Measurement of Time and Motion
Class 07 ScienceHumans got interested in keeping track of time long ago. They started noticing that many events in nature repeat themselves after definite intervals of time.
For example, the rising and setting of the Sun, the phases of the Moon and the changing seasons. They started using the cycles of these events for timekeeping. First, they devised calendars. A day was defined by the cycle of rising and setting of the Sun. Then began the quest to find ways of knowing the time of day.
So, they made many devices which helped them to measure smaller intervals of time within a day. Some of these were sundials, water clocks, hourglasses, and candle clocks.
In a sundial, time is determined with the changing position of the shadow of an object cast by the light of the Sun during the day.
The water clocks used the flow of water out or into a vessel to measure time. In one type, water flowed out from a vessel which had markings for time. In the other type, there would be a bowl, with a fine hole at the bottom, which was floated on the surface of water. It gradually filled up in a fixed time and finally sank. Then, it was lifted up and floated again.
In an hourglass, time was measured on the basis of the flow of sand from one bulb to another.
Candle clocks were candles with markings that indicated the passage of time when burned.
As human civilisation advanced, and as people began to travel long distances, the measurement of time became very critical. This led to the development of increasingly better mechanical devices for the measurement of time, driven by weights, gears, and springs from the fourteenth century onwards. However, the invention of the pendulum clock in the seventeenth century marked a major breakthrough in mechanical timekeeping.
A Simple Pendulum
A simple pendulum consists of a small metallic ball (called the bob of the pendulum) suspended from a rigid support by a long thread.
The pendulum at rest is in its mean position. When the bob of the pendulum is moved slightly to one side and released, it starts oscillatory motion. Its motion is periodic in nature because it repeats its path after a fixed interval of time.
The pendulum is said to have completed one oscillation when its bob, starting from its mean position O, moves to extreme position A, changes direction and moves to another extreme position B, changes direction and comes back to O. The pendulum also completes one oscillation when its bob moves from one extreme position A to another extreme position B and comes back to A. The time taken by the pendulum to complete one oscillation is called its time period.
The time period of a simple pendulum of a given length is constant at a place. This property is used in the measurement of time.
All clocks, old or modern, are based on some process repeating continuously, which can be used to mark equal intervals of time.
SI Unit of Time
The SI unit of time is the second. Its symbol is s. The larger units of time are minute (min) and hour (h).
60 s = 1 min
60 min = 1 h
The distances moved by objects in a given interval of time decide which one is faster or slower.
Speed
By comparing the distances moved by two or more objects in a unit time, it can be found out which of them is moving faster. The unit time may be one second or one minute or one hour. We call the distance covered by an object in a unit time as the speed of the object.
The speed of an object can be calculated, if we know the total distance covered by an object and the time taken to cover it. The speed of an object is the total distance covered divided by the total time taken to cover it.
$$ \text{Speed} = \frac{\text{Total distance covered}}{\text{Total time taken}} $$
Unit of Speed
The SI unit of speed is metre/second and is expressed as m/s. Speed can also be expressed in other units. If we express the distance in kilometre and time in hour, then the unit of speed is kilometre/hour, expressed as km/h.
Example: Swati’s school is 3.6 km from her house. It took her 15 min to reach her school riding on her bicycle. Calculate the speed of the bicycle in m/s.
Distance in meters = 3.6 km = 3.6 × 1000 = 3600 m
Time in seconds = 15 min = 15 × 60 = 900 seconds
$$ \text{Speed} = \frac{\text{Distance}}{\text{Time}} = \frac{3600}{900} = 4 \, \text{m/s} $$
Relationship between Speed, Distance, and Time
To calculate the distance covered by an object, if you know its speed and the time taken.
Total distance covered = Speed × Total time taken
Similarly, you can also calculate the time an object will take to cover a distance, if the distance and speed are given
$$ \text{Total time taken} = \frac{\text{Total distance covered}}{\text{Speed}} $$
Example: Raghav is going to a neighbouring city in a bus moving at a speed of 50 km/h. If it takes him 2 h to reach that city, how far is that city?
Distance covered by bus = Speed × Time
= 50 km × 2 h
= 100 km
Example: A train is travelling at a speed of 90 km/h. How much time will it take to cover a distance of 360 km?
$$ \text{Time taken by the train} = \frac{\text{Distance covered}}{\text{Speed}} $$
$$ = \frac{\text{360}}{\text{90}} $$
= 4 h
Uniform and Non-uniform Linear Motion
When an object moves along a straight line, its motion is called linear motion.
An object moving along a straight line with a constant speed is said to be in uniform linear motion. On the other hand, if the speed of an object moving along a straight line keeps changing, it is said to be in non-uniform linear motion.
An object in uniform linear motion covers equal distances in equal intervals of time, while it covers unequal distances in equal intervals of times when it is in non-uniform linear motion.