Calculating Energy for Water Rocket Car

[Mechpro]

Homework Statement

Hi, i have constructed a rocket car out of a meccano kit and used a 2l water bottle filled with 200ml of water and pressured gas as a means to propel it forwards. The distance that it has to travel is 16.5m

I am having difficulty calculating the energy stored in the device, the energy required to travel this distance and the energy dissipated through the process.

Homework Equations

To calculate the energy stored in the device i decided to record the relationship between an increase in pressure and an increase in volume of the bottle. If I can draw the curve and integrate would this give me the energy stored in the bottle?

To calculate the energy dissipated through rolling friction I have come across formulae and am not sure if they are correct, I know that this can be found out experimentally however I'm not sure how to go about this

Calculating the thrust produced by the car can be done using T=2PA where P is the pressure and A is the cross sectional area of the nozzle.

One equation for rolling friction that i encountered was FR = (Mew)R x weight of wheel. I looked online an got a friction coefficient value of 1. As the weight of the wheel was 0.173kg and there are four wheels i got a value of 0.6935kg

Thrust - rolling friction = resultant force
167.190 - 0.6935 = 166.5N

By using F = m a the mass of the car being 0.6935kg
I calculated the acceleration by doing 166.5 / 0.6935 = 240m/s^2


I know that energy stored can be equated to pressure x volume and the acceleration is not constant throughout so therefore i cannot use the kinematic equations.

The Attempt at a Solution

I calculated thrust by doing 2 x 200,000Pa x (Pi X 0.0115^2) which gave me a value of 166.190N
However as the nozzle is at an angle of 30degrees i multiplied this value by cos30degrees and got a thrust of 143.92N

 

[jbsteele]

To calculate the energy stored in the device i recorded the relationship between an increase in pressure and an increase in volume of the bottle. I then drew the curve and integrated this to give me a value of 4.8J. I calculated the energy required to travel the distance by using E = mgh. The mass was 0.693kg and the height was 0.7m so i got a value of 2.48J. The energy dissipated through rolling friction is something I am having difficulty calculating. Is there an equation that can be used to calculate this?