Mouse-Trap Car Work What is a

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What is a Mouse-Trap
Car and How does it Work?
A mouse-trap car is a vehicle that is powered by the energy that can be stored in a
wound up mouse-trap spring. The most basic design is as follows: a string is
attached to a mouse-trap’s lever arm and then the string is wound around a drive
axle causing the mouse-trap’s spring to be under tension. Once the mouse-trap’s
arm is released, the tension of the mouse-trap’s arm pulls the string off the drive
axle causing the drive axle and the wheels to rotate, propelling the vehicle. This
most basic design can propel a vehicle several meters for any first-time builder.
But in order to build vehicles that can travel over 100 meters or extreme
speed cars that can travel 5 meters in less than a second, you
must learn about some of the different variables that affect the performance of a
mouse-trap car. For example, how does friction affect the overall distance that a
vehicle can travel? How does the length of the mouse-trap’s lever arm affect the
performance? By reading each section of this book you will learn about many of
the different variables that will affect a vehicle’s performance. Also you will
learn how to modify different
variable in order to
build a top
performing
vehicle.
GettingGet
Started
Started!
1
Surface Friction
A ball rolling across the floor will eventually
slows to a stop. The reason the ball slows to a
stop is because of
friction .
Friction is a force that always
opposes motion in a direction that
is opposite to the motion of the
object. An object that slides to the right experiences friction towards the left. If it
was not for friction, the ball would roll forever, as long as there was nothing—like
a wall—to stop its motion. Your mouse-trap car is affected by friction in the same
way as the rolling ball, friction will slow it to a stop. Friction will occur anytime
two surfaces slip, slide, or move against one another. There are two basic types of
friction—surface
friction and
fluid friction. In some situations
12
fluid friction is called air resistance. A
ball falling through the air is
affected by fluid friction and a
block sliding on a table is mainly
affected by surface friction as well
as a little air resistance. The
greater the amount of friction between two
surfaces, the larger the force that will be
required to keep an object moving. In order
to overcome friction, a constant force is needed.
In order to maintain a constant force, there must
be a supply of energy. A ball which is given an
initial push will roll until all its energy is consumed by
friction, at which point it will roll to a stop. The smaller the forces
Friction
of friction acting against a moving object (like a ball or mouse-trap car), the
farther it will travel on its available energy supply. Eliminating
all forms of friction is the key to success
no matter what type of vehicle you are
building.
Surface friction occurs between
any two surfaces that touch or rub against one
another. The cause of surface friction is mutual
contact of irregularities between the touching surfaces. The irregularities act as
obstructions to motion. Even surfaces that appear to be very smooth are irregular
when viewed microscopically. Luckily, during motion surface friction is
unaffected by the relative speed of an object; even though the speed of an
object may increase, the force of surface friction will remain constant. This means
that the same force is required to slide an object at a slow or fast rate of speed on
a given surface. The amount of friction acting between two surfaces depends
on the kinds of material from which the two surfaces are made and how
hard the surfaces are pressed
together . Ice is more
slippery than concrete;
therefore, ice has less friction
or less resistance to slippage.
A heavier brick is harder to
push and has more friction than a
lighter brick only because the
heavier brick pushes into the
ground with more force or weight.
Friction
13
Minimizing surface friction on a
mouse-trap car allows its wheels to spin
with less resistance, resulting in a car that
travels faster, farther and wastes less
energy. The most common area
where surface friction will occur is
between the axle and the chassis.
14
The interface between the axle and the chassis is called the bearing.
A plain bearing can be as simple as an axle
turning in a drilled hole. A bushing is a smooth
sleeve placed in a hole that gives the axle a smother
rubbing surface, which means less surface friction.
Some combinations of material should not be used
because they do not help the cause; for example,
avoid using aluminum as the axle or a bearing sleeve.
A ball bearing is a set of balls in the hole which
is arranged so that the axle rolls on the balls
instead of sliding in a sleeve. A r o l l i n g
ball has very little
friction; therefore, ball
bearings usually provide the
best performance. Ball bearings have the least friction, but
they are the most expensive, so you must evaluate your budget
when thinking about ball bearings.
You can buy small ball bearings at
a local hobby store that deals with remotecontrolled vehicles.
Friction
Changing an Axle Size
Friction
17
Construction Tip
Making A Bushing
18
Friction
Axle Size
and Friction
20
For an object spinning against another
object, like an axle turning inside the
frame of a mouse-trap car, the surface
friction is complicated by torque,
which will be discussed in another part
of this book. A thicker diameter axle or
bearing will stop turning sooner than a
comparable thinner axle or smaller
bearing. The reason is as follows: The
force of friction is the same around the
surface of a large and small diameter axle
but the location of where the force of
friction is being applied from the center of
the rotating axle is not the same. With a larger
diameter axle, the frictional force is applied further from
the center of rotation than with the smaller diameter axle. Torque is a combination
of an applied force and the location of that force from the point of rotation. The
further from the point of rotation, the greater the torque becomes for the same size
force. Smaller diameter axles and bearings translate into
less stopping torque and better performance. A pressure
bearing or even a magnetic bearing where only
the sharpened tip of the axle touches the
bearing is the best solution to beating
friction.
Friction
Construction Tip
Thrust Washers
Thrust washers can be used to eliminate the rubbing friction of a
wheel touching the frame. If a wheel has a side-to-side movement and touches the
frame, a metal washer can be used to prevent the wheel from directly touching the
frame, which will causing poor performance of your vehicle In these pictures, a
rubber stopper is placed on the axle to help eliminate the side-to-side movement
and then a metal washer is placed between the frame and the stopper.
thrust bearing
rubber stopper
Experiment
Try an experiment to learn
about a thrust bearing .
Place a book on the table and
give it a spin. The book should
spin slowly and then stop
quickly. Now place a coin under the book and give it a spin
again. The book should spin
for a considerably longer time
before stopping.
Friction
25
Number of Wheels
The number of wheels your vehicle has does not have an effect on the
overall friction. Vehicles with three wheels will often have fewer friction points
than vehicles with four wheels. However, the distribution of weight for
a three wheeled vehicle is spread out over three wheels rather than four; therefore,
there is more pressure on the turning points and an increase in friction for each
point. The decease in the number of friction points is offset
by the increase in pressure. This is what we call “a
wash.” You get no change! No matter the number of
wheels, the total amount of friction should be the same.
The advantage of using fewer wheels is better wheel
alignment. With three-wheeled cars, it will be
easier to adjust the steering of the vehicle because there are fewer wheels to be aligned.
32
Steering Alignment
Building a vehicle that travels straight can be somewhat challenging, especially when building a long traveling distance vehicle where any small misalignment in the
wheels is exaggerated over a long distance. If your car
does not travel straight, energy is being wasted. Vehicles
turn because of one reason: “THE WHEELS ARE NOT
POINTING IN THE SAME DIRECTION!” What does
wheel alignment have to do with friction? Vehicles turn
because wheels use friction to push a vehicle into a turn.
For example, if a wheel is turned so that it points to the
left, the ground actually pushes the car in this direction. If
the frictional force is not large enough to overcome the
momentum of the vehicle (like a car traveling fast
Friction
Construction Tip
on ice), the vehicle does not make the turn. When building your vehicle, be aware of your wheel alignment. If
your car turns you have to change the alignment of the
wheels in the direction opposite the turn. Sometimes on a big-wheeled vehicle the
string tension can pull an axle out of alignment, so always test your car
under operating conditions. Do not just push your vehicle to see if it travels straight.
Another Idea to Get You Going Straight
Friction
33
Wheel Size
You should consider the
diameter of the wheels in the
construction of your distance vehicle.
The number of turns a wheel makes has
an effect on the energy consumed by a
wheel. Large diameter wheels will
make fewer turns over the same
distance compared to smaller diameter
wheels, thus reducing the total amount
of energy needed to overcome friction.
Larger wheels do not decrease the
amount of friction, only how far the
friction acts. Energy, or work, is
equal to the force times the distance
over which the force is applied. If the
force of friction is applied over a
smaller turning distance compared to
vehicle travel distance, less energy will
be wasted due to friction, which leaves
more energy to move the car.
34
With distance vehicles, you want the smallest possible
force acting over the longest possible distance;
therefore, you want to build a car with a small axle to a
large wheel ratio. A small axle with a large wheel will
cover more distance per each turn of the axle when
compared to a smaller wheel size with the same axle.
Friction
String
The mouse trap only exerts a
force where you direct it to exert a
force. String is often used to transfer
a force from one point to another.
Choosing the proper string is
critical. If you use a string that
48
cannot handle the pulling force, it
will snap as you release the mouse
trap. If you use a string that is too
thick, it will not wind around the
drive axle smoothly, causing the
pulling force to be inconsistent. DO
NOT USE THREAD. Use Kevlar®braided fishing string, not the plastictype fishing string. Kevlar®-braided
fishing string is the best string to use
because it does not stretch and can
wind around a small axle without
tangling. Kevlar®-braided fishing
line is extremely strong and thin so
that it will not break or get tangled
while unwinding from the axle. The
string must not be tied to the axle. If
the string is tied to the axle, it will
begin to rewind itself and will cause
your car to come to a sudden stop.
String slippage during the
acceleration of your vehicle can be
eliminated by a hook-like device
attached to the car’s axle on which a
looped string or hook can be
attached.
Use NO MORE string than
the length required to
reach the pulling axle
when the trap is in the relaxed position.
Inertia, Force, Acceleration
Construction Tip
Inertia, Force, Acceleration
Making the Perfect Loop Knot
wn
49
Construction Tip
Making an Axle Hook
50
In order to achieve maximum performance from your mouse-trap car, it is important that the
pulling string does not slip off the drive axle prematurely. A release hook can be constructed
to allow the string to remain connected to the axle during the pulling phase and then release
once the pulling has stopped. There are two different ways presented in this book to make a
good axle hook. The first method is to drill a small hole into the drive axle and then attach a
small splint to act as a hook. The second method is to wrap a small piece of wire around the
drive axle, twist and glue it in place, and then trim the wire. In either example, if the hook is too
long, it may snag the string and not properly release it from the drive axle.
Inertia, Force, Acceleration
Construction Tip
r
e
h
t
o
An Making an Axle Hook
51
Inertia, Force, Acceleration
Momentum
With a distance car, you want to
build the lightest possible vehicle,
although heavy cars can perform well
due to their momentum. That is, more
massive cars, once moving, will have a
greater resistance to change in their
motion when
52
your mouse trap. This usually means
that the car will not be pulled as far as
slower vehicles with longer lever arms.
With momentum you are counting on
coasting distance, not pulling distance.
The down side is that
heavier vehicles will
have to travel faster
and will therefore
experience
greater
air
resistance. This consumes
compared to more energy and ultimately the vehicle
lighter cars. Massive vehicles will tend will not travel as far as a slow moving,
to maintain their motion, coasting farther energy-conserving vehicle.
than a lighter vehicle (given the same
force of friction acting against the
vehicle). This is because massive cars
can take advantage of what is called
momentum, which means inertia
in motion. This sounds like a good idea
for building a distance car, but there is a
down side to this idea. You have to first
get this more massive car moving in
order to take advantage of its
momentum. Heavy cars will be harder
to start because of their inertia, so the
pulling force will be larger using more
energy at the start to accelerate. In order
to increase the pulling force you
will have to use a shorter lever arm on
Momentum
Construction Tip
Trap Placement
112
A perpendicular push or pull provides the greatest amount of rotation for the
least amount of effort; for this reason, it is important that the drive axle and lever
arm are correctly positioned for the start. When the string is fully wound around
the drive axle, the position where the string is tied to the lever arm should be
directly above the drive axle. At this point the string will be pulled perpendicularly
from the lever arm.
Torque
Construction Tip
Changing the Length of the Lever Arm
113
The length of the mouse trap’s arm can be extended
using a metal rod; this will decrease the pulling
force from the spring. When replacing the mousetrap’s arm with a longer arm, make sure the replacing
arm is strong enough not to bend when under a load.
Choose a rigid material that does not bend as easily
(e.g., copper or brass rods).
Torque
Construction Tip
Attaching the Trap
Key Idea
114
For attaching your mousetrap
to your vehicle I recommend
using DU-BRO 2-56 Blind Nuts
(Cat. No. 133) and DU-BRO 2-56
x 3/4” Socket Head Cap Screws
(Cat. No. 311). These can be
purchased from a local hobby or
crafts store.
Common problems with both
Distance and Speed cars
PROBLEM:
Axle slides back and forth causing wheels to rub against the frame of the car,
slowing or stopping the car.
1. Add thrust washer between wheels and the frame for a smoother
rubbing surface with less friction.
124
2. Make spacers out of brass tubing to hold the wheels in place and
limit the side-to-side play of the wheels or axles.
3. If you are using bearings, the axle may be moving side to side.
Carefully glue the axles to the bearings without getting glue in the
bearings.
PROBLEM:
Can’t find an axle to fit the wheels or bearings.
1. Re-size the axle or change the wheel’s hole
size with a spacer.
Technical Help
PROBLEM:
Mouse trap falls quickly but car moves slowly.
1. Drive wheels are not glued to the axle
and the axle is spinning inside the
wheels. Glue wheels to axle.
2. There is no hook on the axle for attaching
the string and/or the string is slipping off the axle without pulling.
The hook may not be glued to the axle and is slipping under the tension
of the string.
125
PROBLEM:
Mouse trap car does not start or moves slowly.
1. Too much friction in the rolling points. Re-do the rolling points or
try ball bearings.
Technical Help
PROBLEM:
Mouse trap car does not start or moves slowly. (Continued)
2. Not enough tension in the pulling string. Move the trap closer to the
drive axle and adjust the string attachment point on the lever arm
accordingly or try to build up the drive axle with tape. Lastly, test
different mouse traps and use the strongest one.
126
PROBLEM:
Mouse trap car does not travel straight.
1. Wheels are not pointing in the same direction.
The solution is to bring the wheels into alignment. If
the vehicle is a three-wheeled vehicle you need to
focus your efforts on the single wheel. Try to align it
with the other two. This is not easy!
2. This only applies to long distance cars that use one
large wheel as the drive wheel. If the car travels
straight when you push it without the tension of the
lever arm, but turns once under the pulling tension of the trap, there is
too much frame flex or the trap is pulling too hard on one side of the
axle. Strengthen the frame or prevent the pulling side of the axle from
giving too much at its holding points.
Technical Help
PROBLEM:
Mouse-trap car suddenly stops or slows quickly.
1. The string is not releasing from the drive axle because it is either
glued to the axle or it is too long or the hook is too long and
caching the string causing it to rewind.
Cut string a little bit
shorter than what
is needed in order
to insure a proper
release.
2. There is too much friction at the rolling points. Re-work the bearings
to reduce friction or try ball bearings. Try a small diameter axle.
3. Wheels are rubbing on the frame. Prevent wheels from rubbing
with spacers or thrust washers.
Technical Help
127

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