The Mechanics behind a Mousetrap Car

Mousetrap car is a very good illustration of conversion of energy. The energy for moving the car comes from the mouse trap. It is stored as spring potential energy. This is finally converted to kinetic energy of the moving car. Building the mouse trap car is about converting the potential energy to kinetic energy and transferring the energy. Finally all the energy is converted to dissipation energy due to friction.

Many principles are involved in building this simple looking mousetrap car. Applying these principles helped in making mousetrap car more efficient. The mousetrap car is designed keeping in view of these principles to move it faster and a longer distance.  Some changes in the design increased both speed and distance. In some cases the change intended to increase speed also decreased distance and vice versa. There were some factors which contradicted each other as mentioned above. The experience in building the mouse trap car yielded many interesting results.

We used few instruments and materials. They are a wooden mouse trap, CDs, two steel rods of 6mm diameter, aluminum strip, driller, drill bits, epoxy glue, few scrap wood pieces.

Making mousetrap car is not very tough. We got a wooden mouse trap which is supposed to provide the energy. The spring has two arms. One of them is an arm which has to be fixed to the frame the other one is the one that is pushed and which moves. Fixing one of the arms to the frame is a little problem. We placed an aluminum strip and screwed it on to the frame making one of the arm fixed. The other end is left free.

The wheels have to be fitted to the wooden frame. Two steel rods of about 6mm in diameter are taken. We used 4 compact disks (CDs) as wheels. Attaching the wheels (CDs) to the steel rod was a problem initially. We attached wooden pieces to cover the hole in the CD using epoxy. We drilled a hole of 6mm in the wood such that it is the centre of the CD. The rod is inserted into the hole drilled in the wooden portion of the CD. Epoxy is used to keep the wheel and rod intact. The wheels (which are paired using the steel rods) are attached to front and rear ends of the wooden mouse trap by making grooves. These rods have to be fitted properly to the wooden frame work. The rods should be able to rotate freely.

            A string is attached to the free arm of the spring in the mouse trap. The other end is wound around the axle (6mm steel rod) of the rear wheels. The free arm is pushed making it store energy as potential energy in the spring. When the spring unwinds, it rotates the axle around which the string is wound. This makes the wheel rotate which moves the mousetrap car move forward. We have few problems in this process. The string has to be wound tightly round the axle. If it is slack it just loosens without rotating the axle. The string should be rough enough to prevent slippery between the axle and string failing which the car does not move.

            After all the above measures if the floor is too smooth the wheel slips. So we have to attach a rubber strip round each wheel making sure that it won’t slip. This need not be done if the floor is rough enough. When we want to run the car, the arm of the spring has to be pushed and the string has to be wound round the axle (6mm steel rod) ensuring that there is no slack. When the car is released the string unwinds rotating the axle (6mm steel rod) making the car move.

            The speed and the distance traveled by the mousetrap car depends on inertia of the car. As it is known mass is the measure of inertia. The lesser the inertia the faster and longer it travels. We have from Newtons laws that a body having less mass accelerates more which implies it has higher velocity and travels more distance before coming to rest. So we tried to decrease the mass by removing unnecessary wooden material. Though there was no huge difference observed there is still a difference.

            Instead of using big CDs we can use small CDs or wheels if we want better speed rather than longer distance. Big CDs have larger moment of inertia compared to smaller CDs since the radius is larger. We know that Torque is product of rotational inertia and angular acceleration. If the rotational inertia is less for same torque the wheel has high angular acceleration making it rotate at high angular velocities. One may say that though the angular velocity is higher the radius is small leading to decrease in velocity. But the increase in angular velocity is high enough to compensate the decrease in radius of the wheel. This results in a faster car compared to the car with big wheels

            We can also employ gears to change the amount of torque applied and duration of the Torque. Let’s say we attach a circular wheel to the axle and wound the string round the wheel. Since the size of the wheel is larger than the axle there will be more torque but for a lesser duration compared to torque applied to axle directly. This design travels faster initially compared to the axle configuration. So if the times are measured for some distance traveled. The design with wheel attached to axle wins.

            We can also try to reduce losses due to friction but we won’t have much of leverage with regard to this.

References:

            Retrieved 18^th January, 2007 from: http://users.bigpond.net.au/mechtoys/mouse.html

            Retrieved 19^th January, 2007 from:

http://en.wikipedia.org/wiki/Mousetrap_car