a distance–time graph, if the graph is a straight line parallel to the time axis, then the object, here the car, is at rest (i.e. its distance is constant with time).

#### Page No 157:

#### Question 7:

A car with non-uniform motion travels 100 km in 4 hours. What can be said about the speed of the car?

(a) Its speed is constant at 25 km/h during the journey.

(b) Its average speed is 25 km/h during the journey.

(c) Its speed is 25 km/h at the beginning and end of the journey and varies in between.

(d) Its speed is 25 km/h for most of the journey.

#### Answer:

(b) Its average speed is 25 km/h during the journey.

Average speed of the car = $\frac{\mathrm{Total}\mathrm{distance}\mathrm{travelled}}{\mathrm{Total}\mathrm{time}\mathrm{taken}}=\frac{100}{4}=25\mathrm{km}/\mathrm{h}$

#### Page No 157:

#### Question 1:

What are the quantities that can be measured called?

#### Answer:

The quantities which can be measured are called physical quantities.

For example, the length of an object can be determined by comparing it to an object of known length, such as a ruler.

#### Page No 157:

#### Question 2:

The SI system is used all over the world in everyday measurements. True of false?

#### Answer:

True.

The SI system or international system of units is the modern form of the metric system, and it is used all over the world in everyday measurements.

#### Page No 157:

#### Question 3:

Time could not be accurately measured before the principle of the pendulum was discovered. True or false?

#### Answer:

True.

Clocks that measure time accurately could be made only after the discovery of the simple pendulum.

#### Page No 158:

#### Question 1:

Why are standard units used in measurements?

#### Answer:

Standard units are used in measurements because these are reliable and can be uniformly used by everyone. Other units like the footstep and hand span are unreliable because they vary from person to person.

#### Page No 158:

#### Question 2:

The discovery of which principle made the accurate measurement of time possible? What is the principle?

#### Answer:

The discovery of the simple pendulum made the accurate measurement of time possible. According to the principle of simple pendulum, if a weight hung from a string is made to swing, it always completes one to and fro motion in exactly the same time.

#### Page No 158:

#### Question 3:

What do you mean by 'time period' of a pendulum?

#### Answer:

The time taken by a pendulum for one oscillation is known as its time period.

#### Page No 158:

#### Question 4:

Give an example of a periodic change related to the earth that can be used to measure time.

#### Answer:

Our earth completes one rotation on its axis in almost 24 hours. Due to this rotation, we can see sun rise, change its position during the day and finally sun set.

The time between one sunrise and the next is called one day. This concept can be used to measure time.

#### Page No 158:

#### Question 5:

On what principle are modern electronic watches based?

#### Answer:

Modern electronic watches are based on the vibrations of the crystals of substance 'quartz' present in it.

These crystals can vibrate very fast and at a very precise rate. These vibrations are used to measure time accurately.

#### Page No 158:

#### Question 6:

When is an object said to be in uniform motion?

#### Answer:

A body is said to be in uniform motion if it travels in a straight line and covers equal distances in equal intervals of time.

#### Page No 158:

#### Question 1:

What is the SI system of units? Name two other systems of units.

#### Answer:

The SI system or international system of units is the modern form of the metric system and it is used all over the world in everyday measurements.

In this system, the following standard units are used:

(1) Metre (m) for length

(2) Kilogram (kg) for mass

(3) Second (s) for time

(4) Ampere (A) for current

(5) Kelvin (K) for temperature

(6) Candela (cd) for luminescence

(7) Mole (mol) for number of atoms and molecules

Following are two other systems of units:

(1) CGS system

(centimetre, gram and second)

(2) MKS system

(metre, kilogram and second)

#### Page No 158:

#### Question 2:

A car travels a distance of 70 kilometres in $4\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$ hours. Find its speed.

#### Answer:

Speed is given by the expression,

$\mathrm{Speed}=\frac{\mathrm{Distance}}{\mathrm{time}}\phantom{\rule{0ex}{0ex}}\mathrm{Distance}\mathrm{travelled}\mathrm{by}\mathrm{car}=70\mathrm{km}\phantom{\rule{0ex}{0ex}}\mathrm{Tine}\mathrm{taken}\mathrm{to}\mathrm{travel}\mathrm{this}\mathrm{distance}=4\frac{1}{2}\mathrm{hours}=\frac{9}{2}\mathrm{hours}\phantom{\rule{0ex}{0ex}}\mathrm{Speed}=\frac{70}{\left({\displaystyle \frac{9}{2}}\right)}=15\frac{5}{9}\mathrm{km}/\mathrm{h}\mathrm{or}15.5\mathrm{km}/\mathrm{h}\left(\mathrm{approx}\right)$

#### Page No 158:

#### Question 3:

A cheetah runs a distance of 200 metres in 10 seconds.What is the speed of the cheetah in

(a) m/s,

(b) km/h?

#### Answer:

Speed is given by the expression,

$\mathrm{Speed}=\frac{\mathrm{Distance}}{\mathrm{Time}}\phantom{\rule{0ex}{0ex}}\left(\mathrm{a}\right)\mathrm{Distance}\mathrm{travelled}\mathrm{by}\mathrm{cheetah}=200\mathrm{m}\phantom{\rule{0ex}{0ex}}\mathrm{Time}\mathrm{taken}=10\mathrm{sec}\phantom{\rule{0ex}{0ex}}\mathrm{Speed}=\frac{200}{10}=20\mathrm{m}/\mathrm{s}\phantom{\rule{0ex}{0ex}}\left(\mathrm{b}\right)\mathrm{Distance}\mathrm{travelled}\mathrm{by}\mathrm{cheetah}=200\mathrm{m}=\frac{200}{1000}\mathrm{km}=0.2\mathrm{km}\phantom{\rule{0ex}{0ex}}(1\mathrm{km}=1000\mathrm{m})\phantom{\rule{0ex}{0ex}}\mathrm{Time}\mathrm{taken}=10\mathrm{sec}=\frac{10}{60\times 60}\mathrm{hours}\phantom{\rule{0ex}{0ex}}(1\mathrm{hour}=60\mathrm{minutes}\mathrm{and}1\mathrm{minute}=60\mathrm{seconds})\phantom{\rule{0ex}{0ex}}\mathrm{So},\mathrm{speed}=\frac{0.2}{\left(\frac{10}{60\times 60}\right)}=72\mathrm{km}/\mathrm{h}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}$

#### Page No 158:

#### Question 4:

An aeroplane takes 7 hours to travel from New Delhi to London. If the plane travels at a uniform speed of 950 kilometres per hour, find the distance between New Delhi and London.

#### Answer:

The expression for the speed is given by

$\mathrm{Speed}=\frac{\mathrm{Distance}}{\mathrm{Time}}\phantom{\rule{0ex}{0ex}}\mathrm{or}\mathrm{Distance}=\mathrm{Speed}\times \text{T}\mathrm{ime}\phantom{\rule{0ex}{0ex}}\mathrm{Speed}\mathrm{of}\mathrm{aeroplane}=950\mathrm{km}/\mathrm{h}\phantom{\rule{0ex}{0ex}}\mathrm{Time}\mathrm{taken}\mathrm{by}\mathrm{aeroplane}=7\mathrm{hours}\phantom{\rule{0ex}{0ex}}\mathrm{So},\phantom{\rule{0ex}{0ex}}\mathrm{Distance}\mathrm{between}\mathrm{New}\mathrm{Delhi}\mathrm{and}\mathrm{London}=950\times 7=6650\mathrm{km}$

#### Page No 158:

#### Question 5:

Draw a typical distance-time graph for uniform motion.

#### Answer:

In uniform motion, the object covers equal distances in equal intervals of time.

Distance–time graph in this case is a straight line.

#### Page No 158:

#### Question 6:

Draw a distance-time graph from the following data (Table 1) showing the distance covered by a racing car in fixed intervals of time. Calculate the speed of the car.

Time (s) |
Distance (in m) From start point |

0 | 0 |

1 | 15 |

2 | 30 |

3 | 45 |

4 | 60 |

5 | 75 |

#### Answer:

**Distance–time graph**:

To find the speed from the graph, we take any point A on the straight line graph, then

Speed = Distance travelled/Time taken

= OB/OC

= AC/OC

= 30/2

= 15 m/s

#### Page No 158:

#### Question 7:

Draw a distacne-time graph from the above data (Table 2) showing the distance covered by a racing car. Does the car have uniform motion?

Time (s) |
Distance (in m) |

0 | 0 |

1 | 10 |

2 | 25 |

3 | 45 |

4 | 65 |

5 | 90 |

#### Answer:

**Distance–time graph**:

No, car does not have uniform motion. From the above graph, it is clear that the car travels unequal distances in equal intervals of time. So it is in non-uniform motion.

#### Page No 158:

#### Question 8:

Distinguish between uniform and non-uniform motion.

#### Answer:

Uniform Motion |
Non-uniform Motion |

In uniform motion, the object covers equal distances in equal intervals of time. | In non-uniform motion, the object covers unequal distances in equal intervals of time. |

In this motion, the speed of the object is constant. | In this motion, the speed of the object is not constant. |

The distance–time graph for uniform motion of an object is a straight line. | The distance–time graph for non-uniform motion of an object is a curve. |

#### Page No 158:

#### Question 4:

An object moves to-and-fro about its position of rest. What type of motion is this?

#### Answer:

When an object moves to and fro about its position of rest, the motion is known as the periodic motion.

#### Page No 158:

#### Question 5:

The time period of a simple pendulum remains constant even if its length is changed. True or false?

#### Answer:

False.

The time period of a simple pendulum changes if its length is changed because the time period is dependent on the length of a simple pendulum.

#### Page No 158:

#### Question 6:

In a mechanical wristwatch, which part works in the same way as a pendulum in a clock?

#### Answer:

In a mechanical wristwatch, the balance wheel works in the same way as a pendulum in a clock.

#### Page No 158:

#### Question 7:

What kind of watch will you use to accurately measure short intervals of time?

#### Answer:

We will use stopwatch for measuring short intervals of time accurately. This watch is designed to measure the amount of time elapsed from a particular time when activated to until it is deactivated.

#### Page No 158:

#### Question 8:

What is the term used for 'distance covered by a body in unit time'?

#### Answer:

The distance covered by a moving body in unit time is known as 'speed'.

speed = distance/time

#### Page No 158:

#### Question 9:

The _________ in the vehicle gives the distance travelled in kilometres.

#### Answer:

The __odometer__ in the vehicle gives the distance travelled in kilometres.

#### Page No 158:

#### Question 10:

If the direction of motion and the speed of a body do not change with time, what kind of motion does it have?

#### Answer:

If the direction of motion and the speed of a body do not change with time, the body is said to be in uniform motion.

#### Page No 159:

#### Question 1:

Why do you think accurate measurements of time became possible much after accurate measurement of length and mass?

#### Answer:

The earliest measurements made by us were length and mass. Time was measured in terms of length as distance and time.

Position of sun, moon and stars give idea of days, months and years. Later, mechanical clocks were used to measure time accurately up to minutes. These clocks are based on weights. Today, atomic clocks are used to measure time up to nanoseconds by measuring waves emitted by caesium clock.

Therefore, we think that, over the time, improvements in accuracy in measuring length and mass help in measuring time with more accuracy and precision.

#### Page No 159:

#### Question 2:

You have made a pendulum by tying a stone to a string. Its time period is 1 second. You want to reduce the time period. How will you achieve this?

#### Answer:

The time period (T) is directly proportional to the square root of the length (L) of the pendulum.

$T\mathrm{\alpha}\sqrt{L}$

So, we can reduce the time period by reducing the length of the string.

#### Page No 159:

#### Question 3:

A car travels along a circular racing track at a constant speed of 100 km/h. Does it have uniform motion?

#### Answer:

Yes.

If a car travels in a circular path with constant speed, its motion is called uniform circular motion.

#### Page No 159:

#### Question 4:

Figure 1 shows the distance-time graph for two racing cars A and B. Which one of them won the race?

#### Answer:

Speed is given by the expression,

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

It is clear from the above expression that the speed of a car is greater if it covers maximum 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 clear that for a given time *t*, the distance covered by car A is more than car B (i.e. car A is moving faster than car B). Therefore, car A won the race.

#### Page No 159:

#### Question 5:

What can you say about the speeds of the two objects A and B from the distance-time graphs in Figure 2?

Figure

#### Answer:

For object A,

The distance–time graph is a straight line parallel to the time axis. It means the object is at a fixed distance and at rest. In this case, the object has zero speed.

For object B,

The distance–time graph is perpendicular to time axis. It represents infinite speed. Practically, this is an impossible case.

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