What variables need to be considered in slingshots?

[Wroen]

Say I have a basic slingshot (like an elastic band attached to a 'Y' shaped stick), what variables do I need to consider when calculating its ability to launch an object into the air?

So for example, say I load a mass, m, into a slingshot, pull the elastic band back x metres, and then release it, and the object flies off at an initial velocity of 'u' m/s. How can I determine if the slingshot can launch an object of a larger mass?

Do I need to know:
The mass of the initial object?
The 'spring constant' of the sling shot? (and if so, how do I find it?)
The projectile range of the initial object?

Is there more that I need to consider here?

(My little brother asked me if his homemade giant-slingshot could launch a human. I said that it probably wouldn't, but I'd like to work it out purely out of curiosity haha)

Thanks

 

[A.T.]

My little brother asked me if his homemade giant-slingshot could launch a human.

You have a little brother, a giant-slingshot ... physics is an experimental science. But seriously, use a sand bag with the mass of a human.