Do I always have to convert to SI units?

  • Thread starter Bugsy23
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  • #1
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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
 

Answers and Replies

  • #2
34,549
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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
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.
 
  • #3
mgb_phys
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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
 
  • #4
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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.
 
  • #5
HallsofIvy
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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!
 
  • #6
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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
I think as a general rule it's always safest and quickest to stick to SI.
 
  • #7
Redbelly98
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...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.

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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