## Introduction: Mousetrap Car - Explained

The mousetrap car project is a classic physics challenge: Build a vehicle that can travel as far or as fast as possible by using only the energy that can be stored in a single mousetrap. Seems simple enough, but the reason it's so popular is because it's rich in science concepts, applied math, and design possibilities. For example, you can calculate the mechanical advantage to determine how efficiently the car is utilizing the energy from the mousetrap, or calculate the minimum distance the car will travel!

This Instructable will focus on how to build a mousetrap car that travels as far as possible using only the materials that are shared with all of my other STEM Projects for Kids (plus mousetraps of course).

Read on to the end of the Instructable for a detailed explanation about the science and math concepts that are being applied, how to optimize the mousetrap car to go as far as possible, and downloadable classroom resources including a lesson outline and project sheet.

You can find the lesson plan, 1-page project sheet, and more project ideas at STEM-Inventions.com

## Supplies

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## Step 1: Build the Frame

Hot glue craft sticks together to form a rectangle that's 4 sticks long (picture 1). Make sure to overlap the sticks by at least 0.75" or so to ensure the frame doesn't break under the stress of the loaded mousetrap. Additionally, leave one end of the frame open (shown on the left in picture 1).

Glue four craft cubes onto the open end of the frame with a gap between them (picture 2). This gap is where the drive wheels will be installed, so here's a tip: Put the plastic straw between the cubes as you're gluing them in place to space them perfectly.

## Step 2: Make the Drive Wheels

Cut very short pieces (≈0.5 - 0.75") of straw and put it onto the dowel that's supplied with the large wheel set. Next, fit the large wheels onto the ends of the dowel (picture 1).

If the wheels fit loosely, then wrap one layer of of masking tape around the end of the dowel before putting the wheels on (picture 2).

## Step 3: Install the Drive Wheels

Put a generous amount of hot glue into the gap between the cubes. Wait a moment for the glue to begin to cool, otherwise the heat may melt or deform the straw pieces, which will generate lots of unnecessary friction. Once the glue is tacky, set the small straw pieces of the drive wheels into the gap (picture 1).

• Note: the large wheels were removed from this picture for visual clarity, but it's not necessary.

Flip the frame over (picture 2). This helps prevents the drive wheels from slipping out of the gap over time.

Tie a cable tie to the drive wheel dowel, off-center and near the frame (this will be explained when winding up the string). Pull it as tight as possible, then cut off the excess (picture 3). This is where the string will be attached to the drive wheels. If the cable tie is slipping, then apply a small dot of glue onto it.

## Step 4: Make the Idle Wheels

The idle wheels need to be as lightweight and frictionless as possible, so small wheels will be used.

Put a straw piece that's about 5" onto the 1/8" dowel. Fit two small wheels onto the ends of the dowel; wrap the ends of the dowel in tape if the wheels are slipping off. Cut off any excess dowel (picture 1).

Tape the wheels to the other short side of the frame (picture 2). Make sure these wheels are parallel to the drive wheel dowel.

The car frame and wheels are complete (picture 3)! Time to add some mousetrap power.

## Step 5: Attach the Mousetrap

Optional: Use pliers to pull out the latch and trigger from the mousetrap (picture 1). This isn't totally necessary, but it will make the next steps a little easier and may add a small amount of efficiency to the car's performance.

Glue the mousetrap to the frame near the small wheels. Make sure the closed arm is facing toward the small wheels as shown.

## Step 6: Extend the Mousetrap Arm

Mousetraps are not as forceful and dangerous as you might think, especially when the arm is already lowered (at least this is true of the Victor brand trap linked in the supplies). They only pose a significant hazard when the arm is pulled back and released quickly, so don't be afraid to get in there and start building!

Lift the mousetrap arm and position two half-craft sticks under it as shown (picture 1).

Glue two craft sticks perpendicular to the half-sticks, then apply a generous layer of hot glue over all the sticks and the metal mousetrap arm (picture 2).

Glue two more half-sticks on top of the arm as shown (picture 3). This sandwiching technique ensures that the mousetrap car arm has a solid foundation and won't bend or break during use.

Extend the arm so its three craft sticks long and two craft sticks wide (picture 3). Like the frame, make sure to overlap the sticks by at least 0.75" or so (picture 4).

Lastly, adjust the length of the arm: add more sticks or cut it shorter until the very end of the arm touches the drive wheel axle. If the arm is shorter or longer than that, then the mousetrap car won't work as efficiently (see the step The Science and Math of the Mousetrap Car for more info).

## Step 7: Attach the String

Pull the arm back toward the drive wheels. Apply hot glue to the end and lay the end of the string onto it (picture 1).

Wrap tape around the string and the arm (picture 2). The combination of hot glue, tape, and attaching the string to the underside of the arm will prevent the string from coming undone.

Let the mousetrap arm close it points past the front wheels. Unspool the string from the roll and cut it so it's about 2-3" past the drive wheel dowel (picture 3).

Tie the end of the string into a small loop (picture 4).

## Step 8: Wind Up and Go!

Loop the string onto the cable tie from the underside of the dowel (picture 1).

• Starting the string from the underside of the dowel will make the car drive with the large wheels in front. This means the drive wheels will "pull" the car forward instead of "pushing." Pulling the car usually results in a straighter drive.

Carefully wind the string around the dowel by turning the drive wheels. As much as possible, try to wrap just one layer of string around the dowel (picture 1).

• The mousetrap car will be more efficient if the string is wrapped around the dowel as many times as possible. If the string starts to wrap around itself and forms a bundle, the diameter of each wrap will increase, which results in less wraps overall.

To drive, just set the car on a smooth, flat surface, then let go!

## Step 9: The Science and Math of the Mousetrap Car

Now that you've seen how the car is built, here's an overview of the science and math that's behind the car:

The basics

• The mousetrap storespotential energyin the form of the spring.
• That potential energy is converted into kinetic energy in the form of the arm rotating forward.
• The arm pulls on the wound-up string, which turns the drive wheel dowel, which is connected to the wheels, which makes the car drive forward.

• The mousetrap car relies on a form of mechanical advantage to convert the fast burst of energy from the spring into a weaker but longer-lasting output of energy. The mechanical advantage can be calculated as follows:
• The metal spring-arm of the mousetrap is just 2" long and the craft stick mousetrap arm is ≈12" long, so the mechanical advantage of the extended arm is 2/12, or simplified as 1/6.
• This means that the force outputted at the end of the craft stick arm is 6 times weaker than the metal spring-arm, but it also travels 6 times farther.
• Additionally, the drive wheel dowel diameter is 0.25" and the wheel diameter is 4.75", so the mechanical advantage from the dowel to the wheel is 0.25/4.75 = 1/19.
• The total mechanical advantage can be calculated by multiplying the two: 1/6 x 1/19 = 1/114.
• This means that the force output at the outside of the drive wheels is 114 times weaker than the force at the end of the metal spring-arm, but it will also travel 114 times farther!
• 1/114 is a low mechanical advantage, meaning the output force is less than the input force. This is useful for mousetrap cars: a lower mechanical advantage means the energy from the mousetrap is used over a longer period of time so the car can be powered over a longer distance.
• By contrast, a high mechanical advantage would be useful if you wanted to move something heavy, but apply the force over a longer distance.

Calculating minimum distance

• The minimum distance can be calculated as follows:
• The circumference of the dowel axle is π (pi) x 0.25 = 0.785".
• The length of the string is about 25".
• Therefore the string can be wrapped around the dowel 25/0.785 = About 31.8 times.
• The circumference of the wheels is π x 4.75 = 14.92"
• So the minimum distance travelled will be the number of times the string can wrap around the dowel multiplied by the circumference of the wheels, which is 31.8 x 14.92 = 474.456", or about 39.5ft!
• In addition to the minimum distance, the car will continue to travel due to momentum. This extra distance depends on several factors such as the car weight, the surface it's driving on, and the amount of friction at the axles, which are too varied to reliably calculate here.
• When I tested the car at a tennis court, this calculation proved to be fairly accurate. The car traveled the width of a standard tennis court plus a few additional feed under the power of the mousetrap, then coasted for a few more feet for a total travel distance of about 44ft!

Summary of Formulas:

• Mechanical advantage = ([length of metal spring-arm] / [length of extended mousetrap arm]) x ([circumference of axle dowel] / [circumference of drive wheel])
• Minimum travel distance = (String length / circumference of axle dowel) x Circumference of drive wheel

The arm length

• A longer string will result in a lower mechanical advantage. In theory, this would make the drive wheels will turn more times, and the car will drive farther.
• However, longer string also necessitates a longer arm and frame, which adds further weight.
• If the frame is shorter than the string, then part of the string won't be able to wrap around the drive wheels. This defeats the purpose of elongating the string: the total usable length of the string is equal to the diameter of the arm arc, so any string that's not wound around the drive wheels is essentially wasted.
• The added weight generates more friction, which mostly occurs at the point of contact between the straws and the dowels.
• This means that making the string longer is only helpful to a certain point! It's possible to create a car that's too long and heavy.
• Therefore, the sring needs to be long enough to create very low mechanical advantage, but if it gets too long then it'll necessitate a car that's too heavy and slow.

Wheel size

• Large wheels will also theoretically result in greater distance since the a larger circumference will increase the minimum distance travelled
• Remember the formula is (drive wheel circumference) x (number of times the string is wrapped around the dowel).
• However, just like adding a longer arm, larger wheels can be heavier, but this is problematic for a different reason. Larger wheels don't generate friction in the same way as a larger arm; the wheels do not weigh further on the axle.
• Larger wheels still create more friction between the edge of the wheel and the surface it's rolling on. Heavier wheels press into the ground with more force, which increases friction.
• Additionally, large wheels have more inertia, which is the tendency for objects to remain at rest.
• This means it requires more energy to move larger wheels.
• Furthermore, whenever energy is transferred from one source to another, some of it is lost. This means that the larger wheels cannot store that extra energy very efficiently in the form of momentum.

The summary, the science and math concepts behind the mousetrap car manifest as a balance between these principles: lowerfriction and inertia as much as possible and decrease mechanical advantage as much as possible.

## Step 10: Ideas for Maximizing Distance

The example mousetrap car built in this Instructable is a good place to start, but it's not the absolute best design. We can use our understanding of the math and science behind the car to test some ways to optimize its performance.

The best mousetrap car is one that starts by slowly crawling forward, using the smallest amount of energy possible to get moving. This indicates that it has the lowest possible mechanical advantage. As the car moves forward, it begins to build momentum. When the arm reaches the end of its arc, the car has generated enough momentum to continue coasting for some distance. The less friction the car generates, the greater the coasting distance will be.

With that in mind, challenge your students to think about the following categories of improvement:

Weight

• The first and most straightforward optimization is to make the car lighter. A lighter car will generate less friction, which means the car will coast farther using its momentum.
• What materials can be removed and the car will still work?
• Are there lighter materials that could be substituted for the heavier ones?

Friction

• The biggest enemy of a high performing mousetrap car is friction. There are ways to reduce friction without removing weight.
• First, look for any imperfections that might create excessive friction, such as bits of hot glue inside the straws, or the wheels pressed too tightly against the ends of the straws. Cleaning up small things can have a big impact on performance.
• From there, what else could you do to reduce friction?
• How could you modify the design so the side of the wheels doesn't rub against the ends of the straws?
• How can you reduce the amount of contact between the front wheel dowel and the straw?

String Length

• The length of the string (and consequently, the length of the extended arm and frame) plays an important role in determining how many times the drive wheels will rotate under the power of the mousetrap, but making it too long will increase the weight and consequent friction.
• Making the string too long may also create an extremely low mechanical advantage. If the mechanical advantage is too low, then the output force will be too weak to overcome the car's inertia.
• How long can the arm be before it becomes too long?

Drive Wheel Dowel

• Another way of lowering the mechanical advantage is to change the drive wheel dowel thickness. You can experiment with thinner dowels, and still fit them into the wheels by wrapping tape around the ends until it's thick enough to fit inside the 1/4" hole.
• So for example, in theory, replacing the 0.25" (1/4") dowel with one that's just 0.125" (1/8") thick will increase the ratio to 25/(π x 0.125) x (π x 4.75) = about 950", or 80ft.
• However in practice this won't work so well. The thin dowel may bend under the strain of the force of the mousetrap, causing excessive friction or even preventing the car from moving at all. Additionally, this equation assumes that the string will be wrapped in a single even layer over the dowel, but there isn't enough space for that; the string will start to wrap around itself, which increases the diameter of the dowel, and therefore results in a fewer number of wraps.
• What's the ideal dowel thickness?
• What other string-like materials can be used that can be wrapped around the dowel more efficiently?

## Step 11: Lesson Plan & Project Sheet

If you're planning on teaching this project to a group of kids, then download the attached lesson plan and project sheet. Like all of my lesson plans, it contains the project goal, prep, troubleshooting, and a suggested lesson plan. The lesson plan is an outline, and it's provided as an editable .docx file, designed to be elaborated upon to suite your audience. This lesson plan also includes all the details on the math and science behind the car.

To download the Project Sheet, click on the image and then click on the download button in the lower left corner. Or, right click and open the image in a new tab, then right click and save the image. I recommend showing how to build the car step-by-step, and then use the project sheet as a reminder of the steps. Print out one project sheet for every 2 students.

Lastly, this project aligns with the following NGSS:

• MS-ETS1-4 Engineering Design - Develop a model to generate data for iterative testing and modification of a proposed object, tool, or process such that an optimal design can be achieved.
• 4-PS3-4 Energy - Apply scientific ideas to design, test, and refine a device that converts energy from one form to another (stored energy to cause motion).

Thanks for reading this far! If you'd like more science and engineering projects like this, then check out Made for STEAM.

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