catapult goes whoop again...

[iwfc87]

Hello again.

 

Anyway, yeah..i got round to building a catapult which works great. After doing testing with different arm lengths and masses etc, i found it surprising that the shorter the arm length, the further the object thrown lands. Whý is that? I thought that the longer the arm further the object lands.

 

Well yeah..but I'm just guessing, but heavier objects land at shorter distances due to Newton's second law of F=m.a

 

Anyway, any suggestions on the arm length bit??

THanks.

 

(Yes I am posting this again cause no one answered my reply frm my previous post, but erm yeah..so I'm kinda stranded and lost)

 

THanks

[swansont]

A longer arm has more mass, and so has more KE that can't be transferred to the projectile. That's one reason that the trebuchet works better - longer moment arm with essentially no increase in mass.

[Guest GemiNI]

mmm yes that is a tricky question ..

MA = Effort arm length / Resistance arm length?

But then what kind catapult have you built? Are you using a motor or have you stuck to old fashion methods?

Some Important variables for repeatable launch trajectory:

The shape of the "cup" part of the catapult is very important (especially the tip of the cup.)

Stopper position

Arm length

Arm weight (lighter is better as long as the arm is still sturdy. The most important place to cut weight is at the end of the arm)

 

Not having much knowledge on your design or question:

My guess would be that having a shorter arm can be compare that of the centripetal acceleration of a spinning disk. Imagine a dot on your spinning disk. The closer to the centre of the disk(i.e. shorter radius) the faster the dot is spinning , the father out it is the slower it is (ie lager radius). So if you force is constant and so is your relative mass.. Then your long arm as further to travel (and is therefore slower), where your shorter arm is free to travel a lot faster)

Hope that was of some use to ya :S

[Guest GemiNI]

eva thought about a potato gun? they are so much fun!

[iwfc87]

yeah...well..i built one 'powered'by two extension springs...so yeah..exciting. The catapult is about 0.5m long, so it's not huge.

 

I assume the shape of the cup was good enough. I used one of the purple cups used to transport apples/pears in fruit boxes, in order to overcome the shallow depth of the spoon, so yeah. I assume that the cup was alright or not too much of a problem.

[iwfc87]

Well, back to the spoon length.

 

I asked someone else today, and they suggested that it could have something to do with moments about the hinges.

(A link with a smiiliar design of my catapult is http://www.alumni.ca/~thomsoa/3buildingcatapult.html)

 

So yeah. I think it does make a bit of sense when thinking of the moments conept. What do you guys think?

[Pinch Paxton]

I don't know much about catapults, and things, but I do know about personal observations. Throwing stuff! Anyway, if you have an heavy object, and you are a human, with your two arms, legs fingers, you have quite a useful catapult. How would a human throw an heavy object to his best potential? Well, from observation, and years of evolution, he would probably use the Hammer method. I am talking about the hammer on a chain, that you spin around with, and let go! I have never seen a catapult that uses this method of throwing stuff. I think that it would be an interesting experiment to make one. So to conclude... A good catapult method would require a spring mechanism, that winds down to full strength, with an outstretched arm holding the projectile inside a cloth, on the end of a rope, or chain. The entire structure of this catapult would be pulled backwards against yet another spring, to give it forward momentum, as well as its spinning momentum. (A Hammer thrower travels forwards, whilst spinning the hammer.) Now, how do you make the catapult let go of the projectile at the right moment? You would definately want it pointing forwards when it lets go. I'll let you figure that out! LOL!

 

 

Pincho.