Moving Things, People And Ideas
If you can read a clock, you can know the time of day. But no one knows what time itself is. We cannot see it. We cannot touch it. We cannot hear it. We know it only by the way we mark its passing. For all our success in measuring the smallest parts of time, time remains one of the great mysteries of the universe. In the real world - the world with time - changes never stop. Some changes happen only once in a while, like an eclipse of the moon. Others happen repeatedly, like the rising and setting of the sun. Humans have always noted natural events that repeat themselves. When people began to count such events, they began to measure time.
In early human history, the only changes that seemed to repeat themselves evenly were the movements of objects in the sky. The most easily seen result of these movements was the difference between light and darkness. The sun rises in the eastern sky, producing light. It moves across the sky and sinks in the west, causing darkness. The appearance and disappearance of the sun was even and unfailing. The periods of light and darkness it created were the first accepted periods of time. We have named each period of light and darkness - one day.
People saw the sun rise higher in the sky during the summer than in winter. They counted the days that passed from the sun's highest position until it returned to that position. They counted three hundred sixty-five days. We now know that is the time earth takes to move once around the sun. We call this period of time a year.
Early humans also noted changes in the moon. Even before they learned the answers to these questions, they developed a way to use the changing faces of the moon to tell time. The moon was "full" when its face was bright and round. The early humans counted the number of times the sun appeared between full moons. They learned that this number always remained the same - about twenty-nine suns. Twenty-nine suns equaled one moon. We now know this period of time as one month. Time Measurement is a complicated task and is always a challenge for us. How do we measure a minute, an hour, a day? Our ancestors noticed that there were some periodic events that never changed, like the changing of seasons, rising and setting of the sun, etc. These were the first attempts mankind had ever made to measure time, it was pretty simple and the measurements they got were accurate for those times. The first ever device used to measure time in daytime was the sundial.
The oldest sundial was found in Egypt dating back to 3500 BC. A sundial has dial plates with hour lines engraved on it and a gnomon stick which sticks out at an angle. As the sun move in the sky it casts a shadow on the plate and shows the time, but it works only in daytime. So, people had to find other ways to measure time at night time. A candle clock was used by the ancient chinese to measure time at night; a candle with a uniform dimension was used to measure time as the rate at which it would burn for every hour was constant. So markings were made for every inch or so to measure time.
As technology advanced, we devised more and more sophisticated methods to measure time: a simple pendulum was used, as long as the weight of the bob and the length of the pendulum is unchanged, the pendulum would oscillate at a constant rate.
What mechanism does your wrist watch use for time measurement? Modern clocks make use of a piezoelectric crystal called quartz, you must have read quartz written on a wall clock while checking time. Well this crystal vibrates at a specific frequency when voltage is applied to it. So it is designed in such a way that a frequency of one hertz is obtained from it, this is used to measure time.
We encounter different events in our day-to-day life where we can witness different types of motion. Let us take a few examples of a moving object:
Examples of moving object
An auto-rickshaw can move slow or fast depending upon the traffic. An auto-rickshaw can move faster than a bicycle, but an aeroplane can move faster than the auto-rickshaw. How can we decide if one object is moving faster or slower than the other?
On a road we can easily spot the difference in speed of different moving object by simply looking at them. If we look at the position of vehicles at one instance of time and then again at the same vehicles after 5 seconds we can spot which vehicle moves the faster and which vehicle moves slower. The distance moved by an object in a given instance of time will help us decide its speed, like for example in a 100m race, we can say who has the highest speed by looking at who covers 100m in the shortest time. If we have an idea of time and distance we can easily determine the speed.
We are familiar with different moving objects and their speed by now. Higher speed means that a given distance has been covered in a shorter time or more distance has been covered in a given time. The best way to measure speed of two objects is to have uniformity in measuring units. Say, we will take a constant time of 1 hour and measure the distance travelled by the object in that time, we have 3 vehicles,
- A bicycle that can travel a distance of 10 km in 1 hour.
- An auto rickshaw that can travel 50 km in 1 hour.
- An aeroplane that can travel 1500 km in 1 hour.
Now, which of the above has greater speed? The answer is c) An aeroplane, because it covered more distance in the given time. We can denote this, as the speed of the aero plane is 1500 km/hr but that doesn't mean that the aeroplane travels at a constant speed all the time. The speed may or may not vary during the flight, but on average the speed at which the aeroplane has travelled is 1500 km/hr.
We encounter different events in our day-to-day life where we can witness different types of motion. Let us take a few examples of a moving object.
Motion in a straight line is nothing but linear motion. As the name suggests, it's in a particular straight line, thus it can be said that it uses only one dimension.
The linear motion, also called the rectilinear motion can be of two types -
- Uniform linear motion with constant velocity or zero acceleration.
- Non-uniform linear motion with variable velocity or non-zero acceleration.
Linear motion is the most simple kind of one-dimensional motion. As motion suggests, objects will either be in rest or continue to move in a straight line with a uniform velocity unless and until an external force is applied to it.
Uniform motion in a straight line
If a body travels in a straight line and covers an equal amount of distance in an equal interval of time, then it is said to have uniform motion. In simple words, a body is said to have uniform acceleration if the rate of change of its velocity remains constant.
For example: If a car travels at a speed of 60 km/hour, than it will cover a distance of 1 km/minute. In this sense, the motion of car acceleration is uniform.
Non-uniform motion in a straight line
Unlike uniform acceleration, a body is said to have a non-uniform motion when the velocity of a body changes by unequal amounts in equal intervals of time. The rate of change of its velocity changes at different points of time during its movement.
For example: A boy kicking a football might cover 4 meters in the first attempt, 6 meters in the second attempt, 9 meters in third attempt and so on as per the velocity exerted by the boy.
Slow or fast
We know that some vehicles move faster than others. Even the same vehicle may move faster or slower at different times. Make a list of ten objects moving along a straight path. Group the motion of these objects as slow and fast. How did you decide which object is moving slowly and which one is moving fast.
When vehicles are moving on a road in the same direction, we can easily tell which one of them is moving faster than the other. It is common experience that the motion of some objects is slow while that of some others is fast.