I'm a beginner and I'm making a school project to wake heavy sleepers up.
So I'm trying to make something that can vibrate a lot
That's a silly question. You can buy a motor strong enough to kill the person and tear the entire house apart.
What I meant was a motor strong enough to make the person uncomfortable, but not hurt them
Are you intending to attach this motor to the person? If so to which part of their anatomy?
I guess you have an idea inside your head of what exactly you're planning to build but if you want any useful answers you're going to have to provide more detailed information. We're not bad at programming but we're pretty useless at reading minds.
Steve
What you want is a Bass Shaker Tactile Transducer
Bolt that onto their bed frame and they will wake up.
On a maybe more serious note, just spend like a moment of your time and look at vibrating alarm clocks. There are a gajillion of them out there. People who are deaf use them. There was a deaf guy in our dorm building and on some mornings his vibrating device dropped to the floor and made a ruckus on the floor waking a lot of other people up.
Colin Furze bed shaker: Voice Activated Bed Shaker - How To - YouTube and Extreme Bed Shaker - Voice Activated Wake Up Machine - YouTube.