When do we know whether to use gram vs. kilogram, etc.? [ENDORSED]

[Lindsey Lill]

Say you are doing a problem using E = (1/2)mv^2. You are multiplying mass times velocity to receive energy. However, how to we know whether to use grams, or kilograms? Are we trying to keep everything the same (such as doing a problem with kJ, kg, and km/s), or do some units need to be in kilo form while others aren't in kilo? Thank you.

[Ivy Kwok 2I]

I believe we use grams when the problem involves chemistry concepts (e.g. converting substances to moles) for the majority and kilograms when the problem involves physics concepts (e.g. kinetic energy in the photoelectric effect) for the majority.

[Chem_Mod]

Use whatever units you need in order to convert and cancel units properly. In addition, make sure to note what the questions asks. For the purposes of a quiz/test, you will always be told to give the mass in grams, etc...

[Carolyn Huh 1K]

Usually, the problem will specify what the units it wants the answer in. However, also be aware that Joules is given in kilograms, meters, and seconds, so when cancelling units in equations, make sure that other units are converted into these before doing the problem.

[Yamilex Velgara 2I]

The question will usually tell you what units to express your answer in, but if it doesn't, I would say stick to the standard (grams or meters). Also, you should be able to receive credit as long as your answer and units match up. For example, if your answer is 1500 grams, it's ok to write down 1.5 kg. Keeping your units while solving the problem will help you keep track of what units to cancel.