# Picky about initial velocity condition

1. Sep 29, 2008

### fisico30

In simple kinematic problems,

the initial velocity can be zero or some nonzero value.

A friend asked me: what makes it zero in some cases and nonzero in some others.

Take a rocket in take off. It has acceleration moving upward, from the beginning, but its v_0=0.

Instead, if I apply an impulsive force on a tiny block on a table and then let it go, we say that there is a certain v_0.....

Do we have to assume that they both are actually starting from zero speed, but over a very small delta_t, the average velocity of the rocket is still very very small (amost zero), and that average velocity corresponds to the instantaneous velocity at that point.

In the case of the small block being pushed with an impulsing force, the instantaneous velocity is not taken as zero, because the velocity change in that short delta_t is not negligible.....

IS this a correct explanation?
thanks

2. Sep 30, 2008

### jablonsky27

It depends which part of the motion is important for you. I mean depends on the situation. For your rocket example, if I am interested in the motion after the first stage boosters have been exhausted, my initial velocity will not be zero.