What Makes A Cell Phone Vibrate?

Cell phones, and other vibrating devices, work just like the washing machine that is banging around your laundry room.  During the spin cycle, the drum of a washing machine spins really fast and the clothes inside usually spread out evenly along the walls of the drum.  However, there are times when the clothes bunch up along one side of the drum and that is when the banging starts. 

As the clothes spin, they push away from the center of the drum onto the walls of the washing machine.  When the clothes are evenly distributed, the push evenly in all directions and there is balance; like a tug-of-war between Arnold Schwarzenegger and Carl Weathers.  However, when the clothes bunch up, they push harder in one direction and create an imbalance; like a tug-of-war between Arnold Schwarzenegger and you.  The direction of the unbalanced force constantly changes as the drum spins in circles.  That is why your washing machine rocks back and forth instead of just falling over on its side.

Cell phones, and other vibrating devices, replicate this phenomena by using a small electric motor to spin an asymmetrical weight (think semi-circle).  The asymmetrical weight is attached to the axle of the motor which is mounted inside the phone.  When the motor spins, the weight acts like the bunched up clothes and causes your phone to start banging around on the table as if it were your washing machine. 

You can see the vibrating motor from a cell phone in this video.