2D catapult game

I want to make a 2D mobile catapult game but I need some help.
For firing an object the catapult will have to stretch backwards and then like a spring it will fire the an object.
I think I will use the hinge joint with a motor to do the lift of the firing but how can I make it so when I press and hold my finger then the catapult starts to stretch backward?

Thank you in advance.

 

Take a look at these two tutorials, the first talks about spring joints (@ 28:00) in the Angry Bird style game, which is what I think will work best in a catapult scenario, and the second goes into mobile touch input.

https://unity3d.com/learn/tutorials...-angry-birds-style-game-part-1?playlist=17093

https://unity3d.com/learn/tutorials/topics/mobile-touch/multi-touch-input?playlist=17138

 

I found that was I needed was hinge joints, thank you any way