In the expressions below: g = gravitational acceleration; h = height; m = mass;
R = electrical resistance; t = time; v = velocity; V = voltage.
Which of the following expressions have units of power?
A. 1 and 2 only
B. 1 only
C. 2 and 3 only
D. 1 and 3 only
E. 1, 2 and 3
Remember that Power is the rate at which energy is transferred from one system to another. A familiar formula should be P=E/t.
Expression 3 should look the most familiar. Since GPE= mgh, mgh/t is the rate at which gravitational potential energy is transferred from one system to anther; and it is correct.
We can eliminate A and B.
You’ll notice Expression 2 is the formula for Electric Power. It derives from a combination of Ohm’s Law and another power equation. Look here:
P= VI
but also
P= I^2 R
Which means that P= \frac{V^2}{R} and that Expression 2 is correct.
We can eliminate D.
Expression 1 is also correct, as it is the rate at which kinetic energy is transferred from one system to another. Recall that K.E= 1/2mv^2. The rate of which would simply be \frac{mv^2} {2t}.
All 3 expressions are correct and E is our answer.
I’m sorry
please ignore this comment.
I tried to ask a question, but since it was resolved, I tried to delete it, but an error occurred and it could not be deleted. 
For expression 1
We can use
a=v/t
Force=ma=mv/t
P=forcexvelocity=mv2/t
What is happening here?