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Mouse Trap Vehicle: Lever Arms

March 13, 2012

Earth shattering secrets for building record setting and winning mousetrap cars and racers. Here you will find all the latest and greatest untold construction secrets so you can build your very own mousetrap vehicle.


To understand how the length of the lever arm affects performance.

Equipment Needed

  • Stopwatch
  • Meter Tape
  • Graph paper


Torque is used to cause an object to change it state of rotation. Torque is a force that "twists" or "turns" an object around a pivot point. Torque is a relationship that depends on the size of the force and at what point the force is applied to cause the "twisting" or "turning" motion.

As torque relates to a mousetrap-powered vehicle, a torque is applied to the wheels by pulling the wound string from the drive axle. The mouse trap's lever arm applies a force to the drive axle that causes the drive axle to pivot or rotate thereby causing the vehicle to move. By changing the diameter of the drive axle or by changing the length of the mousetraps lever arm you control the torque that is applied to the wheels. For the mouse trap, the torque that the spring on the base of the mouse trap applies to the lever are is always the same no mater how long the lever is but by changing the length of the lever arm you change its radius (the lever arm length) and the pulling force. Shorter lever arm on a mouse trap have more force at there tip than longer lever arms.

Torque = Force * Lever Length

In this activity you will time your mousetrap car over a 5-meter distance. Between each time trail you will shorten the length of the mouse trap's lever arm and reposition the mouse trap on the frame. It is important to design and build a mousetrap vehicle that will work with this activity. It is recommended to use the Doc FIzzix Basic Mousetrap Powered Car kit with hold down bolts.

adjust lever arm: using a dremal or another appropriate cutting tool shorten the length of the mouse trap's lever arm.

reposition: the mouse trap is repositioned closer to the drive axle each time the lever arm is shortened.

Step 1:

Build your mousetrap powered car using a 12-inch lever arm and make sure not to glue the mousetrap to the frame. Use small micro bolts or Doc FIzzix hold down bolts for attaching.

the basic kit: using Doc FIzzix's Basic kit, start with a 12-inch lever arm and attach the mousetrap to the frame with hold down bolts.

Step 2:

Mark out a 5-meter course in a hallway to other large area sufficient enough to test your mousetrap racer.

the course: using tape for a start and finish line measure a 5-meter course.

Step 3:

Measure only the length of string that is normally pulled from the drive axle; in most cases, this is twice the lever arm length. Important: If there is more string than is actually pulled from the drive axle by the mouse trap's spring then make sure to measure only the amount of string that is pulled directly from the drive axle my the mouse trap.

String Length = _____ meters

measure string length: measure only the length of string pulled by the mouse trap from the drive axle.

Step 4:

Design a data table to record your values that has the following columns: Lever Arm Length, Time Over 5-meters, Average Speed. Each row should be labeled as follows: 12-inch, 10-inch, 8-inch, 6-inch, 4-inch, 2-inch.

Step 5:

Place the front of your mousetrap vehicle at the start line and have someone ready to time the vehicle upon release through the 5-meter distance. Record the vehicle's time over the 5-meter distance for a 12-inch lever arm.

travel time = _____ seconds

Step 6:

Using a dremel took or a hobby saw cut off 2-inches of the mouse trap's lever arm. Re-position the mouse trap back on the deck-top making sure to move the mouse trap 2-inches closer to the drive axle (you will need to drill four new holes to attach the mouse trap at its new location).

adjust lever arm: using a dremal or another appropriate cutting tool shorten the length of the mouse trap's lever arm.

reposition: the mouse trap is repositioned closer to the drive axle each time the lever arm is shortened.

reposition: drill new holes in order to attach the mouse trap.

Step 7:

Repeat steps 5-6 until you have completely filled in your data table for all lever arm lengths making the final length of the lever arm 2-inches.

Graphing the Results

make a graph of lever arm length vs. time.

Label your graph so that lever arm length is on the y-axis and time is on the x-axis. Choose a large enough scale to stretch your graph across one sheet of graph paper. Plot all the points for lever arm length in centimeters and time in seconds. Once you have plotted all your points, draw a best fit line remembering that a best fit line may not touch all the points but will show the shaped of the line or curve

Extension - Testing the Results

Your instructor will tell you a time that you need to travel the 5-meter distance. You can not be under or over the time, you must hit the time right on the mark. Once your instructor gives you a time, use your graph in order to calculate the lever arm length in order to achieve the exact time. Once you have the lever arm length from your graph, cut a new lever arm to the correct length, reposition the mouse trap so that the axle just touches the drive axle, then test your results. Calculate your percent error.

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