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Introduction
The building of mousetrap cars allows you to experience the engineering
process first hand. While designing and building your car, you need to
turn your ideas into a working machine. Building the mousetrap car can be
a challenging project for two reasons:
- The problems are discovered as you build your car. Each person will
face different problems, and will solve those problems in different
ways.
- There is not only one correct answer. The variables involved in the
construction of a mousetrap car provide endless possibilities to the
design of a working car.
During the construction of your car, you will have to make some
compromises between performance and efficiency. The goal of the car you
decide to build, whether speed or distance, will help you make some design
decisions. However, the only way to determine your best design is
experimentation. Experiment often, and don’t be afraid to make
mistakes. The best working cars will come from a design that, at one time,
might not have worked. The process of designing your car is a process that
requires testing and re-testing to determine the best performance. A
combination of unique ideas and the application of physics concepts will
lead to a winning design. Do not be afraid to try your ideas.
Quick hints for better performance
- Reduce friction
- Use lightweight materials
- Amount of rotational inertia depends on the goal of the car.
- Test the lever arm length and wheel size to determine the best for
your type of car.
- Experiment early and often
Physics concepts involved in designing the cars
Friction
Friction is the resistance of motion between two objects. Most friction
between the materials in your car reduce the amount of energy that is used
to move the car, so it would make sense that you want to reduce that
friction. However, you actually rely on the friction between your wheels
and the floor to help your car move.
Another, less thought of, friction involved in the performance of your
car is air resistance. Remember, air resistance acts against the motion of
the car and therefore should be reduced to increase performance.
Newton’s Laws
Newton’s first law is also known as the law of inertia. Inertia is
an object’s tendency to resist change. The more massive an object is,
the larger its inertia, and therefore the harder it is to change its
motion. Newton’s second law is most commonly known in its equation
form, F=ma. Both of these laws should show you that the more massive your
car, the more force that will be required to move the car. Remember that,
depending on the goal of the car, you may want the car to have some
inertia so it will keep moving after the mouse trap has used all it
energy. Keep these laws in mind while constructing your car.
Rotational Inertia
Just like linear inertia deals with an object’s tendency to resist
change of its linear motion, rotational inertia is an object’s
tendency to change its rotational motion. The rotational inertia of an
object depends on its mass and the distribution of that mass. Since the
wheels of your car are the parts that are rotating, you may want to
decrease or increase their rotational inertia. To help with this, here are
some of the more common rotational inertia equations.
| Disk |
Thin Ring |
Thick Ring |
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Energy
Energy can be defined as the ability to do work. Work is the applying a
force to an object, and actually moving that object. If the object is not
moved, no work is done. The goal of the project relies on efficiently
transferring the spring’s energy to the car’s wheels. The
concepts involved in the transfer of this energy rely on an understanding
of simple machines. The trap acts as a lever to transfer the energy to the
axle. The axle acts on the wheel as a second transfer of the energy.
Finally, the energy is used to push the wheel against the floor, causing
the car to move. Any friction or resistance in the transfer of energy,
leads to a decrease in the efficiency of the machine and decreased
performance. Keep the use of simple machines in mind as you design the
car, the correct use of these machines will increase your performance.
Information on this page has been reprinted with permission from
Mouse -Trap Cars: The Secrets to Success, written by Al
Balmer.
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