5. Discussion and Conclusion

5 Discussion and Conclusions

5.1 Key findings
How the mousetrap car works
Elastic potential energy stored
The spring of a mousetrap stores elastic potential energy when it is pulled back and its tension is increased. 
Elastic potential energy converted into kinetic energy
Released, the elastic potential energy is converted into the kinetic energy of movement. 
What happens when the energy is converted
As the trap closes, the lever arm attached to the mousetrap arm pulls the nylon string that has been wound around the back axle of the mousetrap car. 
As a result
This causes the back axle and attached back wheels to spin, propelling the car forward.

Friction depends on: 
Roughness of contacting surfaces
Rougher the contacting surfaces, the greater the force is needed to move two surfaces past one another. 

Magnitude of forces holding the bodies together
When a body is moving over a horizontal surface, it presses down against surface with a force equal to its weight. Increase of weight causes increases in amount of resistance offered to the relative motion of the surfaces in contact.

Minimize friction on the axle by minimizing the contact surface area. The larger surface area of contact between the axle and axle holder causes the car to use energy overcoming friction rather than traveling forward.

Friction when used to your advantage. Friction should be maximized where needed (where the string wraps around the axle and where the wheels contact the floor). Slipping string or wheels =  wasted energy.

Lighter vehicles will require less force to start moving, and also experience less friction. However, if car is too light, there is not enough effective friction(traction) to move the car forward. See Newton’s law of Motion: Friction to move the car forward, below. 

Newton law of Motion
Friction to move the car forward
Third law: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

A car is propelled forward because the drive wheels push on the road and the floor pushes back against the wheels of the car causing the car to move, because of the contact/interaction between the drive wheels and the road causing a action-reaction relationship between the two objects. A vehicle's acceleration is limited by interaction the vehicle has with the road, any vehicle that produces a lot of torque also requires good traction with the road or the vehicle will just spin it's wheels. Therefore, if the floor can not push back with the same force as the wheels push, then the wheels will just spin in place and the car will not accelerate to its fullest potential. Hence, friction must be existing so as to start the car and move it forward. 

Wheel Slippage
Occurs when drive wheels do not have enough traction (aka friction used to advantage) on the ground (between wheels and ground). The amount of traction is proportionate to the grip the wheel has and the amount of weight pushing the wheels onto the ground. As centre of mass of a vehicle is moved away from the drive wheels (back wheels), traction also decreases, meaning there is lesser acceleration than that of when the mass of the vehicle is nearer to the drive wheels. 

Simple Machines, Mechanical advantage
Lever arm
The amount of energy released is the same for a short or long lever arm, but the length of the lever arm will determine the rate at which the energy is released and this is called the power output (aka elastic potential energy converted into kinetic energy). Mechanical advantage of the Lever arm can be calculated by the ratio of the distances from the fulcrum to where the input and output forces are applied to the lever. When the distance a from the fulcrum to the point A where the input force is applied is greater than the distance b from fulcrum to the point B where the output force is applied, the lever amplifies the input force. When the distance from the fulcrum to the input point A is less than from the fulcrum to the output point B, the lever reduces the magnitude of the input force.

Wheel & Axel Ratio
When the ratios of the length of string used for each round around the axel to the distance traveled is less than one, the mechanical advantage is small and the car travels slow and far. When the ratios of the length of string used for each round around the axel to the distance traveled is greater than one, the car accelerates very quickly and uses only a small amount of string. 

Size & Type of wheels
Larger wheels cover more distance per rotation. Wheel has to be aligned 90• to chassis so that it will move straight forward. Drive wheels have to create as much friction as possible to grip onto the ground so that the car can move a greater distance, rather than the wheels slipping off the ground. Better traction of wheels ensure that they do not slip. 

Placement of mousetrap
Trap placed farther away from drive axle. Allows for longer length of lever arm so that more distance can be covered. 

Position of the lever arm
As the lever arm starts to release, the weight is centered closer to the rear of the vehicle. As the lever arm makes its way through the power stoke (string), the weight is shifted to the front of the vehicle. This shift in weight can cause the vehicle to turn as the power stoke comes to an end. 

A lighter car requires less energy to move. Lower the mass of your vehicle for best distance.

Hooke’s Law
Force required to stretch spring directly proportionate to amount of stretch in spring (Elastic potential energy)


Potential energy
Energy stored within an object. Elastic Potential energy is stored in the spring of the mousetrap.

Kinetic energy
Energy that a body possesses as a result of its motion. Potential energy converts into kinetic energy as mousetrap begins to move.

Law of Conservation of Energy: Energy cannot be created or destroyed but it can be changed from one form to another but the total amount of energy in a system will remain constant.

Total energy = Elastic potential energy of spring
Elastic potential energy of spring will be converted into KINETIC ENERGY to move the mousetrap & FRICTION (which would convert the kinetic energy of the mousetrap into HEAT & SOUND energy), eventually causing the mousetrap to come to a stop, when ALL KINETIC energy of the mousetrap has been converted into HEAT & SOUND energy. 

Lever Arm
When the string is wound, the place where the string is attached to the lever arm should be just above the axle should be just above the drive axle. This maximises torque as the car takes off, maximum torque occurs when lever arm and string forms 90• right angle. If the lever arm is too long, extending past the drive axle, it will form an angle less than 90• to the axle, decreasing torque. 

Lever arms affecting power output / rate of energy release
In the mousetrap car, the same overall amount of energy is used regardless of its speed, only rate of use aka power output or rate of energy release can change. If the energy is released slowly, the power is used more efficiently, and the car will travel further. To slow this release, lengthen the lever arm, providing more puling distance. A longer arm means that the car travels a greater distance and allows more wraps of string around the axle, meaning the wheels will turn more times. The car will go far, but more slowly, covering more distance.

Perpendicular component of a force that is applied some distance from the point of rotation that causes a change in rotational motion. Greater torque = greater change in state of rotation. In the car, the lever arm applies a force to the drive axle (back) through the string. This in turn causes torque to be produced around the drive axle. 

Rotational inertia

Rotational inertia is the measure of an object’s resistance to changes in rotation.  The more rotational inertia an object has the more torque that will be required to change it's state of rotation.

5.2 Comparisons with other designs based on research
1. Our car's wheels have more friction. Due to the group going out and getting specialised wheels, our car has more friction and can move a greater distance. (Others used CDs & balloons for the wheels)
2. Our car is smaller than the ones used in competitions.
3. Some cars are slanted to the front to put more weight there. So that the car would travel a greater distance forward.
4. We use lego as our body as compared to wood.
5. Some cars only have 1 wheel at the front, making it a 3-wheeler.
6. Some cars attached the string to the front wheel instead of the back. (Front drive wheels)
7. We used stainless steel & hexagon shaped axles. Other cars used Satay sticks or thicker sticks made out of wood as their axles.
8. We allowed our front wheels to be axle free and instead screwed to the chassis (free wheels). Other groups attached their front wheels to axles. This allowed the car to travel a greater distance as friction between the axle and wheels did not exist.

5.3 Evaluation of engineering goals
a) Uses only the MouseTrap provided as the only energy source 
Yes, we used only the spring of the mousetrap, and not other external sources of energy.
b) Has a maximum length of 30 cm, with of 10 cm, and a height of 10 cm
Yes, if the length of the extension of power arm (chopstick) was not included. *it isn't included since it's not the considered as the chassis of the car.
c) Can travel a minimum distance of 5 meters carrying an egg (the egg will be provided by the teacher)
Yes, it travelled about 14 metres on average, carrying an egg. 
d) All time-lines have to be adhered  
Yes, we submitted the work on time. 

5.4 Areas for improvement 

  1. We could have used a lighter and strong material that prevents the car from breaking from impact and prevent it from breaking easily.
  2. Triangular chassis for streamlined body to reduce air resistance.
  3. Use a more centralized object to even out the forced to move the car forward (it was always slanting to the right when the lever was released)
  4. Connect velcro or any similar object to enable the car to be easily wound up. (we used tape which wasn't so effective)
  5. Tracker videos were not calibrated, as such results in the graphs were not accurate at first
  6. One tracker graph which was supposedly velocity over time, was plotted as another type of graph. 
  7. Larger wheel to axel ratio so that more distance can be covered. We used a 3.3cm in circumference wheel for the drive (back) wheels. Other groups with similar designs as ours used about 30cm in circumference wheels and were able to travel further.
  8. A thicker and rougher string to grip onto the axel. The thin nylon string was too thin and smooth thus sometimes it would slip pass the axel without rotating it.

5.5 Practical Applications
The concepts in 5.1 key findings are used in industrials, like mechanical engineering. Examples where these would be used are in F1 race cars and racing bikes. They require the vehicle to be efficient in speed and distance, and using the concepts in 5.1, they can make those vehicles to be used for the racers. 

For example, mechanical advantage. Gears and pulleys in bicycles allow the bikes to go up steep gradients of terrains because of force amplified through mechanical advantage. When the rear gear on a bike is as small as possible (i.e., it's in high gear), the bike can go the fastest on a level road, but as the road begins to slant upwards the bike begins to slow. More torque is required to propel the bike up the hill. By putting the bike into a lower gear, you increase the mechanical advantage of your gearing and consequently get more torque, but at the cost of less speed.

Another example would be the shape of axle used. Instead of round axles, they may use hexagon shaped ones to improve the gripping between axle and wheels so that the wheels do not slip off the axle.

5.6 Areas for further study

We can do some research on the reasons behind the design of the car, the way the car works, the way the mousetrap works, the effects external forces have on the car and how we can improve on the car. 

Mechanical advantage would be an area that could be further studied. Perhaps gears and pulleys could be implemented into the car for better mechanical advantage, allowing the car to travel a greater distance.

Other solutions to minimise the amount of friction acting on the car so that there is more effective frction (traction)

Explore the uses of different shapes of axle - square, hexagon, triangular, circular, rectangular, to find out the most effective one for the drive axle so as to allow the car to move a further distance when the wheels rotate with the axle.

5.7 Bibliography
Mousetrap Car - Robert Koches - Google Sites. (n.d.). Retrieved from http://sites.google.com/site/robertkoches/mousetrap-car 

How to Build a Mousetrap Car | Design Secrets | Distance. (n.d.). Retrieved from http://www.docfizzix.com/topics/design-basics/MouseTrap-Cars/mousetrap-distance-basics.shtml

Mousetrap Vehicles | The Science of Mousetrap Vehicles| Doc ... (n.d.). Retrieved from http://www.docfizzix.com/topics/science-concepts/MouseTrap-Vehicles/

Lever - Wikipedia, the free encyclopedia. (n.d.). Retrieved from http://en.wikipedia.org/wiki/Fulcrum_(mechanics)

NEWTON’S FIRST LAW OF MOTION | ANSHS Physics Classroom ... (n.d.). Retrieved from http://xanmechanics.wordpress.com/mechanics/newtons-first-law-of-motion/

Physics Tutorial: Friction , Force , Acceleration. (n.d.). Retrieved from http://www.physics247.com/physics-tutorial/friction-force-acceleration.shtml

Why is friction important in formula one racing? | Formula ... (n.d.). Retrieved from http://www.formula1-istanbul.com/8/why-is-friction-important-in-formula-one-raci


Teachers' Online Primary Science - Climate Change for ... (n.d.). Retrieved from http://www.climatechangematters.net.au/LOTS/Phy/sub/friction/friction.htm

friction: Factors Affecting Friction | Infoplease.com. (n.d.). Retrieved from http://www.infoplease.com/encyclopedia/science/friction-factors-affecting-fricti


Rotational Inertia - Physics Video by Brightstorm. (n.d.). Retrieved from http://www.brightstorm.com/science/physics/circular-motion-and-rotational-mechan


What is the difference between a good speed-trap racer and a ... (n.d.). Retrieved from http://www.experts123.com/q/what-is-the-difference-between-a-good-speed-trap-rac


How to Adapt a Mousetrap Car for Distance: 5 Steps. (n.d.). Retrieved from http://www.wikihow.com/Adapt-a-Mousetrap-Car-for-Distance

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