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### Correct Units??

Posted: Thu Apr 12, 2018 2:25 pm
How do we know whether a formula needs g or kg? some formulas assume g while others assume kg. What is a good rule of thumb? Is it normally that mechanics equations such as 1/2mv used kg whereas other equations in quantum mechanics use grams?

### Re: Correct Units??

Posted: Thu Apr 12, 2018 2:44 pm
you would normally use grams unless the question uses or states otherwise or if the calculated number is very large, making it more convenient or logical to use kg. Typically, you'll be able to tell what is appropriate.

### Re: Correct Units??

Posted: Thu Apr 12, 2018 5:26 pm
A good rule of thumb is following the units the problem gives when stating the initial amounts. If they are initially given in kg, then they typically expect your answer to also be in kg.

### Re: Correct Units??

Posted: Sun Apr 15, 2018 4:30 pm
Also, look at what units are given in the question. If they are stated in kilograms, you would generally give the answer in kilograms also, no matter how big or small the number is. And vice versa with grams.

### Re: Correct Units??

Posted: Sun Apr 15, 2018 4:46 pm
Unless otherwise specified, the general rule of thumb is to give the answer in the units the givens were presented in. It is also good practice to recognize if your answer contains a 10^ x that better corresponds to another unit (ex: picometers 10^-12, megameters 10^9, decimeters 10^-1, etc.), but always relay back to the general rule of thumb.

### Re: Correct Units??

Posted: Sun Apr 15, 2018 5:32 pm
Unless the formula explicitly states that the formula requires the variables to be inputted in terms of certain units, it does not matter which units you use in the formula. What you should do is to keep track of the units and convert the units to another units using conversion factors whenever you find it is appropriate to do so. Based on the 1/2mv2 term that is being asked, it seems that the final answer you may be looking for is in terms of Joules. Just remember that 1 Joule = 1 kg*m2/s2. This should help you understand that even if you start with g, you can convert g to kg then convert to J if needed.

### Re: Correct Units??

Posted: Sun Apr 15, 2018 9:18 pm
I believe normally, if no unit was already given, you would use grams. What I usually do is use the units provided with the initial amount and converting my answer to fit that.

### Re: Correct Units??

Posted: Sun Apr 15, 2018 10:24 pm
It was recommended by my previous TA to just go off of what the problem begins with unless stated otherwise. For instance, if they give you grams for any part of the problem, your result should reflect grams.

### Re: Correct Units??  [ENDORSED]

Posted: Sat Apr 21, 2018 12:44 pm
As stated above, it is important to always follow the units that the problem gives you. It is also important to note that certain units such as joules are given in (kg(m^2))/s^2 and that the problem will not work if you use grams in it.