Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Cut the pulley cord into 52-cm lengths.
2. Set up a test track by marking a starting point and finish line 4 meters apart with tape. If room is available, make more than one test track to reduce group wait time during experimentation. For the final challenge, all groups should use the same track.

Safety Precautions

The mousetrap can snap with considerable force. Use caution when the spring is under tension. Wear safety glasses. Please follow all laboratory safety guidelines.

Lab Hints

• Enough materials are provided in this kit for 24 students working in groups of 4 or for 6 groups of students. This is a Super Value Kit—all the materials included are completely reusable. Both parts of this laboratory activity can reasonably be completed in two 50-minute class periods. The pre-laboratory assignment may be completed before coming to lab, and the post-lab calculations and questions may be completed the day after the lab. Extra time may be taken to allow students to run more trials or to be more creative in their designs.
• When students change the length of the lever arm by using different eyelets, it is important that they 1) move the mousetrap to the appropriate hole on the chassis and 2) wrap the string around the lever arm until the string just reaches the projection on the hub with no slack in the string. Moving the mousetrap on the chassis ensures that the point of force is directly over the hub. Wrapping the strip ensures that the string is the appropriate length without needing to cut three different pieces. This design ensures that the string will detach from the rear axle just as the lever arm completes its forward swing, expending all the potential energy of the spring. From this point on, the car simply coasts until friction causes it to stop.
• Testing the alignment of the mousetrap car is best done on a tile floor if possible. Students can use the lines of the tiles to determine how straight the car travels. Otherwise, a series of meter sticks lined up end to end may serve as a guide.
• If students cannot deduce why changing the hub size produced different results, instruct them to count the number of revolutions of the axle when winding the string on each hub, respectively. This may be done by placing a marker (small piece of tape) on one rear wheel and counting how many times the wheel makes a complete revolution when the string is wound around the hub until the lever arm is back as far as it will go. Winding the string on the larger hub will cause the wheel to turn fewer revolutions, thus the larger hub transfers more torque (produced by a turning force) to the wheels.
• A third, smaller hub diameter may be incorporated into the trials. Students may attach the loop on the projection of the small hub and then guide the string to wrap around the bare axle.

Teacher Tips

• This is a great hands-on activity for incorporating STEM, simple machines, transfer of energy, Newton’s laws, rotational motion, engineering design, and scientific inquiry.
• Depending on the testing surface, students may find that using the large hub and small wheels causes the wheels to spin in place without much forward motion. Increasing the friction between the wheels and the floor will remedy this. Rubber bands may be wrapped around the circumference of each wheel; however, this is not easily accomplished. We have found that cutting the top and bottom off a balloon, leaving a 2–3 cm band from the middle works well. These bands can be easily slipped around the wheels. Use smaller balloons (Flinn Catalog No. AP6420) for the small diameter wheels and larger balloons (Flinn Catalog No. AP1900) for the large diameter wheels.
• The specific design challenge may be changed from year to year or class to class. Other options may include the following:
  • Tow a certain weight a set distance
  • Run uphill at a specific angle
  • Fastest car over either short or long distance
  • Travel through grass or other uneven surface
  • Entire class challenge where cars travel one behind the other keeping an even pace with lead car designed by instructor.
• Be sure to give students any design constraints you deem appropriate. May they modify the wheels in any way? Increase traction or size? Larger wheels may be made by taping large plastic lids to the wheels, cutting a hole in the center of each. May they add weight to the car? How much time will be given to design the car? Will they be able to run a practice test once the design is final?

Answers to Prelab Questions

1. Read through the Procedure and explain how the lever arm is used to propel the mousetrap car.

   A string is attached to the end of the lever arm and the other end of the string is attached to the hub on the rear axle of the car. As the string is wound around the hub, the lever arm is pulled back, tightening the spring. When the lever arm is released, the spring unwinds, causing the lever arm to snap back to its original position. As the lever arm springs back, it pulls on the string, which in turn rotates the hub and the rear wheels turn, propelling the car forward.

2. When testing one variable, why is it important to keep all other variables constant?

   When only one variable is changed, any changes in the outcome of the test may be attributed to that one variable. If more than one variable is changed, then the cause of any differences in the outcome would not be clear.

3. Identify the safety hazards in this activity and the precautions necessary to protect against these hazards.

   The mousetrap can snap with considerable force. Use caution when the spring is under tension. Wear safety glasses.

Sample Data

A. Lever Arm Observations
Complete the following “If/then” hypothesis to explain how the length of the lever arm will influence the amount of force required to lift the lever to a 90-degree angle.

“If the length of the lever arm decreases, then the amount of force required to tighten the spring should increase because the force is exerted over a shorter distance.”

B. Wheel Diameter Observations

The car travels more slowly with the large wheels than with the small wheels.

C. Hub Diameter Observations

It takes more turns to wind the string around the small hub than the large hub. The car travels faster when the string is wound around the large hub than when it is wound around the small hub.

Data Table C. Hub Diameter D. Going Further
Predict how changing the point of force from eyelet 1 to the middle eyelet on the lever arm (closer to the fulcrum) will affect the car’s performance over a 4-meter distance. Write your prediction as an “If/then” statement (see the prediction in Part A as an example).

If the point of force is closer to the fulcrum, then the car will travel faster over a 4-meter distance because the shorter lever rm will transfer the energy over a distance in a shorter period of time.

Answers to Questions

1. Consider a placement of the spring scale on the lever arm at the first bend between the spring and eyelet 3.
   1. Predict how the force would compare to the force recorded at eyelet 3.

      The force required to pull the lever arm from a point closer to the spring would be greater than the force required to pull the lever from eyelet 3.

   2. Explain the reasoning for your answer.

      As the distance from the effort force to the spring decreases, the force needed to tighten the spring increases.

2. Calculate the average time for each trial and record the average in the data tables.
   1. Explain the effect the diameter of the wheel has on the speed of the car over a 4-meter distance.

      The larger the wheel, the slower the speed of the car with all other variables remaining the same. The larger wheels turn more slowly at the start and complete fewer revolutions in a given distance than the smaller wheels.

   2. Explain the effect the diameter of the hub has on the speed of the car over a 4-meter distance.

      Winding the string on the larger hub results in fewer revolutions of the wheels. The larger hub transfers more torque per revolution of the wheels than the smaller hub, and the car travels faster.

3. Consider the amount of potential energy stored in the spring when the lever arm is pulled all the way back.
   1. Does the eyelet that is used in pulling the lever arm back affect the amount of potential energy stored in the spring?

      No—the potential energy of the spring is the same no matter which eyelet is used.

   2. Explain the reasoning for your answer.

      The spring is tightened the same amount each time. When a longer lever arm is used, less force is required, and when a shorter arm is used, more force is required. The total amount of energy transferred to the spring remains the same as long as the spring is tightened as far as it will go.

4. Consider a mousetrap car designed to win a short race in the fastest time.
   1. Which point of force on the lever arm would you choose?

      The point of force should be closest to the fulcrum.

   2. Explain your choice.

      When the point of force is closer to the fulcrum, more energy will be transferred over a distance in a shorter amount of time.

   3. Which set of rear wheels would you use?

      The smaller wheels should be used for greater speed.

   4. Explain your choice of wheels.

      With a smaller wheel, more rotational force is exerted per revolution. The car will have a faster start.

   5. Which hub diameter would you choose?

      The larger hub would be better for a short race.

   6. Explain your choice.

      The string would be wound fewer times on the larger hub, so the wheels would turn fewer revolutions. Since the amount of energy transferred is the same, more torque is transferred per revolution.

References

Special thanks to Brad Christensen, Center for Mathematics, Science, and Technology, Normal, IL, for providing the idea and the instructions for this activity to Flinn Scientific.

Introduction

Design and build a car that runs on energy provided by a mousetrap! Use a prototype mousetrap car to test different variables and then modify the design of the car to optimize its performance. It’s off to the races!

Concepts

• Energy transfer
• Problem solving
• Dependent and independent variables
• Engineering design

Background

A mousetrap car works on the principle of a lever. One end of the lever is connected to a spring. When a force is used to pull up the other end of the lever up, the spring tightens, storing energy. When the lever is released, the stored energy in the spring is transferred back to the lever, and the end snaps back. In order to use the energy stored in the spring to propel a car, the mousetrap must be modified.

Figure 1 illustrates a basic mousetrap car. The mousetrap is mounted on a chassis with front and rear wheels. The bar attached to the spring on the trap is extended or replaced by a longer lever arm. A string is attached to the lever arm and wrapped around the rear axle. As the rear axle is rotated backwards to take up the string, the lever arm is pulled back, winding the spring more tightly. The car is placed on the floor and released. As the lever arm snaps back, the potential energy from the tightened spring is transferred to kinetic energy, pulling the string, which in turn rotates the rear axle and the car is propelled forward.

Many factors, or variables, may affect the performance of the car. The final design of the car will depend on the desired outcome, or design criteria, and any limitations to the solution, or design constraints. Is the car’s function to be speed, distance, towing power, or some other purpose? Is there a limit to the type or cost of materials? A decision on the final design cannot be made until the design criteria and constraints are understood. Since more than one solution may be possible, brainstorming to generate several ideas is an essential part of the design process. Next, experiments with a simple model are carefully planned and carried out to test variables that may affect the outcome. Only one factor should be varied during an experiment with all other factors remaining the same. The independent variable is the variable that is intentionally changed or manipulated for the test; whereas, the dependent variable is the variable being measured or observed, sometimes called the outcome or the responding variable. Any problems encountered during testing should be noted, a possible explanation made, and a remedy proposed. Then the model can be altered and a side-by-side comparison made with the data and observations from before and after the modification.

Experiment Overview

The purpose of this activity is to collect data and identify patterns in the motion of a mousetrap car. The activity is divided into three parts. Part I describes how to assemble the mousetrap car chassis and wheels. In Part II variables that can affect the performance of the car will be tested: length of lever arm, diameter of rear wheels, and hub diameter. After testing is complete, the data will be analyzed to design a mousetrap car that performs best for the design criteria and constraints given by the instructor in Part III.

Materials

Prelab Questions

1. Read through the Procedure and explain how the lever arm is used to propel the mousetrap car.
2. When testing one variable, why is it important to keep all other variables constant?
3. Identify the safety hazards in this activity and the precautions necessary to protect against these hazards.

Safety Precautions

The mousetrap can snap with considerable force. Use caution when the spring is under tension. Wear safety glasses. Please follow all laboratory safety guidelines.

Procedure

Part I. Mousetrap Car Chassis Assembly

1. Place one plastic bushing on the front axle.
2. Place the 3-cm diameter front wheel on the front axle. Use the wheel to push the bushing further along the axle.
3. Place the other bushing on the front axle.
4. Adjust the bushings so the wheel is centered on the axle and spins freely (see Figure 2).
   {12381_Procedure_Figure_2}
5. Insert each end of the rear axle into the hole in the wide end of a rubber stopper. Note: If the diameter of the hole in the stopper is too large, increase the diameter of the axle by wrapping masking tape tightly around the axle until the stopper fits snugly on the end.
6. Push the rubber stoppers onto the axles until the ends of the axle are flush with the narrow end of the stopper.
7. Select one set of rear wheels and place them onto the rubber stoppers on the rear axle. Gently yet firmly push each wheel as far as it will go onto the stopper (see Figure 3).
   {12381_Procedure_Figure_3}
8. Test the alignment of the front axle by placing the car on the floor and giving it a push.
9. If the car does not roll straight, but veers to one side, hold the front of the chassis tightly and carefully bend the long arm of the front axle slightly in the opposite direction.
10. Repeat steps 8 and 9 as needed until the car rolls in a straight line.
11. Set the chassis aside until Part IIB.

Part II. Testing Variables A.Mousetrap Car Lever

1. Obtain the mousetrap with the attached lever arm, a metric ruler, and a spring scale.
2. Observe that the lever arm has three eyelets.
3. Measure the distance from the spring (fulcrum) on the mousetrap to the furthest eyelet (1). Record the distance to the nearest centimeter in the Lever Arm Data Table on the Mousetrap Car Worksheet.
4. Repeat step 3 for the middle (2) and closest (3) eyelets.
5. Hold the mousetrap in one hand and grasp eyelet 1 with the other hand.
6. Pull the lever arm up to a 90-degree angle and then push the lever arm back to its resting position while still holding on to the eyelet.
7. Repeat steps 5–6 with the other two eyelets, noting any changes in the amount of force required to lift the lever arm.
8. Complete the sentence under Observations for Part A on the Mousetrap Cars Worksheet.
9. Zero the spring scale.
10. Attach the hook of the spring scale to eyelet 1.
11. Holding the mousetrap down with one hand, pull the lever arm up by pulling on the spring scale until the lever arm is at a right angle (90 degrees) to the mousetrap. Caution: Be careful to place your hand away from where the lever may snap.
12. Observe and record in the data table the force in newtons required to hold the lever arm at 90 degrees.
13. Repeat steps 9–12 for the remaining two eyelets, checking to make sure the spring scale is set to zero before each test.

B. Mousetrap Car Wheels

To test the effect of wheel diameter on the performance of the car, time how long it takes for the car to travel 4 meters with each set of rear wheels. Mark a starting line and 4-m finish line on the floor with tape. Use the longest lever arm and the small diameter hub for each trial.

1. Obtain the chassis with the front and rear wheels attached (from Part I of the Procedure), the screw, wing nut and two metal washers.
2. Place the mousetrap on top of the chassis and align the hole in the mousetrap with the hole in the chassis that is closest to the front axle. Note: The lever arm will extend over the front wheel.
3. Place one metal washer onto the screw.
4. Insert the screw with the washer through the aligned holes from the top.
5. Place the second washer onto the threads of the screw under the chassis and secure with the wing nut.
6. Obtain a 52-cm length of string.
7. Tie a loop knot at one end of the string as close to the end as possible (see Figure 4).
   {12381_Procedure_Figure_4}
8. Tie the free end of the string to eyelet 1, so that the string measures 40 cm between the eyelet and the projection on the small hub of the rear axle.
9. Attach the loop on the end of the string to the projection on the small hub. Note: Do NOT pass the string through the other two eyelets (see Figure 5).
   {12381_Procedure_Figure_5_Rear axle}
10. Carefully rotate the rear axle backward, giving the lever arm a little lift with your hand as you continue to rotate the rear axle, winding the string around the small hub.
11. Continue to rotate the rear axle until the lever arm is pulled all the way back and the eyelet is just above the hub.
12. Holding the lever arm in place, set the car down so the front wheel is at the designated starting line.
13. One partner should stand at the 4-meter mark with a stopwatch.
14. Time how long it takes for the car to travel 4 meters and record the time in the Wheel Diameter data table on the worksheet.
15. Repeat steps 9–14 two more times with the same diameter wheels.
16. Remove the rear wheels and attach the other set of wheels.
17. Repeat steps 9–15 for the second set of wheels.
18. Record any patterns in the performance of the car that were observed for Part B on the worksheet.

C. Mousetrap Car Hub

To test the effect of hub diameter on the performance of the car, time how long it takes for the car to travel 4 meters with the string wrapped around each diameter hub. Mark a starting line and 4-m finish line on the floor with tape. Use the longest lever arm and the large diameter wheels for each trial.

1. Attach the loop on the free end of the string to the projection on the smaller diameter hub on the rear axle.
2. Making sure the loop remains on the projection, carefully rotate the rear axle backwards, until the string is taut.
3. At this point, give the lever arm a little lift with your hand as you continue to rotate the rear axle, winding the string around the small hub.
4. Continue to rotate the rear axle until the lever arm is pulled all the way back and the eyelet is just above the hub.
5. Holding the lever arm in place, set the car down so the front wheel is at the designated starting line.
6. One partner should stand at the 4-meter mark with a stopwatch.
7. Time how long it takes for the car to travel 4 meters and record the time in the Hub Diameter data table on the worksheet.
8. Repeat steps 1–7 two more times with the small diameter hub.
9. Repeat steps 1–8 with the larger diameter hub. Note: Attach the loop of the string to the projection on the small diameter hub, and then guide the string over to the large hub as the rear axle is rotated.
10. Record any patterns in the performance of the car that were observed for Part C on the worksheet.

D. Going Further

1. Using the data you have gathered so far, predict what would happen to the performance of the car if you used the large diameter wheels and the large hub, but changed the point of force to the middle eyelet on the lever arm. Record your prediction on the worksheet for Part D.
2. Test your prediction.
   1. Remove the mousetrap assembly from the chassis and reposition the mousetrap so the hole lines up with the middle hole in the chassis. Attach the mousetrap to the chassis by following steps 3–5 of Part IIB.
   2. Keep the string tied to eyelet 1 and wind it around the lever arm between eyelets 1 and 2 until the loop end of the string just reaches the projection on the small hub. Thread the string through eyelet 2. Note: The string should measure 30 cm from the hub to eyelet 2 (see Figure 6).
      {12381_Procedure_Figure_6}
   3. Time the car over a 4-meter distance for three trials. Design a data table on a separate sheet of paper to record the time for each trial and any observations made.
   4. Repeat steps a–c with the point of force closest to the rear axle. Attach the mousetrap to the chassis at the hole closest to the rear axle. Wind the string around the lever arm between eyelets 2 and 3 until the string measures 20 cm from the hub to eyelet 3. Thread the string through eyelet 3.
   5. Answer the remaining questions on the worksheet.

Part III. Design Challenge

The challenge is to design a mousetrap car that will cross the finish line of a 6-meter route last. Use the data you have collected to determine with your group which variables you will use in designing your car.

1. With your group, consider the following questions relating to the effects of point of force, wheel diameter and hub diameter on the speed of the mousetrap car.
   1. What is the relationship between distance, time and speed of the car?
   2. Do any of the variables affect the distance the car travels until the lever arm snaps, not including coasting distance?
   3. Will increasing friction between the wheels and the floor affect the speed of the car in any way? If so, how might this be accomplished within the design constraints given by the instructor?
2. With your group, plan, discuss, test and evaluate your design.
   1. Decide upon the variables you will use: hub diameter, wheel size, length of lever arm and any other modifications allowed within the design constraints.
   2. List any safety concerns and the precautions that will be implemented to keep yourself, your classmates, and your instructor safe during testing.
   3. Consider the strengths and limitations of your design.
   4. How will the testing data be recorded?
   5. How will you analyze the data to determine a successful design?

Student Worksheet PDF

12381_Student1.pdf