# Kinematic equations question

1. Sep 30, 2008

### fishingspree2

Hello,

I have a question concerning the kinematic equations

http://www.glenbrook.k12.il.us/gbssci/phys/Class/1DKin/U1L6a1.gif [Broken]

vi and vf stand for initial and final velocity, my question is: do these velocities need to be the initial (t=0) and final (end of the motion)?

can't I just use two velocities in the motion, even if its not initial and final?

I have the same question for the t.. does the t value need to be calculated from t=0?

Last edited by a moderator: May 3, 2017
2. Sep 30, 2008

### Galileo's Ghost

These equations are valid for any motion undergoing constant acceleration (or zero acceleration). They are good for any time interval. The vi and vf need to correspond to the initial and final values of velocity for any time interval you are interested in examining. So, no, you do not need to begin your analysis at t=0, but often it is convenient to do this.

3. Sep 30, 2008

### LowlyPion

You can take certain liberties with the equations in a gravity field because say the time to max height is equal to the time falling from max height if originally at rest.

But a more useful approach is understanding how to apply them in various situations and not to forget about the terms that may apply depending on the situation. For instance the d= ... equation may at times have an initial d that needs to be accounted for but isn't in the equation you show.