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Speed, Time, Distance

Kinematics and dynamics are the two branches of mechanics concerned with the phenomena of motion. A complete description of a moving particle or object would involve its mass, forces producing change of speed, direction of motion, time, speed, the distance travelled, etc. Kinematics describes motion with respect to speed, time and distance only, while dynamics deals with the causes or laws of motion, involving the nature of the particle or object whose motion is under study.

Here, we are to define motion only from the standpoint of measurement of speed as amount of change of position per unit time. Generally, the word “per” indicates division. For example, "mass per unit volume” means mass divided by volume.

The definition of speed gives us a method of measuring it. We divide the amount of change of position (i.e. the distance covered) by the time it takes for the change to happen. We can say that

units of speed = units of distance (length)/units of time

Compared with every other form of motion with which we are familiar the speed of light appears to be an upper limit of. velocity. In a single second light crosses a space equal to eight times the circumference of the Earth and in travelling from any visible object on the Earth to the eye of an observer on the Earth, light occupies, however small, but a real interval of time.

We see objects not as they are at the moment we notice them but as they were the smallest fraction of a second before that. From the Moon light takes a little more than a second and a quarter in reaching us. But light occupies more than eight minutes to reach us from the Sun; a longer or shorter interval in travelling to us from Mercury, Venus, Mars, etc. The information brought by light about the various members of the solar system belongs to different times and it is clear that events of the greatest importance might have happened and we don’t know anything about them.

If, however, we pass beyond the limits of the solar system, we understand better how different in time are the events presented to us. We may assume that the light of many stars occupies thousands of years in coming to us. That’s why we use the so-called astronomical unit (symbol AU) or a still larger unit known as a light-year (symbol lt. yr.) while speaking of galaxies. AU is defined as the mean distance from the Sun to the Earth (1.4964 x 10^13 cm) while lt. yr. is defined as the distance travelled by light in the course of one year and is equal to 9.463 x 10^17 cm.

Kinematics and dynamics are the two branches of mechanics concerned with the phenomena of motion. A complete description of a moving particle or object would involve its mass, forces producing change of speed, direction of motion, time, speed, the distance travelled, etc. Kinematics describes motion with respect to speed, time and distance only, while dynamics deals with the causes or laws of motion, involving the nature of the particle or object whose motion is under study.

Here, we are to define motion only from the standpoint of measurement of speed as amount of change of position per unit time. Generally, the word “per” indicates division. For example, "mass per unit volume” means mass divided by volume.

The definition of speed gives us a method of measuring it. We divide the amount of change of position (i.e. the distance covered) by the time it takes for the change to happen. We can say that

units of speed = units of distance (length)/units of time

Compared with every other form of motion with which we are familiar the speed of light appears to be an upper limit of. velocity. In a single second light crosses a space equal to eight times the circumference of the Earth and in travelling from any visible object on the Earth to the eye of an observer on the Earth, light occupies, however small, but a real interval of time.

We see objects not as they are at the moment we notice them but as they were the smallest fraction of a second before that. From the Moon light takes a little more than a second and a quarter in reaching us. But light occupies more than eight minutes to reach us from the Sun; a longer or shorter interval in travelling to us from Mercury, Venus, Mars, etc. The information brought by light about the various members of the solar system belongs to different times and it is clear that events of the greatest importance might have happened and we don’t know anything about them.

If, however, we pass beyond the limits of the solar system, we understand better how different in time are the events presented to us. We may assume that the light of many stars occupies thousands of years in coming to us. That’s why we use the so-called astronomical unit (symbol AU) or a still larger unit known as a light-year (symbol lt. yr.) while speaking of galaxies. AU is defined as the mean distance from the Sun to the Earth (1.4964 x 10^13 cm) while lt. yr. is defined as the distance travelled by light in the course of one year and is equal to 9.463 x 10^17 cm.

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