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Mousetrap Car Won't Move Far

  1. Feb 13, 2011 #1
    Okay, so I'm not really in High School (second year of college), but this is still a High School level problem. Anyway, I have a mousetrap car competition coming up in which we must get the car to move 15 feet and have the ability to calibrate it to move any distance under 15 feet as well. My problem is probably simple, and I don't need help figuring out how to get it to stop.

    My problem is that the Mousetrap Car doesn't seem to have enough energy to move very far. I'll provide pictures below but I believe the problem lies with the friction between the wheels and the carpet because when I lift the car the axle spins quite easily. The lever arm is a single ruler in the pictures, but had intended to increase the length by connecting one or two more. However, the ruler stops pulling the string about when it nears a 90 degree angle and the car ceases to move. When using two rulers, the angle the rulers reach is even less than 90 degrees. And when using three rulers the car does not move at all.

    Now all of this makes sense to me because a longer lever arm would mean that the spring would have to do more work to move the arm. That makes sense. What doesn't make sense is the fact that this thing won't even move with one ruler. I've seen other mousetrap car designs with long chassis' like mine with long lever arms and they move far. Mine hardly moves with a ruler length lever arm. I don't get it. I have pictures of the car at several angles, I would be most appreciative if you could tell me the flaws in the design and my thinking.

    Thank you in advance for the help.

    Note: The front axle is on the TOP of the car and the rear axle is on the BOTTOM. The front axle has one wheel while the rear axle has two. The car is rear wheel drive.

    Mousetrap car rear axle upside down. Axle is made of brass. String wraps around vinyl tubing the is over the center of the brass axle. Hot glue around the ends of the vinyl inhibits the string from moving off the vinyl onto the brass. Note: The hot glue is not connected to the wood and the axle is free to turn. It only touches the wood when the axle moves from side to side but I don't think this is the problem as this occurs when the car is being held and the wheels don't stop in the air.

    The front of the mousetrap car upside up. The front axle is glued to the chassis, so it is fixed. The rubber grommet is firmly attached to the axle. The CD wheel spins around the grommet. There is more friction than I would have liked between the CD and the rubber, but it does spin fairly easily. I don't think this is THE problem, but it is probably a poor arrangement. Note: the mousetrap itself is being held down with rubber bands (they're blue) currently but we intend to glue it on once we can confirm that the car works.

    Full view of the mousetrap car on a bed. It is right side up in the picture.

    Full view of the mousetrap car on a bed. It is upside down in the picture.

    This picture is a bit blurry, but it's a view of the mousetrap car from the front. You can see how the ruler is attached to a piece of wood attached to the mousetrap spring.

    Okay this is the opposite angle of the last picture. This picture is looking at the front of the car from the back. You can see the glue holding the mousetrap spring to the piece of wood that attaches to the ruler on the other side.

    Okay I hope I have given you a good idea of the design of the car. If I need to provide additional information just ask. And once again thanks for the help any and all of it will be greatly appreciated.
  2. jcsd
  3. Feb 13, 2011 #2
    Have you tried using it on a surface other than the carpet? Like a hard flat surface?
  4. Feb 14, 2011 #3

    jack action

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    Check the angle between the string and the ruler throughout the motion. The force will be most effective when the angle is 90°.

    Also, you can play with the diameter of the axle or the one of the rear wheels to adjust the final gear ratio, not just the ruler's length.

    Rubber grommets does not seem to be the best bearing material. Use something else or maybe a lubricant.
  5. Feb 14, 2011 #4
    That's a lot of vehicle for that tiny mousetrap to move.

    At 90 you may found there's not enough energy left in the spring to continue the motion. Resistance (from bearings and vehicle weight) is greater than the output of the spring.

    I say this, because you note that when adding rulers it moves less or not at all - this indicates that the additional mass of the system by adding rulers is too much for the spring to handle.

    You need to make everything as light as possible.
  6. Feb 28, 2011 #5


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    It looks like the mouse trap is too far away from the driven back wheels. For the first 80 degrees of mouse trap arm movement, the string is not really pulled much at all.

    I would move the mouse trap closer to the back wheels so that it starts to rotate the back axle immediately when it is released.

    If it doesn't have enough force to do this, then move the string down the ruler towards the mouse trap.
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