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

• Andy24
The drag equation is a mathematical model that calculates the force exerted by the air on an object in motion. The air resistance equation is a mathematical model that calculates the force exerted by the air on a surface (like a rocket) in motion.

#### Andy24

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)?

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
A.T. said:
That's reminds me that I forgot to add that it is the velocity with respect to the surrounding air.

DrClaude said:
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

Andy24 said:
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.

## 1. What is the significance of the 'v' in drag=kv^2?

The 'v' in the drag equation represents the velocity of the object. This means that as the velocity of the object increases, the drag force acting on it will also increase.

## 2. How does the 'v' affect the drag force?

The 'v' in the drag equation is squared, meaning that it has a significant impact on the overall drag force. This means that as the velocity of the object increases, the drag force will increase exponentially.

## 3. Is there a limit to how much the 'v' can affect the drag force?

Yes, there is a limit to how much the 'v' can affect the drag force. As the velocity of the object approaches the speed of sound, the drag force will reach its maximum value and will not increase any further.

## 4. How does the 'k' constant affect the drag force?

The 'k' constant in the drag equation represents the drag coefficient, which is unique to each object and determines how much drag force it experiences at a given velocity. A higher 'k' value means that the object experiences more drag force at a given velocity.

## 5. Can the 'v' in the drag equation be negative?

No, the 'v' in the drag equation cannot be negative. Velocity is a vector quantity and cannot have a negative value. The direction of the velocity may change, but the magnitude (or speed) will always be positive.