公式幾時用

When there is Resistance,
like given rated value then you can use P=V^2 /R or P=R I^2
But if give only power supplied by mains, then you shall you P=IV

For mechanics,
No specific cases to use. Just see which variables you have to find and what information given and you shall use the one that suit the requirements.
eg,
give displacement, initial velocity and time, find acceleration,
then of course us
s=ut+1/2 at^2

Hope you understand!

The three equations, P = I^2.R, P = V.I and P = V^2/R are all used to calculate the power consumed by a device with resistance R. They are all equivalent and would give the same result.

Which equation is to be used depends what data are given. Select the most convenient equation.