Knowledge of Science to Design Balloon-Powered Car?

Question

In science, we made balloon-powered cars. Question 6 asks, "Did you use your knowledge of science to guide your design? If so, in what way?"

I don't really understand the question. We know that the heavier the car is, the slower it will move, but what principle of science is that? Also, we know that the faster the air can escape from the balloon, the faster the car will move. What do I write for our knowledge of science, though?

Sorry to bother you with such a stupid question. Thanks for the help.

Answer 1

As far as the design of the car is concerned - the force of the air out of the balloon is pretty much fixed. Therefore as the previous answer explained the force generated is fixed.

So to get the best out of your car it needs to be as light as possible .  as F=ma  the smaller the value of m the greater the value of a (aceleration) . Also the car needs to be as frictionless as possible so the force generated by the balloon isnt used up in making the wheels turn but in making the car going faster

Answer 2

Newton's second law is summarised by F=ma. The force on an object is equal to the mass times the acceleration. For the same force by the baloon, bigger mass will mean smaller acceleration and vice versa.

With the baloon pushing the air back, the conservation of momentum comes into play. The momentum in a closed system (the car) is constant so the momentum of the air moving back is equal to the momentum of the car moving forwards. Momentum M=mv where m is the mass and v is the velocity. The faster the mass of air is released the bigger the momentum and therefore the greater the velocity of the car.

Because the air is being let out over a period of time, not all at once the momentum transferred is not as simple as above. Newton's second law comes into play as the car will have positive accereration so long as there is air being let out.

Always remember that friction will be slowing the car down.