# Basic question

1. Apr 24, 2007

### chrisjj

For my Engineering III class we're designing trebuchets.

But, specifically, my question is:

If I have the arm swing forward and let it go as it does normally, where it swings through and then relaxes back to a 90 degree position, will there be more force present in throwing the object there; or option B: would there be more throwing force if I had an immediate stopping point along the axle where the throwing arm would hit into stopping the swing at the right point for the sling to release the object I'm throwing?

2. Apr 24, 2007

### Staff: Mentor

What are your thoughts on this? Graph the velocity of the projectile for the two different options and see what you get. And keep in mind that the shape of the projectile holder will have an effect on your answer.

My guess is that there is a tradeoff in launch velocity and accuracy for the trebuchet design tradeoff that you are exploring....

BTW, homework and coursework questions should normally be posted in the Homework Help forums here on the PF, but more complex project work can sometimes be posted in the general forums. I'll leave your thread here for now.

Welcome to the PF, BTW.

3. Apr 24, 2007

### chrisjj

I haven't taken any kind of physics class yet, being I'm only in eleventh grade and physics is next year. So, I'm not sure where to begin on any of that. If you wouldn't mind, could you get me started, if not just the equations needed.

But just from a guess, I would think that there would be either the same if not more force with having a stopping block, as long as it's located at a point that the throwing arm isn't exerting any more force and that the stopping block will add excess force to that amount.

Sorry about wrong section, didn't notice the Homework/Coursework area before hand. And thank you.

Thanks for response!

4. Apr 24, 2007

### Staff: Mentor

I'd say that's a good guess. The force from the arm is what accelerates the projectile, according to the classic equation F=ma (m is the mass of the object). The force will vary through the arm motion, and depend on what your driving mechanism is (elastic band, or whatever).

If there is no more force from the arm to the projectile, then it is not accelerating anymore, so stopping the arm there does no harm.

The basic equations of motion are the "kinematic" equations:

http://en.wikipedia.org/wiki/Kinematic

You can understand the basics without calculus, but they will make more sense after you take some calculus and physics next year.

5. Apr 24, 2007

### chrisjj

The driving mechanism is just a 6 lbs. weight. So, the throwing arm is driven by the weight. But then there is a sling included as well -- attached to the end of the throwing arm. The sling is then driven by the throwing arm, which is driven by the weight, correct? So the force produced by the sling and put onto the projectile is relative to the throwing arms output of force... or is it just dealing with the sling?