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Thread: mach number

  1. #1 mach number 
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    why the speed of sound is considered in mach no??why not speed of light??


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    Physicists often use units where the speed of light is 1. Will that do?

    On the other hand, you can go at multiples of the speed of sound (Mach 2, Mach 3, etc) but you can't go faster then the speed of light.


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    Quote Originally Posted by JIM001 View Post
    why the speed of sound is considered in mach no??why not speed of light??
    All those . followed by zeroes are too hard to follow.
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    of course we cannot go beyond the speed of light...but they can say that an aeroplane is travelling at a speed this many times less than the speed of light.right??
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    if u say more zeroes create more problem then we can assume anything we want to reduce our calculations....there is no need to of physics...only our own assumptions stands.....
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    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by JIM001 View Post
    of course we cannot go beyond the speed of light...but they can say that an aeroplane is travelling at a speed this many times less than the speed of light.right??
    Well, you could. It's just not very practical. For example, a 747 typically cruises at about 0.0000013 c
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    Quote Originally Posted by JIM001 View Post
    if u say more zeroes create more problem then we can assume anything we want to reduce our calculations....there is no need to of physics...only our own assumptions stands.....
    Huh?

    We use terms that we can relate to and are easiest to hold in our short term memories. That consideration isn't important in a physics calculation.

    Of course we could use simplified conventions with the speed of light....but I'm not sure that helps either.
    Which is easier for that 747?

    0.0000013 c
    1.3E-6 c
    650 mph
    or mach 0.9?
    (we could make up other ones for fun...how about parsecs per year or Plank lengths per second.etc)

    Call me crazy, but the mach 0.9 is the easiest by far which is why for aircraft, rocket cars, or I guess stratospheric balloon jumpers (that's one crazy crazy SOB--in a good way!) we use it.
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    Mach is not a unit of speed : it is a number that is the ratio between the speed of a mobile and the local speed of sound. As the speed of sound in air depends on factors such as temperature and pressure, the Mach number will also vary with these factors. So it cannot be converted (exactly) into knots or km/h or c witch are true units.

    Pilots of airplanes use Mach rather than IAS (indicated air speed in knots) when cruising at hight altitude. For instance, some model of Boeing 737 may have a recommended cruise speed of Mach 0.74 The actual speed will be different according to the altitude of the aircraft but the pilot don't have to care, he just set the automatic pilot on Mach 0.74. When he starts descending for landing, the pilot will switch the speed display from Mach to IAS (knots) in order to control the descent speed (avoid overspeed, respect of air regulation and of ATC instructions).
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