Introduction: Mousetrap Car for Less Than $5
As a physical science project, I had to create a mousetrap car that would go 3m and push an empty soda can the last meter. I based my design on geek27's Mousetrap Car Racer.
Sir Isaac Newton discovered three laws of motion. Newton’s first law states that “Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.” (Newton's Three Laws of Motion) If an object is not moving, it will not move unless a force moves it, and an object in motion will not accelerate or decelerate unless a force accelerates or decelerates it. Newton’s second law states that force = mass * acceleration “Acceleration and force are vectors … in this law the direction of the force vector is the same as the direction of the acceleration vector.” (Newton's Three Laws of Motion)The force that changes an objects state of motion is the product of the objects mass and the amount that the object accelerates. Newton’s third law states that “For every action there is an equal and opposite reaction.” (Newton's Three Laws of Motion) For every force applied there is an equal and opposite force.
Energy is conserved. The spring in the mousetrap has potential energy because the spring is pulled back and retains that energy. The car will not move unless the spring is released. (Newton’s First Law) When the spring is released, the potential energy is converted into Centripetal force, moving the arm. The arm acts as a lever, and also has Centripetal force. The arm pulls the string, giving it tension. The string turns the axle and the wheels, giving it Centripetal force. The wheels contact the ground and because friction works in the opposite direction as the car, the car is able to move forward (Newton’s Third Law). Friction is required. Without friction, the car would not be able to move forward.
According to Newton’s First Law, the car would remain in motion for ever, as long as external forces are not applied. The car eventually stops, because of the friction that was required to start to move the car. The spring has to have enough force bring the car the three meters, after overcoming inertia. Friction is the applied force vector for the car, to the car will accelerate in the direction of the friction. (Newton's Second Law) Friction is the equal and opposite force from Newton's Third Law, so the direction of the friction vector is opposite of the direction of the car's vector. The car decelerates because the friction's opposite direction causes negative acceleration.
Step 1: Schematics
Theses images are the schematics from my original and final designs.
3d models are available here (uses Google Sketchup)
Step 2: Materials & Cost
-one mousetrap ($1.69 for a 2 pack)
-half a pool noodle ($1 for a whole)
-four 1/4" nuts
-one 1/4" wing nut
-epoxy (that works with metal)
-lengths of 1"X1/4" wood
- 1 3/4"
- 10" (2x)
- 1 3/8" (3x)
-3/16" copper tubing (27" needed) ($3.49)
- 1/4" copper tubing (15" needed)
Step 3: The Body
After clamping the mousetrap, use pliers to take out everything but the spring and the lever arm on the mousetrap. Cut the lever arm (see picture).
Drill 1/4" axle holes 3cm from one end and 5cm from the other end of the 10" lengths.
Assemble as shown with wood glue or epoxy. 1 3/8" lengths should go under either end of the mousetrap and one should be centered under the mousetrap. 7" and 1 3/4" lengths should be centered on both ends of the car with the 7" length 5cm away from the axle hole. The mousetrap arm should face the front of the car (the 7" piece)
Step 4: Axles
Cut 3/16" copper tubing into a 5" and a 7" length. Insert into axle holes, with the 5" axle in the front and the 7" axle in the back. Put nuts on the axle on either side of the car, and put the wing nut on the back axle, between the wood sides.
***Leave 1/4" between the nuts and the wood.
Step 5: The Wheels
Cut the pool noodle into two 2" pieces and two 1" pieces. Make sure that the ends of the pool noodle are square.
Wrap the axle with electrical tape. This takes a long time.
The 2" wheels go onto the back axle, so the back axle needs two widths of electrical tape on each side.
Slide the wheels into place. They should go on snugly but not to a point that it is impossible to get them off.
Step 6: Extend the Arm
Take the last 2 pieces of copper tube and put the 3/16" tube into the 1/4" tube. Use the epoxy to attach it to the mousetrap's arm.
Tie a knot in the thread, so that there is a small loop at one end. Cut the string so that it is at least 33cm long.
Wrap electrical tape around the string 10cm up the arm, with 30 cm of string and the loop facing the back axle. The loop on the string should not reach the axle.
Step 7: Use the Car
Pull back the mousetrap arm and turn the back axle backwards, after slipping the loop over the wing nut. Set the car on the ground and let it go.
Below is a table of the data and a graph of the data from my home testing.
Step 8: Modifications to the Origional Design
I modified my original design a few times during the building and testing process. (This Instructable shows the final design).
-Structural supports were added under the mousetrap and on the end of the car.
-A wide piece of wood was added to the front to help push the soda can.
-thread was used instead of floss
-pool noodles were used instead of pipe insulation
Step 9: Works Cited
geek27. (2009, December 5). Mousetrap Car Racer. Retrieved February 27, 2010, from Instructables: https://www.instructables.com/id/Mousetrap-Car-Racer/
Newton's Three Laws of Motion. (n.d.). Retrieved March 21, 2010, from University of Tennessee Knoxville: http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html