# Model Rocket- What does the 'v' in drag=kv^2 really mean?

Hello! I am predicting the peak altitude of a model rocket based on some ground tests. I know that Total Force on the rocket=Thrust-mg-kv^2 but am stuck as to which value for 'v' to use? Is it the velocity of the rocket with the effects of gravity and air resistance taken into account or the velocity purely from the motor (without any air resistance or gravity taken into account)?

Your help is appreciated, thanks.

## Answers and Replies

DrClaude
Mentor
The velocity at any given time is the rate of change of position over time, how fast the object is actually going. There is no such thing as "the velocity purely from the motor": the motor does not give a velocity, it gives a force. The velocity will be the result of all the forces acting on the object (and its previous velocity if it is already moving).

Andy24
DrClaude
Mentor
That's reminds me that I forgot to add that it is the velocity with respect to the surrounding air.

That's reminds me that I forgot to add that it is the velocity with respect to the surrounding air.

Thanks for your response.. I understand now. I actually calculated the velocity based on some ground testing (where I found out the impulse of the motor) and was hoping to incorporate drag.. This may give you context: https://www.physicsforums.com/threads/coefficient-of-drag-on-a-model-rocket.871810/
Thanks for your help though, I think I might just leave incorporating drag into my calculations as I have 2 unknowns- the force due to drag as well as the actual velocity. Thanks again,
Andrea

Chestermiller
Mentor
Hello! I am predicting the peak altitude of a model rocket based on some ground tests. I know that Total Force on the rocket=Thrust-mg-kv^2 but am stuck as to which value for 'v' to use? Is it the velocity of the rocket with the effects of gravity and air resistance taken into account or the velocity purely from the motor (without any air resistance or gravity taken into account)?
In this context, the terms "drag" and "air resistance" are synonymous.