1. Kristin McCoy Academic Coordinator, CSU Fresno MESA
Mousetrap Cars
Kristin McCoy
Academic Coordinator, CSU Fresno MESA

2. What is a Mousetrap Car?
Vehicle powered by the spring device of a mousetrap
Mousetrap is a simple machine – uses mechanical advantage to multiply forces
Mousetrap acts as a third-class lever with the spring as the fulcrum and the hammer as the load

3. Third Class Lever which a lever turns. Resultant force (load)
Fulcrum
Resultant force (load)
Applied Force
Applied Force
A fulcrum is the point or support on 
which a lever turns.

4. What is a Mousetrap Car? How does the power source work?
The spring propels the hammer, which causes an enormous release of energy (Kinetic)
The hammer is connected to a string that is wound around the drive axle
The string unwinds as the hammer snaps – making the car roll!!

5. Scientific Concepts
Important concepts for building a mousetrap car to consider:
Potential Energy
Kinetic Energy
Force
Friction
Torque
Power

6. Scientific Concepts Potential Energy:
Energy that is stored within an object, not in motion but capable of becoming active
Have stored potential energy (in the spring) when your mousetrap is set and ready to be released.

7. Kinetic Energy: Energy that a body possesses as a result of its motion
Potential energy becomes kinetic energy as the mousetrap car begins to move
Some of this energy goes to friction – the rest makes the car go

8. Force: An action that causes a mass to accelerate
To change the motion of your mousetrap car, a force must be applied.
To increase the acceleration of the car, the force must be increased or the mass decreased (Newton’s Second Law)

9. Friction:
The force that opposes the relative motion of two surfaces in contact
Friction will slow- and eventually stop – the mousetrap car
Friction occurs between the wheels and the floor and between the axle and the chassis

10. Torque:
Can informally be thought of as “rotational force” or “angular force” that causes a change in rotational motion
In the mouse trap car the snapper arm applies a force to the drive axle through the pulling string. This in turn causes a torque to be produced around the drive axle.

12. Power Rate at which work is done or energy is used
In the mousetrap car, the same overall amount of energy is used regardless of its speed – only the rate of use changes
distance – to use energy slowly
power – use it more quickly ( lots of energy needed at the start to get car moving up a ramp)
Accuracy – balance is important (enough power to reach target, but not a lot of energy saved for the end so braking will be easier)

13. Construction Hints!!
When building a mousetrap car, there are a number of variables to consider:
Weight of car
Placement of mousetrap
Length of the snapper arm and the string
Size and type of wheels
Wheel-to-axle ratio
Depends on the goal of the car – distance, accuracy, or power

14. Weight of the Car Build the lightest possible vehicle
Lighter will require less force to begin moving and will experience less friction than heavier cars
If a car is too light, it will not have enough traction
This causes the wheels to spin out as soon as the trap is released

15. Length of the Snapper Arm and The String
Long snapper arms and short snapper arms release the same amount of energy
The difference is in the rate at which the energy is released (power output)

16. Distance try long arm. This will provide less force, but more distance
Accuracy try shorter arm. This will provide more force and power output, but less distance
Power try a shorter arm. Will provide more force and power output, but less distance.
These cars need power to get up a ramp

18. For all cars, the lever arm should just reach the drive axle when its in the ready position
When the string is wound, the place where the string is attached to the snapper arm should be above the drive axle
This maximizes torque as the car takes off (max torque occurs when you lever arm and string form a 90⁰ angle)

19. For all Cars
Lever arm should just reach drive axle when in ready position
When string is wound, place string to the snapper arm above the drive axle
This will maximize torque – max torque occurs when lever arm and string form a 90°angle

20. Correct Length: lever arm just reaches axle
Correct Length: lever arm just reaches axle. Lever arm and string form a 90°angle, allowing for max torque

21. For Distance and Power Cars:
String length shorter than distance from leer arm to drive axle when the trap is in relaxed position
Allows string to release from hook preventing tangles

22. Accuracy Cars:
String serve as a braking mechanism – string length is very important and must be exact.
String can be tied to drive axle so when string runs out, car will come to a sudden stop
String length can be set so that it runs out exactly when car reaches the target

23. ** string tension can also cause misalignment
All cars:
if wheels are misaligned, car will be working against itself – energy will be lost
Most visible sense, misaligned wheels also mean care won’t go in desired direction
Power cars:
Misaligned can cause car to leave ramp
Accuracy cars:
Misalignment can cause car to miss target
** string tension can also cause misalignment

24. Wheel-to-Axle Ratio Power cars: Smaller wheel-to-axle ratio best
Increasing size of axle will decrease wheel-to-axle ratio
This will increase torque giving more pulling force for every turn of the wheel

25. Placement of Mousetrap
Distance Cars:
Place trap farther from drive axle
Accuracy Cars:
Placement of mousetrap depends most on the length of the string
Power Cars:
Place trap closer to the drive axle – get more pulling force

26. Size and type of Wheels For Accuracy Power: Accuracy:
Make sure wheels have good traction
Traction is a good type of friction
Increase traction by covering edges of the wheel with a rubber band or middle of a balloon
Accuracy:
Traction will be important ensuring the car can come to a sudden and accurate stop without skidding