I am trying to start a project, and have no experience in electronics only in software, but want to learn as much as I can.
I want to do an arm to punch a wooden wall at different speeds and break it. Is it preferable to use an electronic actuator or pneumatic?
A pneumatic would only need a valve and a compressor?
Where can I get a linear actuator strong enough for this, and how can I calculate the force needed?
A side question I have: How to determine which stepper is needed to lift a long metal arm?
You will break your linear actuator very quickly trying to punch something with it.
And stepper motors have very low torque, you are not going to successfully lift a long metal arm with one.
Does anyone has any better recommendation of an actuator to break something?
A pneumatic actuator should do it, but the only way you have of controlling speed is by controlling the air pressure and that could be tricky. You're not going to find an inexpensive, electronically controlled pressure regulator; such things are industrial use only and thus have industrial pricing. Your best bet would be to use off-the-shelf pneumatic regulators and basic ball valves to release the air if you're trying to keep your costs reasonable.
To save on the inevitable abuse of your actuator you should arrange your system so that the actuator "launches" the punching mechanism instead of being directly attached. And then for the best measurement of punching force you should set up some photogates to calculate the speed of the punch right before it hits. I'd suggest using a couple TSSP4038 and 38KHz modulated pen lasers for the light beams that the punch passes through. The will allow a wide yet precise light beam.
I gotta ask; why the heck are you making a device that punches walls? Did Robin Givens hire you to make her home safer?
Does it have to be a wall (ie vertical) or can you lay it flat? Then you could lift something and drop it like a pile-driver.
Or if vertical, some kind of pendulum would be feasible, like an Izod tester.