Mousetrap-Powered Car

Can you take the energy from a mousetrap and use it to power a car? Try it out!

This project results in a very simple mousetrap car. It probably won't go very far or fast, but it'll help you learn the basics of how this type of car works. Then you can try your hand at building a more complex car. See some ideas at the end of the project.

Note: Mousetraps are dangerous! If one snaps back on your hand it could break a finger. This project requires adult permission and supervision.

Materials:

• Wooden snap-back mousetrap
• Duct tape
• 4 eye hooks
• Wooden dowel that fits inside the eye hooks
• Heavy cardboard
• Large and small rubber bands
• Foam board (usually found at a craft store)
• String
• Ruler or straight edge
• Utility knife
• Pliers

What to do:

1. Have an adult help you use a utility knife to cut four wheels out of a piece of foam board or corrugated cardboard. Make the back wheels about double the diameter of the front wheels. (Use a compass to draw the circles, or trace around a bowl or cup.)
2. Give your wheels some traction by stretching large rubber bands around each wheel.  For the small wheels you could also try using a section of a balloon.
3. If there are metal or plastic teeth on the mousetrap, remove them carefully using a pair of pliers. Also remove the rod that is used to set the trap.
4. Start building the base, or chassis, of the car by cutting a piece of strong cardboard so that it is slightly bigger (about 1/2") than the mousetrap on every side.  Use duct tape to attach the mousetrap to the base. Don't cover up the spring in the middle of the trap or the "snapper arm." 
5. Screw the eye hooks onto the bottom of the cardboard chassis, one in each corner. Use a ruler to make sure that the eye hooks are aligned with each other.
6. Cut the wooden dowel so you have two pieces that are both about two inches longer than the width of the chassis you have made.  These will serve as your axles that rotate the wheels.  Stick the dowels through the eye loops.  Make sure that the axles are straight and that there is room for them to spin in the eye hooks.
7. Cut holes a little bit smaller than the dowel through the center of each wheel, then attach the wheels to the base.  Put the large wheels on the back of the car, opposite the snapper arm.  Wrap a small rubber band around the axle on either side of each wheel so the wheels can't fall off.
8. Tie a string very tightly to the snapper arm on the mouse trap.  The string should be long enough to just reach to the back axle.
9. You may need someone to help you with this last step.  Carefully pull back the snapper arm until it reaches the other end of the trap.  Hold it in place while your helper wraps the string tightly around one side of the axle.  Holding the string tightly, set the car on the ground and carefully let go of the trap - the string should be wound tight enough that it holds the trap in place. Next, make sure everyone's hands are out of the way and then let go of the string. The trap will snap forward, propelling your car. 

What's happening?

A set mousetrap is full of potential energy which, when released, is converted to kinetic (motion) energy. The design of your car allowed that energy to be transferred to the axle to make the wheels turn. When the trap snapped closed, it yanked the string forward. As the string was pulled, friction between it and the axle caused the axle to rotate, spinning the wheels and moving the car forward.

There are many different ways to build a mousetrap car. Your simple model moves forward a few feet, but how could you design it to go longer distances? Or how could you design it to go faster? Here are some things to think about:

• wheel-to-axle ratio. For distance cars, larger wheels are best. Every time your axle turns one time, so do your wheels - if the wheels have a much larger diameter than the axle, the car will go further on each turn of the axle than it would if the wheels were smaller. It takes more force to accelerate a car with a large wheel-to-axle ratio, so smaller wheels will work better if you want your car to be fast.
• inertia. Newton's first law of motion states that objects at rest tend to stay at rest, and objects in motion tend to stay in motion unless acted on by an external force. Inertia is the tendency to resist changes in motion, and the more inertia something has, the more force will be necessary to change its state of motion. If your mousetrap car is very heavy, it will require greater force to get it moving. To avoid too much inertia, think about how you can build a lighter car.
• rate of energy release. If the energy from the mousetrap is released quickly, your car will accelerate quickly and run faster. However, it will also run out of energy sooner. If the energy from the mousetrap is released slowly, the car will move slower, but be powered for a longer distance. One way to try making the energy release slower is to lengthen the lever arm by attaching something (pencil, dowel, etc.) to the snapper arm and tying the string to the end of that. (This will give you a longer piece of string than the one tied directly to the snapper arm.)
• friction. Analyze all the points of friction on your car, where two substances rubbing together can slow the car down or bring it to a stop. Think especially of how to reduce friction between your axle and the eye hooks attaching them to the body of the car. Some friction is good, however - the friction that enables the wheels to grip the floor is called traction, and without it the force of the trap may make your wheels "spin out" instead of propelling the car forward. The above procedure used rubber bands to provide traction; can you think of a better way?

Other ideas for improving the car:

• make it more durable by using lightweight wood such as balsa or basswood instead of cardboard.
• use CDs or records as the wheels.
• glue a small hook to the axle and connect the string to it with a small loop, then wrap the string by turning the wheels in reverse.

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