Launching a projectile

[gwiyomi17]

Hi

I have a project and it's obviuously about projectiles.

I need to build my own mechanical machine to launch my projectile, like a catapult or something.

The goal is to hit a target that is 5-8 m away from the starting point, and I will be tested by it's accuracy and distance.

 

I'm thinking about a machine which I can adjust the angle, and measure the force that will be applied to the projectile..

any ideas how can I do this?

I need plenty suggestions. THANKS!

[imatfaal]

I built a trebuchet once - this uses a heavy weight on one arm of a lever to flick a longer arm around. Once you get the release mechanism sorted out then it can be very accurate and will drop a missile in a metre circle about a 30-40 metres away.

 

But - I have just re-read your question and it seems you have a shorter range, and probably want greater accuracy. If the target is vertical (like in archery) I would tend to go for something that looks like a crossbow (they can fire pellets as well as bolts); ancient name ballista. If it is a quick build then I would use lots of elastic bands for my propulsion, if you have time then I would use bendable arms - from plastic rulers to sprung steel. If you need a trigger mechanism it can be very tricky so I would pull it back by hand to a marked point and release.

 

If your target is on the floor - you need accuracy in dropping at a certain distance; then I would tend to make a trebuchet. Rather than a release mechanism just have the swinging arm stopped suddenly and the projectile will continue in a straight line - I would have this at around 45 degrees. You can change the weight to vary distance - or have a slightly adjustable stopping point of the swinging arm

 

For accuracy make sure everything is tight, well supported, lots of bracing, and most importantly that you can hold the thing absolutely still for release and firing - best way I have seen is that base can be knelt upon to hold steady (ie you kneel down with device in front of you to operate and being held absolutely steady by your weight)

[rktpro]

If you are using rubber bands or elastic strings for providing the initial energy to the projectile then it would be pretty accurate to find out yourself the elastic constant by hanging weights. Since you have the range fixed, you can fix the angle of projection and using kinematical equations get the initial velocity. Now all you would have to know is the stretch required on the rubber band. By energy conservation principal you can find the amount of stretch. Your machine would be precise.

[imatfaal]

If you are using rubber bands or elastic strings for providing the initial energy to the projectile then it would be pretty accurate to find out yourself the elastic constant by hanging weights. Since you have the range fixed, you can fix the angle of projection and using kinematical equations get the initial velocity. Now all you would have to know is the stretch required on the rubber band. By energy conservation principal you can find the amount of stretch. Your machine would be precise.

 

I have never actually checked - but to what extent do elastic bands follow a nice F=-kx Hooke's Law ideal? My vague memory from school physics last millennium was that good quality springs obey hookes law for small extensions - and that for everything else it was a very very rough approximation

[studiot]

 

imatfaal

but to what extent do elastic bands follow a nice F=-kx Hooke's Law ideal?

 

rktpro

it would be pretty accurate to find out yourself the elastic constant by hanging weights.

 

Does Hooke's law matter?

 

By the looks of things rkt was suggesting calibration.

 

I was going to suggest something like that myself, along with what we used to call a spud gun (that used a coil spring).

[imatfaal]

 

Does Hooke's law matter?

 

By the looks of things rkt was suggesting calibration.

 

I was going to suggest something like that myself, along with what we used to call a spud gun (that used a coil spring).

 

If it isn't predictable and repeatable then there is no point in looking for the elastic constant (which was rocketpro's suggestion) and there would be no calibration to be had.

 

In the spirit of empirical enquiry I set out to see if my elastic band (the office's pride and joy) obeyed Hooke's Law to any extent.

 

 

First lot of results to follow

For low masses and little extension the results (ok very rough and ready) look pretty good and consistent with a nice straight line and a linear relation between mass in bucket, thus force applied and extension.

 

 

 

 

 

[rktpro]

The validity of the law depends on how stretched the band initially is. The mass of projectile, I assume is small. I think, it would be incorrect to assume that the elastic constant measured for a band when vertically stretched would be same, in practice, for a band tied like a string to a bow; the stretching force not being verticalthis time.There is no denying the fact that hookes law is limited to materials and to the stretching force. Btw, rkt in my name is not for rocket.

Edited February 20, 2014 by rktpro