Building a Model Car: Maximizing Efficiency with Newton's Law

[XJL488Hax]

Homework Statement

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 traveled by the car depends on relevant parameters and maximize the efficiency of the car.

Homework Equations

Only Newton's Law so far (F = ma)

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 traveled (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.

 

[phinds]

Nicely thought through. I can't give you much help, but I would point out to you one thing about your discussion.

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).

Think about this: If your car's tires were experiencing kinetic friction, that would mean that the surface of the tire that is in contact with the road it's on would be moving relative to that surface. Now, at first thought one might be inclined to think "well of course it is moving" but in fact it is not. If it were then the wheels would, at least to some extent, be spinning uselessly. You CAN make car tires do this; it's called "burning rubber". You can also do it more easily by driving on ice; this is called "how to kill yourself" . Google "static friction and car tires" for more discussion.EDIT: one suggestion: put the balloon on top of the car and put a shield in front of the balloon such that the air friction is pretty much the same against the car whether the balloon is inflated or not. Might be awkward with the relative sizes you have, but something to think about. The effect of the air friction on the balloon slowing the car probably only matters if you are comparing trial runs where in one run the car goes as far a possible (no weight added) against one where it goes a very short distance (heavier weight added). In the longer run the balloon has to push aside more air than in the shorter run.

 

[CWatters]

As for air resistance... I note that the shape of the balloon isn't specified.

 

[XJL488Hax]

Well the shape is pretty much that of a typical balloon... it's an ovoid(3D oval shape). It's not a perfect sphere, at least.

 

[XJL488Hax]

Nicely thought through. I can't give you much help, but I would point out to you one thing about your discussion.Think about this: If your car's tires were experiencing kinetic friction, that would mean that the surface of the tire that is in contact with the road it's on would be moving relative to that surface. Now, at first thought one might be inclined to think "well of course it is moving" but in fact it is not. If it were then the wheels would, at least to some extent, be spinning uselessly. You CAN make car tires do this; it's called "burning rubber". You can also do it more easily by driving on ice; this is called "how to kill yourself" . Google "static friction and car tires" for more discussion.EDIT: one suggestion: put the balloon on top of the car and put a shield in front of the balloon such that the air friction is pretty much the same against the car whether the balloon is inflated or not. Might be awkward with the relative sizes you have, but something to think about. The effect of the air friction on the balloon slowing the car probably only matters if you are comparing trial runs where in one run the car goes as far a possible (no weight added) against one where it goes a very short distance (heavier weight added). In the longer run the balloon has to push aside more air than in the shorter run.

Hi, regarding your suggestion, there are a few things to note. Firstly, what do you mean by shield? If you mean any object which physically blocks the balloon from the air, well attaching it to the car would add on to the mass, and even if it did manage to decrease the air resistance, the added mass means the overall efficiency of the car might decrease as well. There are also practical limitations, for example, given the relatively small size of the car, it would not be practical to add say, a cardboard piece of dimensions 30cm x 30cm to it. Also, I can't attach the balloon to the car physically, if I use scotchtape, it could affect the inflation/deflation of the balloon and even cause it to rip, while if I use blu-tack, it would be difficult to actually attach the balloon to the car.

Nevertheless, thanks for your suggestion.

 

[CWatters]

Well the shape is pretty much that of a typical balloon... it's an ovoid(3D oval shape). It's not a perfect sphere, at least.

Balloons come in different shapes. I think I might choose a different one.

 

[phinds]

Yeah, I was suggesting a rectangular "wind shield" and you are right in what you say about it. I understand about the balloon attachment.

 

[XJL488Hax]

Balloons come in different shapes. I think I might choose a different one.

Well I wasn't really aware of that... Still, don't most balloons have roughly the same shape? And how would changing the shape (hypothetically) affect the distance of the car?

 

[CWatters]

I thought perhaps one like this...

http://www.thenakedscientists.com/HTML/uploads/RTEmagicC_balloon.jpg.jpg

Perhaps compare how balloons with the same volume but different shape perform? Drag is proportional to frontal area.

 

[XJL488Hax]

I thought perhaps one like this...

http://www.thenakedscientists.com/HTML/uploads/RTEmagicC_balloon.jpg.jpg

Perhaps compare how balloons with the same volume but different shape perform? Drag is proportional to frontal area.

Ah I see. Thanks!