# What does sub i mean?

by The riddler
Tags: derivitive, equation, kinetic energy, time
 PF Gold P: 7,120 The subscript 'i' refers to the component x,y, and z. It is an index that goes from 1 to 3, 1 = x, 2 = y, 3 = z that is used for the sum. In this case, it means the sum of $$mV_x A_x + mV_y A_y + mV_z A_z$$ in component terms. In more advanced studies, you'll see the summation term dropped and when you see a repeated index such as the one you have, that kind of sum is implied. On the other hand, if you have something like $$A_i B_j$$ where A and B are just two indices, the index runs from 1 to 3 on both i and j so you get 9 terms that include $$A_1 B_3 , A_2 B_3 , A_1 B_1$$ etc etc.