Living Science 2020 2021 Solutions for Class 7 Science Chapter 14 - Motion And Time

Answer:

The SI unit for mass, length, time and temperature are as follows:

(i) Mass = Kilogram (kg)
(ii) Length = meter (m)
(iii) Time = second (s)
(iv) Temperature = Kelvin (K)

Answer:

No, measuring length is more difficult than measuring time. Because to measure time, we just need the time interval. i.e. the starting value and the end value of the time. But to measure the length, we need the actual path and the direction in which the length has to be measured. That's why it is more difficult to measure. 

Answer:

The time period of a pendulum is directly proportional to the square root of the length of the pendulum. So, if the length increases, the time period of the pendulum increases accordingly. 

Answer:

Speed of a moving object is given by the rate of change of distance travelled by the object. i.e. Speed = $\frac{\mathrm{Distance} \mathrm{travelled}}{\mathrm{Time} \mathrm{taken}}$

Answer:

If a body travels in a straight line and travels equal distances in equal intervals of time then the body is said to be in a uniform motion. 

Answer:

In a uniform motion, a body travels equal distances in equal intervals of time. Hence, if we plot a graph between distance and time for uniform motion, the graph would be a straight line with a constant slope.

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.

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.

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.

Answer:

(a) sand clock
It only measures the fixed time interval and not the time of day.

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

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).

Answer:

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

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

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.

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.

Answer:

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

Answer:

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

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.

Answer:

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

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.

Answer:

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

Answer:

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

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.

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.

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.

Answer:

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

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.

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.

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.

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)

Answer:

Speed is given by the expression,

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

Answer:

 Speed is given by the expression,

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

Answer:

The expression for the speed is given by
$$\begin{gathered} \mathrm{Speed}=\frac{\mathrm{Distance}}{\mathrm{Time}} \\ \mathrm{or} \mathrm{Distance}=\mathrm{Speed} \times \text{T}\mathrm{ime} \\ \mathrm{Speed} \mathrm{of} \mathrm{aeroplane}=950 \mathrm{km}/\mathrm{h} \\ \mathrm{Time} \mathrm{taken} \mathrm{by} \mathrm{aeroplane}=7 \mathrm{hours} \\ \mathrm{So}, \\ \mathrm{Distance} \mathrm{between} \mathrm{New} \mathrm{Delhi} \mathrm{and} \mathrm{London}=950\times 7=6650 \mathrm{km} \end{gathered}$$

Answer:

In uniform motion, the object covers equal distances in equal intervals of time.
Distance–time graph in this case is a straight line.

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

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.

Answer:

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.

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.

Answer:

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

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.

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.

Answer:

1. As Mr. Nair let his car pick up speed slowly, the graph describing his motion would be graph B. Because in the graph B speed is rising slowly with increasing time. 

2. As Ms. Patnaik drive the car with the same speed. Hence, the graph describing her motion would be graph A. Because, in graph A, the speed of the car does not change over time.

3. Graph C would perfectly describe the motion of Ms. Rushmore because he speeds up his car in a very short span of time. The same is shown in graph A. 

Answer:

Speed limits are set with a motive to keep everyone safe. A high-speed car undergoes more damage than a low-speed car if it collides with other vehicles on the road. So, moving at high speed is dangerous not just for the driver, but for others on the road (including passengers, other drivers, bicyclists, and pedestrians). Therefore, according to the rush on the road, the condition of the road, turns on the road, and slope of the road, the speed limits are sets nowadays. If speed limits will not be obeyed then more accidents will happen every day and people will lose their lives, it will also create traffic problems.