# Velocity as a fundamental quantity.

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1. Jan 10, 2016

### siddhukrish

Time is a fundamental quantity. but we actually measure time with change in the position of objects. for example one orbit which earth completes is a year. one rotation of earth is a day, one second is defined as 9192631770 cycles of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium atom. so time is based upon on motion. why do have time as the fundamental quantity is it a convention or is there a reason for having time as a fundamental quantity?

2. Jan 10, 2016

### Ssnow

The fact is that if an object doesn't change its position, the time continues go on because is a primitive concept. So this way of measuring time is not always very convenient...

3. Jan 10, 2016

### Bandersnatch

No it's not. It's based on change of state of a system.

For example, in measuring a year you don't care about the rate of change and direction of velocity of Earth, which is anything but constant, but define it as a separation between two sufficiently similar states (e.g. when Earth is back to the 'same' position w/r to background stars).
Similarly, when talking about hyperfine transitions, the notion of motion doesn't even have much sense (there's nothing there with a measurable position or velocity). All that counts is that there exists a state the system periodically returns to.

4. Jan 10, 2016