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Mousetrap Physics
 In this issue:
- Mousetrap Physics
- Mousetrap Marshmallow Catapult
- Mousetrap-Powered Car
- Science Links
Mousetrap Physics
A simple snap-back mousetrap is an ingenious machine. With just a few parts
(a wooden base, a spring, a metal bar, and a trigger mechanism) it can do its
job quickly and efficiently. It is so simple and functional that it has created
a cliche: "to build a better mousetrap" means to improve on the best, or reach
the heights of achievement. But it isn't just a figure of speech; people
continue trying to build a better mousetrap. There are already 4400 mousetrap patents
issued by the Patent Office, and 400 people apply for new patents every year!
But only a couple dozen of those thousands of mousetrap designs have ever made
money, and the simple snap-back is still selling strong more than a hundred
years after it was patented in 1899. Not only can this machine get rid of mice,
it can teach us a lot about physics, too!
When a mousetrap is set, the spring in the center is compressed, becoming a
source full of potential energy. This energy is being stored, not used,
but as soon as the trap is released, it is converted to kinetic energy
(the energy of motion) that propels the snapper arm forward.
 A mousetrap makes use of a simple machine called a lever. There are
three different classes of levers. A first-class lever is like a
teeter-totter at the park. The pivot point is called the fulcrum, the person being
lifted is the load, and the person on the other end is the effort
force. A lever makes doing work easier. You can lift someone with a teeter
totter much easier than if you tried to pick them up! Second- and third-class
levers have different arrangements of the components of a lever. The fulcrum, or
pivot point, is at one end, instead of in the middle. In a second-class lever
the effort force is at the other end, with the load in the middle. In a
third-class lever, the load is at the end and the effort force is between the
fulcrum and the load.
When you set the mousetrap, you are using a second-class lever. The load is
the arm of the spring that is being pushed down to compress the spring. The
effort force is your fingers on the end of the snapper arm, and the fulcrum is
the pivot point in the middle of the trap. When the mousetrap is released,
however, it acts as a third-class lever. The snapper arm becomes the load, and
the spring arm becomes the effort force moving the load.
Of course, all that potential energy can be put to other uses besides getting rid of
mice. In the projects below, you'll use a mousetrap as a power source for a
catapult and a car!
Mousetrap Marshmallow Catapult
Get ready to launch marshmallows across the room with the power of a
mousetrap! Print out our
Marshmallow
Catapult instruction sheet with step-by-step pictures.
Note: Mousetraps are dangerous! If one snaps back on your hand it could break
a finger. This project requires adult permission and supervision.
Materials:
What to do:
 - With a pair of pliers, carefully remove any metal teeth or bait
platforms from the trap. Also take out any staples that are not connected to
the spring.
- Carefully pull back the snapper arm until it reaches the other end of
the trap, and hold it down firmly. Have a helper wrap the
strong rubber band
around the end of the snapper until it holds it down to the base of the
trap, preventing the trap from springing.
- Use a loop of duct tape to attach one of the erasers to the base of the
trap so its long side is right next to the fulcrum (the spring in the middle of the
trap).
- Tape the second eraser on top of the first one, letting it hang over the
fulcrum slightly. Secure both erasers to the base with duct tape, then carefully remove the rubber band and slowly let the snapper arm move up until it rests against the erasers.
- Tape one of the tongue depressors horizontally along the top of the
snapper arm. Place the second tongue depressor perpendicular over the first
and tape it so it extends vertically above the snapper arm.
- Tape the spoon to the second stick. Make sure that the arm of the
catapult will hold. You may need to reinforce it with more duct tape.
- To shoot the catapult, take any small soft object, such as a marshmallow,
and then pull back the arm, put the object in the spoon, and let go!
Always be sure to hold down the base of the catapult when you release the
arm so the structure doesn't topple over.
What's Happening?
Newton's first law of motion states that objects in motion tend to remain in
motion, unless acted on by an outside force. When you released the catapult,
both the lever arm and the "ammunition" moved forward with energy from the
spring. When the lever arm hit the erasers, it came to a sudden stop. The
marshmallow, however, remained in motion until it hit something else or until
the force of gravity overcame its motion and brought it to the ground. The same
principle applies to driving in a car - both you and the car are moving
together, but if the car comes to a sudden stop (as in a collision), your body
will keep moving forward. This is why you should always wear a seatbelt!
This project is adapted from
BOAST
hands-on science lessons.
Make a 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:
- 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.)
- 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.
- 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.
- 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."
- 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.
- 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.
- 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.
- 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.
- 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.
More Projects:
Science Links
Build fantastic contraptions with this fun
online physics puzzle game!
Check out videos and photos of many different mousetrap cars at the
mousetrap car challenge page.
Visit the
Inventor's Workshop to learn "gadget anatomy" and see some of Leonardo da
Vinci's "mysterious machines."
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