# Calculating Force from Pressure in Water Rocket Building

• DruJohnson
In summary: Yes, that is correct. Liquids are considered incompressible because they have a very small change in volume when subjected to pressure. Gases, on the other hand, are considered compressible because they can be compressed to a smaller volume when subjected to pressure. This is due to the fact that gas molecules are more spread out compared to liquid molecules. In summary, the conversation discusses the equation for the force generated by a water rocket, which is dependent on the mass flow rate and velocity of the fluid at the nozzle exit. As the water is expelled, the pressure and flow rate decrease, leading to a decrease in thrust over time. The temperature of the water is largely unaffected by its travel through the nozzle, but the pressure drops dramatically while
DruJohnson
Hello I am currently building a water rocket. I would like some help on finding the equation for the force that is exerted when I add air pressure to an cylindrical tube filled with water. How can I figure out the force that the water would create when I release it. I hope that someone can help with this problem of mine. I appreciate the help that anyone can offer me.

Thank you,
Dru Jouhnson

For a rocket, the force (thrust) generated is given by:

Thrust = mdot * v_exit

where,

mdot = mass flow rate
v_exit = velocity of fluid at exit of nozzle

For a water rocket pressurized with air, you could figure out the mass flow and velocity from fluid mechanics. As the water is expelled, the gas expands, pressure drops, flow rate drops, and the thrust drops. Thus, the thrust decreases with time. But the weight of the rocket decreases too, so the acceleration may increase or decrease depending on the variables.

edgepflow said:
As the water is expelled, the gas expands, pressure drops, flow rate drops, and the thrust drops.

Does the nozzle of the rocket act like the throttle valve in a refrigerator? The pressure of the refrigerant is sharply reduced by the throttle valve which also reduces the temperature of the refrigerant.

Jagella

Not exactly. Since the water is basically incompressible, the temperature will largely be unaffected by the water's travel through the nozzle. The pressure will drop dramatically with a corresponding increase in velocity though (as a rough estimate, you could assume that it is an ideal nozzle, and thus the water's exit velocity is equal to sqrt(2P/rho)).

cjl said:
Not exactly. Since the water is basically incompressible, the temperature will largely be unaffected by the water's travel through the nozzle. The pressure will drop dramatically with a corresponding increase in velocity though (as a rough estimate, you could assume that it is an ideal nozzle, and thus the water's exit velocity is equal to sqrt(2P/rho)).

Are liquids typically incompressible and gases typically compressible?

Jagella

## 1. How do you calculate the force of a water rocket?

The force of a water rocket can be calculated by multiplying the pressure of the rocket's compressed air or water by the area of the rocket's nozzle. This can be represented by the equation F = P x A, where F is the force, P is the pressure, and A is the area.

## 2. What units are typically used to measure pressure in water rocket building?

The most commonly used units for pressure in water rocket building are pounds per square inch (psi) or kilopascals (kPa). These units are used to measure the amount of force exerted on a given area.

## 3. How does the size and shape of the rocket's nozzle affect the force and pressure?

The size and shape of the nozzle can greatly affect the force and pressure of a water rocket. A larger nozzle will allow for a greater volume of air or water to be released, resulting in a higher force and pressure. The shape of the nozzle can also impact the direction and distribution of the force.

## 4. What other factors besides pressure contribute to the force of a water rocket?

In addition to pressure, the mass of the rocket, the amount of water and air used, and the angle and direction of launch can all impact the force of a water rocket. The design and construction of the rocket can also affect its overall force.

## 5. How can I increase the force and pressure of my water rocket?

To increase the force and pressure of a water rocket, you can increase the amount of compressed air or water used, use a larger nozzle, or change the design of the rocket to optimize its launch. It is important to carefully consider safety precautions when making any modifications to a water rocket.

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