Initial Velocity: Kinemetic Equations | When is 0 m/s OK?

[Sylis]

I just have a general question. When is it okay to assume that Initial Velocity is 0 m/s?

For instance I have this question:
A football is kicked straight up into the air; it hits the ground 3.8s later.

Do I assume that the initial Velocity is 0 m/s? Or do I assume that I'm only concerned at the point the football has left the kickers foot at which point it would have a velocity.

 

[jfizzix]

$\vec{}$If you are concerned with the point the football has left the kicker's foot, it will have a nonzero velocity. If it did not, then it would stay on the ground where it was.

The problem gives you the additional information that the ball is in flight for 3.8 seconds. You can use this information in your kinematic equation for the vertical component of position:
$y_{f} = y_{i} + v_{0y}t - \frac{1}{2}g t^{2}$

Given that the initial and final height are both zero,
$0= v_{0y}t - \frac{1}{2}g t^{2}$
and you can plug in the time of flight (3.8s) to find the vertical component of the initial velocity $v_{0y}$.

Hope this helps:)

 

[jfizzix]

Generally, the problem will tell you if something is "released from rest", at which point, you are free to assume that the initial velocity is zero.