Mousetrap Cars

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Mousetrap Cars

• Unit 11

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What is a mousetrap car?

• A mousetrap-powered racer is a vehicle that is
  powered by the energy of a wound-up mousetraps
  spring.
• A mousetrap cars basic design implies many of
  the basic laws of physics.

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How does it work?

• The most basic design is to tie one end of a
  string to the tip of a mousetraps 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 vehicles intended motion. As
  the string is wound around the axle by the
  turning of the wheels, the snappers lever arm is
  pulled closer to the drive axle causing the
  mousetraps 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.

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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.
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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
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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 traps
  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).

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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.

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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.

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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.

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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.

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Torque

• Torque depends on the length of your cars lever
  arm and the strength of the mousetraps 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.

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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
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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 snappers 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.

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Increasing the drive wheels 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.

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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 axles 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.

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Reducing the mouse trap cars weight

• By reducing the weight of the car, you are
  efficiently converting the potential energy of
  the snappers 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.

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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.

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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.

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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.