derive a fundamental equation that is used to determine the speed of a rocket.

This vague statement is bordering on the undefined. There are many equations that are used in determining the speed of a rocket.

Indeed, there are several key equations used to determine the speed of a rocket. One of the most fundamental equations is the Tsiolkovsky rocket equation, which relates the speed of a rocket to the properties of its engine and the mass it carries.

To derive the Tsiolkovsky rocket equation, let's start with Newton's second law of motion, which states that the force acting on an object is equal to its mass multiplied by its acceleration (F = m * a). In the context of a rocket, the force is produced by the expulsion of propellant material through the rocket engine, resulting in a change in momentum.

According to the law of conservation of momentum, the change in momentum of the rocket is equal and opposite to the change in momentum of the expelled propellant. This can be expressed as:

m * Δv = -Δm * ve

Where:
- m is the initial mass of the rocket (including both the rocket itself and the propellant it carries),
- Δv is the change in velocity (or speed) of the rocket,
- Δm is the change in mass of the rocket due to the expulsion of propellant, and
- ve is the velocity of the expelled propellant relative to the rocket.

Now, let's rearrange this equation to solve for the change in velocity:

Δv = - (Δm * ve) / m

Since the mass of the rocket, m, decreases as propellant is expelled, we need to consider the difference between the initial mass and final mass of the rocket, which can be expressed as Δm = m0 - mf, where m0 is the initial mass and mf is the final mass.

Substituting this into the equation, we get:

Δv = - ((m0 - mf) * ve) / m

Finally, to determine the speed, we can divide both sides of the equation by the change in time:

Δv / Δt = - ((m0 - mf) * ve) / (m * Δt)

The term Δv / Δt represents the average acceleration of the rocket, which we can approximate as its speed. Thus, the final form of the equation becomes:

v = - ((m0 - mf) * ve) / (m * Δt)

This equation, known as the Tsiolkovsky rocket equation, provides a fundamental relationship between the speed of a rocket, the mass of the rocket and its propellant, and the velocity of the expelled propellant. Keep in mind that this equation assumes no external forces acting on the rocket and neglects factors such as air resistance.