# Do I always have to convert to SI units?

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

Mark44
Mentor
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.

mgb_phys
Homework Helper
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.

HallsofIvy
Homework Helper
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!

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.

Redbelly98
Staff Emeritus