PT2.2 - 3 designs and decision making matrix


Length : 30 cm 
Width : 8.5 cm
Height : 11.9 cm 

3 designs

Design 1

Design 2

Design 3

Decision matrix

Our chosen design is number 3

Design Rationale

Chassis :
The mousetrap car should be light and rigid so we decided to use wood as our frame because it is sturdy and light. In our chosen design, the mousetrap is attached to two wooden pieces to form the chassis. There is no wooden plank to hold the mousetrap. Having a wooden plank will add unnecessary weight to the car. The unnecessary weight will slow down our car. A lighter car requires less energy to begin. This means that lesser potential energy is used and more potential energy can be converted into kinetic energy. Having more kinetic energy allows our car to travel further. Our chassis would be cut into triangular shapes as it is more aerodynamic than a rectangular shape. This would allow the mousetrap car to have a laminar air flow as the air flow would be smoothly divided by the triangular shape. Hence, there would be lesser drag, as when the speed increases, the drag would increase slower than compared to a lesser aerodynamic shape. The mousetrap car would then require lesser energy to overcome the drag. Thus, the mousetrap car would move faster with a triangular shape. We also chose the design with triangular sides instead of a the chassis that is a triangle base this is because we feared that there would be lesser distribution of weight and force which might cause the mousetrap car to be unstable.

Wheels :
The wheels must be rigid and thin to minimise rolling resistance with the ground. The number of turns a wheel makes has an effect on the energy being used by a wheel. A wheel with a large diameter will make fewer turns as compared to smaller diameter wheels. Hence, reducing the total amount of energy needed to overcome friction and allowing the mousetrap car to travel a longer distance. The wheel should be smooth too. This will enable the mousetrap car to travel a longer distance. In the chosen design, CDs are used and they are covered in duct tape to make it smoother. Also, our group decided to use 4 wheels instead of 3 wheels. 3 wheels would allow the car to travel faster than a mousetrap car of 4 wheels as it would require less energy to turn the wheels. However, we chose the mousetrap car with 4 wheels. This is because, the testing site is made out of tarmac, which is uneven and might cause the thin CDs to wobble a lot. Thus with 4 wheels, the mousetrap car would be more stable as the weight would be more evenly distributed and would not be affected by the uneven tarmac. 4 wheels also provide lesser total friction than compared to 3 wheels, this would then allow the car to move better.

Axle :
The axles are made of wooden chopsticks and plastic capsules (pen) to prevent the wheel from wobbling too much. The axles we are using are smaller than the wheels. This enables the wheel to cover more distance per each turn of the axle as a smaller axle would spin much more than an axle that is thicker.

Lever arm:
We should extend the lever arm as this would allow more string to be coiled around the axle thus increasing the number of revolutions the wheels would make. Increasing the lever arm would also decrease the pulling force needed while shorter lever arm would decrease the amount of string and increase the pulling force. Thus in order for the car to travel further, we chose to create a longer lever arm. The extended lever arm should be a chopstick as it is very cheap and durable enough to withstand the force applied on it.

When the string is wound, the place where the string is attached to the arm should be directly above the rear axle, it should not be too long or too short but directly above. This would maximise the torque produced as the car takes off. The string should also not be longer than the distance from the lever arm to the rear axle this is because, there would be too much slack which could cause the lever arm to have lesser pulling force thus the car would not move very well and when the lever arm is completely closed, there would still be remaining string left on the axle, this would then cause the axle to encounter friction as it turns and would not travel as far.

Position of mousetrap:
We also chose the design with triangular sides instead of a the chassis that is a triangle base this is because we feared that there would be lesser distribution of weight and force which might cause the mousetrap car to be unstable. The mousetrap would be placed along the shape of the triangular chassis and further away from the rear axle this would allow more weight to be placed downwards, allowing the car to move forward even more.

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