Picky about initial velocity condition

AI Thread Summary
In kinematic problems, initial velocity can be zero or nonzero depending on the context. For a rocket at takeoff, it starts with zero initial velocity but experiences upward acceleration. Conversely, when a block is pushed with an impulsive force, it has a nonzero initial velocity once released. The distinction lies in the time frame and the specific phase of motion being analyzed. Understanding the importance of the motion's context is crucial for determining initial velocity conditions.
fisico30
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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
 
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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.
 

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