# Is there a case (besides light speed in any given medium) where speed is experimentally measured rather than theoretically calculated? [closed]

I studied physics throughout college, but I cannot recall a single time where I directly measured the velocity of an object or force. Every time I measured the components of velocity (distance and time) rather than the actual velocity instead. This got me thinking as to whether or not there is some instance where I would have a known velocity, but not components by which to calculate it.

Light, being constant within any given medium, is the only velocity I can measure. Knowing the velocity of an object could be useful for measuring distance, but I want to know if there is a way of directly measuring velocity.

## closed as unclear what you're asking by ACuriousMind♦, sammy gerbil, CuriousOne, Gert, Qmechanic♦Aug 1 '16 at 18:25

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

• what qualifies as a direct speed measuring device? – Wolphram jonny Jul 31 '16 at 0:27
• The speed of light is being "measured" the same way, by using a length and a time normal. Science takes complex phenomena and breaks them down to easier to understand/characterize ones. That's how science works. In case of the speed of light we have simply swapped one normal (a unit length) against a more stable normal (the speed of light). That's a legitimate operation as we are allowed to pick our base normals/phenomena any which way we like. – CuriousOne Jul 31 '16 at 0:35
• I (think) Maxwell calculated the speed of light, so the speed of light was both measured and calculated. – userLTK Jul 31 '16 at 0:36
• @userLTK Maxwell calculated it using the measured values of the permittivity and permeability of free space. – dmckee Jul 31 '16 at 0:56
• How do you think you can measure the speed of light directly? – OrangeDog Jul 31 '16 at 8:49

The term "direct measurement" is tricky. It's easy to take a philosophic position and say no measurement is a "direct" measurement. We're always interacting something with something else to do our measurements. Accordingly, I have to play a little loose with the "direct measurement" concept.

• A speedometer measures velocity by converting it into a rotational motion, then using that to spin a magnet to cause a torque, which is measured by comparing its forces against those of a spring.
• A radar measures velocity by measuring the Doppler shift caused by the velocity of the object.
• One might argue you measure the velocity of cars on the highway as you accelerate up an onramp, trying to predict where they will be.

Every measurement compares "something" (the thing to be measured) with "something else" (a standard). Since there are only a handful of true standards in physics, and everything else is derived from that, nearly every measurement involves a series of steps that help you find the relationship between the quantity of interest, and the fundamental constants.

Let's take velocity. It is expressed in meters per second. Today, [the meter is defined as]

The length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second

The duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom.

Everything else that relates to the measurement of either distance, or time, has to somehow go back to that definition.

Now let us assume that a police car has a well calibrated (traceable to NIST standards) speedometer. If you are trucking along the highway at 153 km/h (illegal in many countries), that police car could drive behind you until the distance between you and it does not change (perhaps using a LIDAR to measure the distance, and find it is constant). At that point, the policeman can confidently say you were going 153 km/h without measuring either distance, or time.

How, then, would that calibrated speedometer come into being? That calibration might involve measuring the speed of the car a number of times, and recording distance and time.

In the end, all measurement is a comparison. It's just a question of what the units are of the thing you are comparing with - and whether that allows a "direct" or an "indirect" measurement.

I talked about the same thing when everyone got shocked: we do have load cell, acceleration transducer and velocity transducer. I think this is not an engineering question because there are transducers for force, velocity and acceleration measurement. But we you dive into it, you will find force, velocity and acceleration are derived value. They are from Hook's law etc. Direct measurement of these doesn't make any sense because its essence is derivation. When looking at CFD, FEA. They are doing the same thing. Direct measurement of velocity/force are for convenient purpose. When you starts to investigate the measurement of distance, you will open the door to philosophy.

I think that calculation is indeed measurement. When you measure length it means that you are calculating it. The tape is calibrated with something else and those calculations give the length. Simmilarly time is also calculated. In ancient times it was calculated in terms of fraction of days and year. Mass is also calculated in terms of other mass.

Velocity can also be calculated from Doppler effect where change in wavelength of emitted/scattered light gives the velocity. In this situation you are not measuring directly time or distance but spectral shift.

I hope this will help