4 Steps MOUSETRAP CAR (1/240 Mechanical Advantage)

The mousetrap car project is an awesome way for young students to learn how to tweak different variables in order to enhance the performance of a vehicle design.

We will learn how to build a mouse trap car that can go as far as possible in 4 easy steps, and how to calculate the mechanical advantage of your design.

Sorry in advance if I make some mistakes in my explanations, I write you this tutorial from France ^^

If you liked it, you can vote for it in the "simple machines" contest from March 30 :)

Supplies

What you need :

-A mouse trap obviously ^^
-Leight wood or rigid cardboard for the car body
-Skewers or metal rods for the wheels axles and the lever arm
-Straws or small plastic pipe to guide the wheels axles
-CDs,bottle caps and tape for the wheels
-Zip ties
-Thin rope
-Tools : Glue gun, drill, scissors, cutter (or saw)

Step 1: Lever Arm - Car Body

First and foremost, you will attach the metal rod (or skewer) to the mouse trap with zip ties.

This will set the size of the car so that the lever arm ends at one side of the car body as shown in the pictures.

You can then cut the wood (or cardboard) regarding the dimensions of your mousetrap. You must cut a hole at the end of the lever arm because it is where the rope will be rolled.

You can finally glue the mousetrap on the car body with the hot glue gun.

Step 2: Axles

Cut the metal rods (or skewers) so that they are larger than the car body width in order to make the two wheels axles.

Cut 4 small pieces of straws (or small plastic pipe) in order to guide the wheels axles and glue them under the car body.

Step 3: Wheels

To make the wheels, we will use CDs and bottle caps.

First drill the 8 caps to the size of the wheels axles. Be carefull that the holes are on the middle of the caps.

Then glue them on each side and on the center of the CD's. Once again, be carefull that the wheels axles are orthogonal to the CDs. To do that, you can put the axle through on glued cap, and align it before gluing the second one.

Once the 4 wheels are done, you can assemble them on the car, and glue the tips of the axles to the wheels. The axles must turn freely in the straws, with the least friction.

Step 4: Mechanism

Finally, you will build the mechanism used to run the car.

To do that, zip tie the axle on the location of the hole. You can also glue it to make sure it will not move.

Then tie the rope on the end of the lever arm. When the trap is not engaged, the lenght of the rope must be a little less than the lenght of the all car. Make a knot as shown in the pictures so that it can be rolled on the zip tie and get away at the end of the car run.

Your car is ready !!

Step 5: Mechanical Advantage - Final Run

Here we are with our awesome mousetrap car ! But why is this design used to build that kind of car ?

In fact, the mousetrap spring stores potential energy, which is converted to kinetic energy when released. But here we used two ways in order to release it gradually, and not in one time because in this case a lot of the energy is lost.

First, the metal rod (skewer) used as a lever arm, is 10 times longer than the mousetrap spring arm, so the mechanical advantage of the lever arm is 1/10.

Secondly, we used a ratio of diameters. The diameter of the the CDs is 24 times the diameter of the axles. So if we used the axles as the wheels, the car would have run 24 times less far than with the CDs wheels. The mechanical advantage of the diameters ratio is 1/24. As you can see on the pictures, I added tape around the CDs to enhance the friction on the ground.

The final mechanical advantage is the multiplication of all the mechanical advantages so 1/240.

It means that 1/240 of the mousetrap spring arm force is applied to the ground with the wheels.

To make your car go further, you can increase the lenght of the lever arm or use bigger wheels. As a consequence, the mechanical advantage will decrease. But if it is to small, the force applied to the ground may be not strong enough to move the car because of its one mass, and friction. There is so a balance between friction, inertia and mechanical advantage ! You can try to tweak these different variables in order to make your car go further or faster.

Enjoy !!

Bonus : You can calculate the maximum distance your car will go regarding the number of rope turns around the axis and the wheels diameter. I let you calculate it ^^

You can also calculate the energy stored in the trap with various methods, here is mine: the lever arm on the spring is able to support 15mL of water (in a bottle), so a mass of 0,15kg. The force applied is 1,5N (multiplication with Earth acceleration) and the lever arm go through a half circle of PI*0,2m~0,628m. The energy stored is the force multiplied with the path of the lever arm end. I finally found ~1J stored in the mousetrap.