Willz
Former Expert
Posts: 44
Joined: Tue Oct 02, 2007 10:28 pm

### Catapult Help

Hi everyone!
For my physics class, I am required to build a catapult. I am just wondering how the axle and the throwing arm of my catapult should move. The axle goes through a hole in my throwing arm, and it is supported on the ends by two supports. The springs on my catapult are on attached to the bottom of the throwing arm. What I am wondering is this: do both the axle and the throwing arm have to be able to move/rotate freely? Or does only the axle have to move, while the throwing arm doesn't move (or vice versa: throwing arm moves, axle doesn't move)?

Craig_Bridge
Former Expert
Posts: 1297
Joined: Mon Oct 16, 2006 11:47 am
Hopefully it is obvious that the throwing arm has to rotate to work. What enables it to rotate is up to you. If it is rigidly attached to an axle and the axle rotates, the arm can rotate. If it rotates on an axle, then the arm can rotate. If both the axle and the arm can rotate independently, then arm can rotate. Your building it, you get to choose.

Now if one wants to do some engineering analysis on what might work better, follow along:

The accuracy in terms of controlling the throwing plane might have a lot to do on out of plane forces and tolerances. An axle clear span length probably has more distance out of the throwing plane than the throwing arm which is centered on the throwing plane and only has its width out of plane.

If one compares the amount of out of plane displacement
1) an arm mounted with a loose fit tolerance hole through the arm that enables it to rotate, to
2) a throwing arm that is force fit or glued in the center of an axle with no wobble and the axle is mounted in two loose fit holes with the same fit tolerance as case #1
the rigid arm on axle will have less wobble when the clear span length is > 4 times the width of the throwing arm. Further, this wobble can be reduced further by using a longer clear span axle up until the axle is long enough so that it bends, deflects, or has a torque displacement. Note: All of these axle deflection problems can be solved by using a bigger diameter axle.
-Craig

KeijuKainen
Posts: 1
Joined: Tue Nov 25, 2008 8:47 am
Occupation: Student

### Re: Catapult Help

Hi everyone!
I need to build a catapult that roughly fits the dimensions of 0.5 of a cubic meters. the catapult must be a simple machine that can be adjusted to fire projectiles at targets at distances of 5,6,7,8,9, and 10 meters away. this is a graded project and i have absoultely no idea how to even start it! There will be calculations involved, and i cant remember which ones to use! Please help me any way you can! Physics is not exactly my strongest subject!

Thanks,
Melody

Gizzmo46
Former Expert
Posts: 22
Joined: Tue Aug 19, 2008 9:38 am
Occupation: Sr. Engineer

### Re: Catapult Help

Melody,

Hi! It sounds like you are working on a very exciting project! You should begin with research on catapults. There are numbers of books and websites on the subject. One website that may be a good starting point is ...
http://www.stormthecastle.com/catapult/how-to-build-a-catapult.htm. You will need lists of references and these books and websites should give you a good BEGINNING. You will need to decide on the size of you catapult in comparison to the distance that you need the object to be tossed.

As for the equations. You should begin by reviewing kinetic and potential energy and remember that energy is conserved. You also need to consider the mass of the object you will be throwing. For example a ping-pong ball will fly much further than a rock. This will be easily understood with the study of the energies involved. (I personally think eggs would be very fun to catapult ... and if you have any left over toy castles to fire at ... )

Keep me updated and if you have any more questions feel free to ask! Most of all have fun with it!

Heidi