1.4 - Design Rationales

Design Rationals: 

Wheels :
Large wheels have greater rotational inertia than small wheels. In practical terms, this means that once they start rolling, they're harder to stop rolling. This makes large wheels good for distance-based contests — theoretically, they'll accelerate less quickly than smaller wheels, but they'll roll much longer and they'll travel a greater distance overall. So, for maximum distance coverage, the wheels on the drive axle (the one the mousetrap is tied to, which is usually the rear one) has to be very large. 

Axle :
Each time the rear axle turns, the rear wheels turn. If the rear axle is extremely thin, the mousetrap car will be able to turn it more times for the same length of string than it would if it were wider. This translates to turning the rear wheels more times, meaning greater distance.

Lever Arm & String Type:
A string tied to the arm of the mousetrap is carefully wrapped around one of the wheel axles, and, when the trap is sprung, the swinging arm of the trap (kinetic energy) transfers its energy to the axle to turn the wheels (rotational kinetic energy). Since the arm of the trap is fairly short, if the car isn't carefully constructed, it can pull on the string too rapidly, causing the wheels to slip and energy to be lost too fast (strings needs to have a large surface area to increase friction so that energy would not be wasted). For a slower, steadier pull, we would have to attach a long pole to the arm to act as a lever, then tying the end of the string to this, better than to the arm itself. It is important to use the right material for the lever. The lever shouldn't bend at all under the stress of the string — this represents wasted energy. Many guides recommend sturdy balsa constructions or balsa reinforced with metal to give a sturdy yet light lever.

Position of Mousetrap :
The longer the distance between the trap and the wheels, the better it is — more distance means that we will be able to loop more string around the axle for just a little extra slow and steady pulling power. Hence we positioned our mousetrap nearer to the front of the car as the lever arm and the string could then have been longer and could then cover a greater distance to the back of the car, thus the rope/string can be released less quickly and the car would be able to move.

The chassis where the mousetrap sits on and mousetrap's force is really strong, hence we need the chassis to be sturdy and strong, not flimsy, so that the chassis can take up the mousetraps's force and not snap/break when the tension of the string and the force of the mousetrap is released. 
But bearing in this in mind, we also had to make the chassis as less complicated as possible, so as to keep it light-weight, which would mean that the car can travel the furthest it can.

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