Science Buddies: "Ask an Expert"

Hi,

My physics project is building a car that is powered by elastic energy, run on a long table, stop as close to the end as possible, (about 1~1.5m, given on run date) and launch a projectile (max 5x5x5cm) to a target 1.5 m away from the end of the table, 1.2 m below the table.

As for powering, im thinking either mousetrap or rubberband; the braking is probably going to be a string on the front axle that stops it from turning after a set distance. Or maybe use a mousetrap and experiment with the string length.

since speed is not important and the distance is short, the wheelsize and axle is not limiting; i'll probably use smaller wheels since the distance is short.

my main concern is the triggering and lauching of the projectile thing. I have two ideas but they dont seem very good. 1 is to use the string for stopping to somehow pull a something. 2 is to use the mousetrap arm to set somthing off when it finish its swinging motion, but will it be too soon as in the projectile will be launched while the car is in motion. (it doesnt have to launch only when the car stops)

also, im not sure how to make a launcher. the only idea i have is to use a catapult powered by rubberband, not sure if theres enough power.

lastly, the projectile is also a concern. the mark given is a combination of the stopping distance from the end of the table and the distance from the projectile to the target. the projectile is mesured after it stop moving. so a rolling or bouncy projectile could be more random in its results.

any suggestions/tips?

Hi,
I like your idea with the string for both the braking and the launching of the projectile. I think that if you set it up correctly, you could make the string stop the car and then, once stopped, launch the projectile. as for your projectile, i think that maybe a dart could work because aren't they projectiles anyway? i hope this helps. good luck!

Hi, asdf1234567!

Perhaps you should consider two forms of energy - something to drive the car and something to simultaneously brake and launch the projectile. You'd have to work out the details, but I was thinking something along the lines of a rubberband to power the car, a string around the axle to trip a mousetrap when the car has travelled far enough, and then the mousetrap to simultaneously launch the projectile and stop the vehicle. Or maybe a catapult at the end since a mousetrap might be too strong.

These are a few things to consider as you plan your project. It sounds really cool.

One thing, what do you mean by launching with a mousetrap? i dont see how it can be done

For the projectile laucher maybe you can use a rubber band shooter, if that's allowed. It would probaly be easy to make and more perdicatble than a catapult. For the release mechanism, maybe a string tied the axle could pull on a pair of scissors to have it cut a string tied around the rubber band.

Hope that helped.

asdf1234567 wrote:One thing, what do you mean by launching with a mousetrap? i dont see how it can be done

A mousetrap could be used as a kind of catapult. You could rig the mousetrap to "snap" when the vehicle had gone a specific distance, thus launching the projectile. If you position the trap such that it hits a brake during the launch, you'd launch and stop at the same time.

This is just one of the options you could consider for launch time, as is the rubberband/scissors launcher suggested by robertzhou. There are many ways to attempt this, and I'm not sure which would work best. That's why this is an experiment - you're trying to figure out a way to accomplish this project in the most effective way.

I hope this helps.