Mousetrap Cars
Unit 11
What is a mousetrap car?
A mousetrap-powered racer is a vehicle
that is powered by the energy of a
wound-up mousetrap’s spring.
A mousetrap car’s basic design implies
many of the basic laws of physics.
How does it work?
The most basic design is to tie one end of a string to
the tip of a mousetrap’s snapper arm and then the
other end of the string has a loop that is designed
to “catch” a hook that is glued to a drive axle.
Once the loop is placed over the axle hook, the
string is wound around the drive axle by turning
the wheels in the opposite direction to the
vehicle’s intended motion. As the string is wound
around the axle by the turning of the wheels, the
snapper’s lever arm is pulled closer to the drive
axle causing the mousetrap’s spring to “wind up”
and store energy. When the drive wheels are
released, the string is pulled off the drive axle by
the mousetrap causing the wheels to rotate.
Basically, for a mouse traps to be
effective, it must store a sufficient
amount of potential energy which
should be translated efficiently to
kinetic energy. This is important
because the kinetic energy must
produce enough torque to create a
rotational inertia that will move the
wheel and axle of the vehicle.
All mousetrap cars follow the same
basic principles regardless of their
design. Whether designing a car built
for speed or one made to travel long
distances, the following physics
concepts will be used:
Energy
Energy is what moves your vehicle. The
energy of the mousetrap car is originally
stored in the potential energy of the
wound-up mouse trap’s spring. The
spring releases its energy and the
potential energy is changed into kinetic
energy of motion. Along the way energy
is lost to the surroundings in the form of
work (heat and sound).
Power output
Power output is how quickly the energy stored in the
mouse trap is released. There are really only two
approaches to consider when building a vehicle:
 Build a fast moving car that releases its energy
quickly and then coasts as far as possible.
 Build a slow moving car that releases its energy
slowly over the entire pulling distance.
Inertia
Inertia is the resistance that an object has
to a change in its state of motion. The
more inertia an object has, the more
force that will be required to change its
state of motion. In theory, a heavy car will
require more pulling force than a lighter
car for equal acceleration. Lighter cars
will be easier to accelerate but ideally will
have less coasting distance than a heavy
car at the same speed.
Rotational inertia
Rotational inertia is the resistance that a
wheel has to changing its state of motion,
similar to inertia but dealing with a
rotating object. The less rotational inertia
an object has, the less the torque that will
be needed to change its state of rotation
or the easier it will be to accelerate.
Friction
 Surface friction is caused by the rubbing of two
surfaces in contact with one another. Where
your axle connects to the frame of your vehicle
is one place you will find surface friction.
 Traction is a wanted surface friction that is
between your car wheels and the floor.
Increasing your traction will allow for greater
accelerations because it will take more torque
to make the wheels spin out or break loose.
 Fluid friction is caused by an object trying to
move the air out of the way as it is moving.
Torque
Torque depends on the length of your cars
lever arm and the strength of the
mousetrap’s spring. A long lever arm has
the same torque as a shorter arm. The
difference between a long arm and a
short arm is that you get more pulling
force with a short arm than a long arm.
As you begin building your mousetrap
car, you will need to make adjustments
along the way to better the performance
of your car. Here are some adjustments to
consider:
Extension of the lever arm
By extending the lever arm, the force required to
turn the drive axle decreases. By doing so, the
stored mechanical energy in the snapper’s lever is
conserved while constantly turning rotating the
drive axle. This adjustment would guarantee your
car to travel longer distances because no energy
is wasted in moving the vehicle.
Inversely, if the lever arm is short, the speed of the
car will be faster. Remember that a shorter lever
arm will require a greater amount of force, and
speed is relative to the amount of force applied.
Increasing the drive
wheel’s diameter
A drive wheel with a bigger drive wheel will result to
traveling farther. The reason for this is that a wheel
with a bigger diameter will cover a longer distance
before it could make a complete turn. But there is a
downside to this: your vehicle will travel slower.
A bigger tire will require more force for it to start
moving from rest, more power to accelerate but at
the same time it will also take more force for it to
stop as compared to a smaller one.
Inversely, a smaller drive wheel diameter will produce
greater speeds.
Increasing the diameter of
the drive axle
Increasing the diameter of the drive axle
will variably increase the speed of the
car. The increase in the axle’s diameter
will increase the torque applied by the
same amount of force. In effect, you will
increase the power used to turn the
wheel and since you will increase the
power used to turn the wheel, and since
speed is correlated to power, this
adjustment will generate higher speed.
Reducing the mouse trap
car’s weight
By reducing the weight of the car, you are
efficiently converting the potential energy of
the snapper’s spring into kinetic energy. A
lighter vehicle will have less inertia and
therefore will require less force to
accelerate it. This also implies that since it
will only require less force to accelerate it,
the time required for the vehicle to reach its
maximum speed will also be shorter.
Using thinner and lighter
drive wheels
This will increase the maximum distance
that your vehicle could travel and
increase the acceleration of the vehicle. A
lighter set of wheels will mean that the
rotational inertia of the particular part is
also less. By using a thinner and less
massive drive wheel, less energy is
displaced for the rotational inertia and
more energy is displaced in the forward
movement of the whole car.
Reduce friction
Friction is another type of force that is acting on
your car, particularly the parts of the car. This
will affect both the speed and distance output.
If friction is allowed to act on the vehicle, the
kinetic energy that moves the car will be turned
into heat energy because of the chemical
reaction between the contacting surfaces. This
transformation of energy to heat will consume
the energy that could be used for the desired
motion of the vehicle.
Increased traction
Increased traction will require an increase in
surface area that will be subject to static friction.
Static friction is the type of force that prevents two
bodies to slide when in contact. This means that
the rotational force coming from the spinning
drive wheel requires traction so that it would be
able to apply a pushing force on the ground to
make the vehicle move forward. Without traction,
the wheels of the car would slip and cause an
energy expenditure without any work output.