Picky about initial velocity condition

[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

 

[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.

 

[astrokreng]

Yes, your explanation is correct. In simple kinematic problems, the initial velocity can be either zero or some nonzero value. This is because the initial velocity is dependent on the specific situation and the forces acting on the object. In the case of the rocket, the initial velocity is zero because it is initially at rest and must accelerate upwards to take off. However, in the case of the small block being pushed with an impulsive force, the initial velocity is not zero because the force applied on the block causes a change in its velocity over a short period of time. This is why the instantaneous velocity is not taken as zero. It is important to consider the specific conditions and forces at play in order to determine the correct initial velocity in kinematic problems.