SST 2014 S3-08 Physics EDP Group Delta ∆: 1. Planning

1.3 Brainstorming

1.3.1 Engineering Goals

Develop a MouseTrap Car with the following specifications:

(a) Uses only the MouseTrap provided as the only energy source

(b) Has a maximum length of 30 cm, with of 10 cm, and a height of 10 cm

(c) Can travel a minimum distance of 5 meters carrying an egg (the egg will be

provided by the teacher)

(d) All time-lines have to be adhered

(e) Has as little surface friction as possible.

1.3.2 Design Requirements

1.3.2.1 Chassis Requirements
The chassis has to be made of a strong, durable but lightweight material. This is to ensure that the car can travel as far a distance as possible, since a reduced mass with the same given amount of force will produce a greater acceleration. The material has to be strong in order to a) withstand the weight of the mousetrap itself, b) resist any impact against an obstacle that may occur while the car is traveling.

1.3.2.2 Drivetrain Requirements
The drivetrain in the context of this project is the system which allows the transfer of energy from the mousetrap to the axles of the car. The drivetrain has to a) ensure there is maximum moment about the trapping mechanism such that one shutting of the mousetrap will turn the axle more. b) sustain structural integrity, so that it can withstand the force of the shutting mousetrap

1.3.2.3 Wheel Requirements
The wheels has to produce very little friction, in order to avoid slowing the mousetrap car down during transit. In order to achieve that, the wheels are to be as thin as possible without compromising traction that may affect the maneuvering of the mousetrap car.

1.3.2.4 Egg Container Requirements
The egg container has to house a whole egg shell during the testing of the vehicle and has to protect the egg from external impact which may break it. It should be able to prevent the egg from moving around in the container, to prevent the eggshell from falling off the car. The egg is the representation of the passenger, of whose safety is a priority. Therefore, the egg container has to protect the egg shell completely.

1.3.3 Preliminary Ideas

1.3.3.1 Preliminary Idea 1:

Our first design is a very basic and default design similar to the typical mousetrap car. The cotton twine is used to tie to the rear axle and to the trapping mechanism of the car, and has to be coiled around the rear axle. Compact disks are used as wheels because of its reduced friction and the egg holder is mounted in the front of the vehicle. When the trap mechanism is pulled back fully and released, the mousetrap releases a massive amount of energy which is transferred by the drivetrain to the rear axle. This pulls the cotton twine which spins the axle and thus moves the car.

1.3.3.2 Preliminary Idea 2:

Our second design is mostly similar to the first idea except for one crucial change - the twine has been tied to an extension, which was made to allow a greater moment about the trapping mechanism. This works as when the trapping mechanism is shut, the ends of the cotton twine is pulled forward over a greater distance, allowing more revolutions of the axle and thus a greater distance traveled by the car. As of the first design, compact disks are used as wheels. However, the egg container has been moved to the side to prevent the egg from being smashed by the drivetrain.

1.3.3.3 Preliminary Idea 3:

Our third design revolves around the concept regarding the size of the wheels. Since the driving wheels are the rear wheels, we designed it to be larger than the front wheels so that one rotation of the axle can allow the vehicle to travel a greater distance because of the rear wheel's larger circumference. Considering that it is not feasible to have wheels larger than the compact disk, the front wheels will be downsized to old plastic wheels due to their much smaller size, therefore preventing an excess length increase that will cross the set limits.

1.4 Decision-Making Matrix

1.4.1 List of Criterias:

Weight, Size, Safety, Energy Saving, Time to Produce, Cost of Production, Availability of Materials, Environmental Impact, Ease of Use.

1.4.2 Matrix

Criteria	Weight	Size	Safety	Energy Saving	Time to Produce	Cost of Production	Material Availability	Environment Impact	Ease of Use	Row Total	Normal Value
Weight	XXXX	0	4	4	0	2	1	0	1	12	0.0833 (3 s.f.)
Size	4	XX	4	4	2	1	2	0	0	17	0.118 (3 s.f.)
Safety	0	0	XXXX	2	0	1	0	0	0	3	0.0208 (3 s.f.)
Energy Saving	0	0	2	XXXXXX	0	1	2	2	0	7	0.0486 (3 s.f.)
Time to Produce	4	2	4	4	XXXXX	2	2	4	2	24	0.167 (3 s.f.)
Cost of Production	2	3	3	3	2	XXXXXX	2	4	2	21	0.146 (3 s.f.)
Availability of Materials	3	2	4	2	2	2	XXXXXXXX	2	2	19	0.132 (3 s.f.)
Environmental Impact	4	4	4	2	0	0	2	XXXXXXX	2	18	0.125 (3 s.f.)
Ease of Use	3	4	4	4	2	2	2	2	XXX	23	0.160 (3 s.f.)
Column Total	xxxx	xxx	xxxxx	xxxxxxx	xxxxxxx	xxxxxxxx	xxxxxxxxxxx	xxxxxxxxx	xxxx	144

1.4.3 Comparison Table

NOTE: The row with the smaller normalised value is considered to be more important.

Criteria	Normal Value	Design Idea 1	Design Idea 2	Design Idea 3
Weight	0.0833 (3 s.f.)	2 (0.167) (3 s.f.)	1 (0.0833) (3 s.f.)	4 (0.334) (3 s.f.)
Size	0.118 (3 s.f.)	3 (0.354) (3 s.f.)	3 (0.354) (3 s.f.)	2 (0.236) (3 s.f.)
Safety	0.0208 (3 s.f.)	5 (0.104) (3 s.f.)	5 (0.104) (3 s.f.)	3 (0.0624) (3 s.f.)
Energy Saving	0.0486 (3 s.f.)	4 (0.194) (3 s.f.)	1 (0.0486) (3 s.f.)	4 (0.194) (3 s.f.)
Time to Produce	0.167 (3 s.f.)	3 (0.501) (3 s.f.)	3 (0.501) (3 s.f.)	4 (0.668) (3 s.f.)
Cost of Production	0.146 (3 s.f.)	3 (0.438) (3 s.f.)	3 (0.438) (3 s.f.)	4 (0.584) (3 s.f.)
Availability of Materials	0.146 (3 s.f.)	5 (0.730) (3 s.f.)	5 (0.730) (3 s.f.)	5 (0.730) (3 s.f.)
Environmental Impact	0.125 (3 s.f.)	3 (0.375) (3 s.f.)	3 (0.375) (3 s.f.)	3 (0.375) (3 s.f.)
Ease of Use	0.160 (3 s.f.)	4 (0.640) (3 s.f.)	4 (0.640) (3 s.f.)	3 (0.480) (3 s.f.)
Total		3.503	3.2739	3.6634

1.4.3 Comparison Table

Note: The row with the smaller normalised value is considered to be more important.

Criteria	Normal Value	Design Idea 1	Design Idea 2	Design Idea 3
Weight	0.0833 (3 s.f.)	2 (0.167) (3 s.f.)	1 (0.0833) (3 s.f.)	4 (0.334) (3 s.f.)
Size	0.118 (3 s.f.)	3 (0.354) (3 s.f.)	3 (0.354) (3 s.f.)	2 (0.236) (3 s.f.)
Safety	0.0208 (3 s.f.)	5 (0.104) (3 s.f.)	5 (0.104) (3 s.f.)	3 (0.0624) (3 s.f.)
Energy Saving	0.0486 (3 s.f.)	4 (0.194) (3 s.f.)	1 (0.0486) (3 s.f.)	4 (0.194) (3 s.f.)
Time to Produce	0.167 (3 s.f.)	3 (0.501) (3 s.f.)	3 (0.501) (3 s.f.)	4 (0.668) (3 s.f.)
Cost of Production	0.146 (3 s.f.)	3 (0.438) (3 s.f.)	3 (0.438) (3 s.f.)	4 (0.584) (3 s.f.)
Availability of Materials	0.146 (3 s.f.)	5 (0.730) (3 s.f.)	5 (0.730) (3 s.f.)	5 (0.730) (3 s.f.)
Environmental Impact	0.125 (3 s.f.)	3 (0.375) (3 s.f.)	3 (0.375) (3 s.f.)	3 (0.375) (3 s.f.)
Ease of Use	0.160 (3 s.f.)	4 (0.640) (3 s.f.)	4 (0.640) (3 s.f.)	3 (0.480) (3 s.f.)
Total		3.503	3.2739	3.6634

1.5 Design Rationale and Notes

1.5.1 Wheels:

Compact Discs (CDs) are used as rear wheels due to its larger but thinner size. Having a small width reduces surface friction between the ground and the wheels, allowing it to move further. In order to avoid compromising traction, the edges are taped to slightly increase the friction.

Recycled plastic wheels are used for the front wheels due to its smaller size. It is also slightly thicker, with the edges lined with rubber to increase traction and ensure that the mousetrap car is stable.

1.5.2 Chassis:

Balsa wood/ corrugated plastic is used to make the chassis as it is firm, stable but cheap. It is also easily cut, making it ideal for the chassis.

The chassis is cut to a rectangle to hold the mousetrap and is slanted downwards to accommodate the larger rear wheels. The mousetrap is positioned at the front of the chassis in order to increase the perpendicular distance between the trapping mechanism and the axle, such that the moment about the trapping mechanism will be increased.

1.5.3 String:

Cotton twine is a chosen material for the string used in the drivetrain due to its durability and ability to withstand larger amounts of force acting on it than conventional strings.

1.6 Materials Used