(2012-11-04) Mousetrap Powered Car

Number Two Son has a science project (PBL) to make a mousetrap-powered car. It needs to go 3m.

He did some "research" online, and settled on working with K Nex pieces, with a string to transfer energy to the axle.

Due to poor communication over the phone, we built with a rather small mousetrap rather than the bigger size. That (a) reduces the power available, and (b) providers a thinner platform, which resulted in some splitting when screwing in the eyehooks. This also resulted in some confusion over measuring how long to make the string. (We may start over with a bigger trap.)

First run: string pulled, but 0 motion. The wheels were allowed to spin freely on the axle, so the axle was being rotated by the pulling of the string, but no energy transferred to the wheels.

• Change: find new connector that locks wheels in place on the axle.

Second run: a few inches of slack in the string when the trap is undone. Car moved quickly a certain distance, stopped instantly. The string would unwind, the continue on to re-wind (in the opposite direction) until it was fully tight, at which point it stopped the wheels.

• Change: loosen the loop around the axle a bit, in hopes that axle will spin freely once string is unwound, rather than re-winding it.

Third run: same result as previous - the looser loop doesn't help, it still rewinds.

• Change: make the strong shorter, so that there's almost no slack when the trap is sprung.

Fourth run: Seems to run more weakly, and stops sooner. Smaller mousetrap means not just less power, but less linear pull on the string.

• Change: go the other direction, make very long (~24") string so that it will be ok to have it rewind because it will have gone 3m before hitting its limit.

Fifth run: weak, doesn't even fully unwind. Theory: the K Nex piece we have just outside each eyehook has a nub that sticks out rather far, and the axles have a lot of wiggle-room because the eyehook is a little too big - so that nub may be hitting the body of the mousetrap and creating friction.

• Change: replace that nubby K Nex piece. Can't find a good K Nex replacement, so just wrap a skinny rubber band multiple times around.

Sixth run: rather weak again, suspect rubber band is rubbing against eyehook as axle turns, creating friction.

• Change: move the rubber bands further out.

Seventh run: success!

Eighth run - by Number Two Son alone setting the trap (have been avoiding that so far out of fear for his fingers, but now that we have a successful model it's time for him to learn). Rather weak, need to try again.

• with some repetition/training, get decent results. Could definitely do better, but out of time/patience.

Meta: would have been good to have quick way to Video/zoom results. Pretty crude/subjective trying to figure out what's going wrong in each experiment that only lasts a second.