Do I always have to convert to SI units?

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Discussion Overview

The discussion centers around the necessity of converting units to SI when calculating the minimum mass of a star using the formula M=PV³/2πG. Participants explore whether to maintain the given units of kilometers and hours or to convert them to meters and seconds, considering the implications for the calculation.

Discussion Character

  • Debate/contested

Main Points Raised

  • Some participants suggest that if the values are given in kilometers and hours, it may be reasonable to keep them in those units rather than converting to SI.
  • Others argue that consistency in units is crucial, implying that all measurements should be in the same unit system, whether that be SI or otherwise.
  • A participant notes the potential for numerical factors, such as 3600 seconds per hour, to complicate calculations if units are not compatible.
  • There is a general sentiment that using SI units is often the safest and quickest approach, especially when in doubt.
  • One participant emphasizes that the choice of units should align with the units used for the gravitational constant G.

Areas of Agreement / Disagreement

Participants express differing views on whether to convert to SI units or to retain the original units provided. No consensus is reached on the best approach, as opinions vary on the importance of unit consistency versus the practicality of using the given units.

Contextual Notes

Participants highlight the importance of ensuring that all constants and measurements are compatible in terms of units, but do not resolve the specific issue of whether to convert to SI or not.

Bugsy23
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Hi, I'm trying to calculate the minimum mass of a star, where
M=PV³/2πG

The values I've been given are V=400km s^-1 and P=34.4 hours.

Does this mean that I have to convert these values to SI units, i.e. give the values in terms of metres and seconds rather than km and hours? This would obviously give a very different answer so I'm not sure
 
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Bugsy23 said:
Hi, I'm trying to calculate the minimum mass of a star, where
M=PV³/2πG

The values I've been given are V=400km s^-1 and P=34.4 hours.

Does this mean that I have to convert these values to SI units, i.e. give the values in terms of metres and seconds rather than km and hours? This would obviously give a very different answer so I'm not sure
I would say that if you are given units in km and hrs, it makes sense to keep things in those units, not switch to meters and seconds.
 
Mark44 said:
I would say that if you are given units in km and hrs, it makes sense to keep things in those units, not switch to meters and seconds.

Assuming that any constants are also in those units -in this case you also have G
 
If you solve both numerically and with the units, you may also get factors of 3600 s h-1 floating around, so I'd suggest ensuring compatibility there.
 
In other words, just be consistent in your use of units- if part of your problem has measurements in "miles" and "hours" and another in "meters" and "seconds", convert to one or the other.

Does it matter which you use? Yes, if your teacher says so!
 
Bugsy23 said:
Hi, I'm trying to calculate the minimum mass of a star, where
M=PV³/2πG

The values I've been given are V=400km s^-1 and P=34.4 hours.

Does this mean that I have to convert these values to SI units, i.e. give the values in terms of metres and seconds rather than km and hours? This would obviously give a very different answer so I'm not sure

I think as a general rule it's always safest and quickest to stick to SI.
 
Bugsy23 said:
...M=PV³/2πG...

...Does this mean that I have to convert these values to SI units,...
It really depends what units you are using for G. The other quantities' units should be consistent with that.

Dadface said:
I think as a general rule it's always safest and quickest to stick to SI.
Agreed, when in doubt go with SI units for everything.
 

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