MASTERS of the Mousetrap Car

Maiya and I built a mousetrap car to put our total Physics knowledge to the test, applying most of the concepts we've learned throughout the year so far.

Our car came in 1st place overall

with a record-breaking speed of 5 meters in 2.2 seconds

Therefore, we reached a speed of 2.27 m/s

-Maiya Eldridge and Catherine Eckerd-

Masters of the Mousetrap Car

Yes, 2.2 seconds.  Don't believe it? Watch this video:

We took many factors into account, most of which we connected to physics concepts (in red) we've learned so far, all labeled on the picture below:

1. Light frame - We used wood (from Lowe's) for the base to keep our car light.  We did not want a heavy frame because acceleration = force / mass, therefore if we decrease our total mass, knowing that the force would remain constant, we would have a greater acceleration
2. Small wheels - we used small wheels (from a toy car I found) because of rotational inertia.  Small wheels distribute their mass closer to the axis of rotation, therefore will spin much faster than larger wheels.  Also, the small wheels kept the car closer to the ground, therefore keeping it's center of gravity lower, keeping our car more stable, therefore faster.
3. Axel - We used a metal rod (also from toy car) as an axel because the metal is slippery, so it would decrease friction between the axel and the wooden frame.
4. Rubber bands - We used rubber bands around the wheels (from Carson) because the rubber would increase friction between the wheels and the ground, so no power would be lost by the wheels slipping
5. Mouse Trap - The mouse trap (from Mrs. Lawrence) increased the energy we put into the car by winding it up because of it's tightly coiled spring, so when we let it go, it snapped together quickly, pulling the string with a great speed
6. Wooden Arm - We used a wooden plank (also from Lowe's) as a lever arm.  We made our lever arm long because lever arms increase the distance the string connected to it travels therefore increasing the velocity at which the string is pulled
7. Fishing Line - The fishing line (from Carson) connects the lever arm to the wheels' axel (by hot glue), pulling the wheels with the speed from the lever arm.

PHYSICS BEHIND MOUSETRAP CAR

This project really showed how Physics applies to mechanics and real-life machines, such as cars.  You can see the importance of each of Newton's 3 Laws in the performance of a car.

• Newton's 1st Law: An object in motion will stay in motion unless acted upon by another force.  This is important to a car because, to make a car as efficient as possible, you want to eliminate as much as possible all these outside forces that will slow it down.  We did this by trying to eliminate the force of friction between the frame and the axel to keep it from slowing down
• Newton's 2nd Law: Acceleration is proportional to force, and inversely proportional to mass.  This was very important to our car because we really focused on creating a light frame so our car could accelerate quickly.
• Newton's 3rd Law: For every action there is an equal and opposite reaction.  This is important because we were able to create an action reaction pair with the car and the ground.  As the car moves forward, it's pushing on the ground, and the ground is pushing back.  This is what makes the car moves.  Knowing this, we realized how important it was for the wheels to have a great force of friction with the ground.

The two frictions present are the friction between the ground and the wheels and the friction between the axel and car frame.  Between the wheels and the ground, we used friction to our advantage by adding rubber bands to the wheels to increase their relation with the ground.

We decided less wheels was better because there will be less friction between axels and the car itself, therefore less energy lost, therefore more efficient.  Using large wheels seemed to increase distance, but not speed.  Small wheels increase rotational inertia, therefore increase speed.

Conservation of energy was important in designing our mousetrap car.  Because a mousetrap car only emits so much energy, it was important to realize that if our car wasn't moving very far, we were losing energy in some way making the car less efficient.

We created a lever arm about 5 inches long.  Because our lever arm was so long in comparison with the rest of the car, we were able to increase the velocity our axel was rotating at.  The lever arm increased the radial distance from the axis of rotation of the mousetrap's arm.  Increasing this radial distance increases the tangential velocity at which the string is moving, therefore moving the wheels with more speed.

As stated above, tangential velocity was important in the movement of our lever arm.  For our wheels, rotational inertia was especially important because we kept the mass of the wheels distributed closer to the axel by using small wheels, therefore keeping a smaller rotational inertia.  Less rotational inertia means a greater rotational velocity.

The spring does no work on the car.  We do work on the spring by winding up the lever arm, but the spring snapping the trap shut is simply due to the energy we gave it in winding it.  We cannot calculate the potential or kinetic energy of the car because it's energy does not come from motion.  The spring did not exert a force on the car because it was part of the car, creating a system.

REFLECTION
Our final design was actually very close to our original design:

One change we made was rather than suspending the mousetrap inside the frame, we connected it to the frame.  This was because our frame didn't allow the space to suspend the mousetrap because of the small width of our wheel axels.  Another aspect we were unable to do was the cork on the back axel of the car.  We were unable to do this because there was too little friction between the cork and the axel, and it was too slippery.  It was much more effective to connect the string directly to the axel.One aspect we didn't incllude in our plan but was extremely vital to the sucess of our car was adding a lever-arm, which we did last in construction.  One major difficulty we encountered was in our choice of string.  We began with thick pink string that was slippery and difficult to work with.  The string got tangled in our wheels, and sometimes kept the car from moving at all.  After we switched to the yellow fishing wire, we kept in mind how important it was to CAREFULLY wrap the string around the axel so it wouldn't get tangled and cause the car to stop.

If I were to do this project again, I would have been more careful in aligning the holes on both sides of the planks for where the axels go through because one was higher than the other, and this may have caused the car to be unsteady.  Also, I would have found a better way to add friction to the wheels because the rubber bands slipped off almost every test run.  Perhaps we could have secured them better.