1. The problem statement, all variables and given/known data Hi, this isn't so much a specific question, but rather a project which I'm embarking on. As part of my coursework, I'm required to take up and work on a Physics project, and I've chosen a problem fro the 2011 IYPT. The problem statement goes like this: Build a model car powered by an engine using an elastic air-filled toy-balloon as the energy source. Determine how the distance travelled by the car depends on relevant parameters and maximize the efficiency of the car. 2. Relevant equations Only Newton's Law so far (F = ma) 3. The attempt at a solution I'm not quite sure how to go about tackling the project. So far, I've defined efficiency as the distance the car travels for a fixed amount of air input into the balloon (Mechanical efficiency itself is defined as output energy/input energy) and a car simply as a four-wheeled vehicle. I've also come up with mechanical diagrams which show how the car works. Basically, when inflated, the tension of the balloon exerts a force on the air, which causes the air to be pushed out, and the air exerts a reaction force that causes the whole car to move. Forces which oppose the motion of the car include air drag and friction force(although I'm not sure if it's static or kinetic friction which acts on the tires). I've also come up with a brief statement of the energy conversion process in the system: Basically, the inflated balloon possesses elastic potential energy, which is then converted to kinetic energy and transferred to the air and then the car. The energy is then converted to heat and sound energy, and lost from the system. The main problem here is that I'm not quite sure how to proceed with the experiment. I initially planned on doing a series of experiments on how altering the physical dimensions of the car would affect the distance travelled (e.g. mass, length, height) but my science mentors told me I couldn't just put any random variables in there, each independent variable would have to be related to the measured variable(the distance) in some way. To make some progress for the experiment, I came up with a basic design for the car(see below). I have tried to conduct some experiments so far, measuring mass against distance, but a few problems have cropped up. Firstly, I have no real way to ensure that the amount of air I pump into the balloon is the same for every experiment, as I have no real accurate way of measuring the amount of air I pump(at least not that I know of). I'm using a balloon pump. Secondly, the balloon may also obstruct the motion of the car, due to the balloon being much bigger than the car when inflated, it may sometimes rub against the floor, generating friction and preventing the car from moving forward. I have tried to remedy this by using a curved tube/pipe which points upwards, but the balloon may still be pulled down by gravity at certain times and obstruct the car. Thirdly, even when the car does manage to travel, it may only travel a short distance(roughly 35-55cm for 2000 cm3 of air pumped). I suspect this may be due to the tires having too much static friction to overcome. So how should I solve (or go about solving) these problems? How should I continue with the experiment in general?Is my method of experimentation right? Thanks very much for helping.