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How does a mobile phone vibrate without any external force? By Newton's law, any body can't move without any external force

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That's like saying food can't move inside your body, because your not pushing it down from the outside... –  Ivo Flipse Nov 18 '11 at 11:21
If your statement "any body can't move without any external force" would be true, how would cars be able to move? –  klennepette Nov 18 '11 at 11:34
The center of mass of the phone does not move (at least, it wouldn't if there were no friction). The parts of the body of the phone move relative to the stationary center of mass of the phone, hence vibration. @klennepette Friction acts on the wheels to move the car forward, which is also why a vibrating phone may wander over a flat surface. –  Peter Morgan Nov 18 '11 at 12:56
This is the same thing as saying "no engine should vibrate" - even though they "try" to eliminate vibration, small variance in the weight of moving parts prohibits this. –  Mark Schultheiss Nov 18 '11 at 17:09
Say I'm holding a bowling ball in space, and we're not moving (in some frame of reference). I throw the bowling ball - both I and the bowling ball are now moving in opposite directions, without any external force. –  BlueRaja - Danny Pflughoeft Nov 18 '11 at 18:45
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4 Answers

up vote 36 down vote accepted

That's not true, Newtons's laws do not say that. What's important here is conservation of momentum. Inside the phone, there is an oscillating mass. While the mass inside has a momentum and thus a velocity in one direction, the (friction-free) phone has to have the same momentum in the opposite direction. It "vibrates".

Homework: Get on a skateboard (best kneeling, not standing), take a decent mass with you (e.g. a cobblestone) and move it forth and back in front of your chest. Now, put a large cardboard box over your head (e.g. from a refrigerator) and you have a box that moves back and forth without any external force.

If you want translation instead of oscillation, you have to divide the object, making one part go in one direction and the other in the opposite direction (again, with the same momentum). That's how rockets work, by expelling the reaction products of their fuel at high speed in the opposite direction. Again, without "external" force.

Alternatively, you can just sit in a chair, and punch the air really fast. When your arm moves out, your body moves back, when your arm moves back in, your body moves toward the arm.

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please take video of this experiment and post it for us to see. :-) –  David Murdoch Nov 18 '11 at 13:08
I think you should make the homework more extensive. Gaffer tape a length of drain pipe to the side of the box first to mimic an aerial. –  Greg B Nov 18 '11 at 14:42
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At least one mobile phone I've heard about uses an unbalanced spinning weight. As the weight moves in one direction, the phone moves in the other, in accordance with Newton's Third Law of Motion.

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I'd say that all or most use such an excentric drive. Any other construction of such a low frequency electroacustic transducer would be rather voluminous and ineffective. –  Georg Nov 18 '11 at 10:32
Newer iPhones apparently use a linear oscillator for vibration, but I think excentric drives are more frequent. The photo at ifixit.com/Guide/Image/meta/ubIhmlniERMlpwKy shows both (linear oscillator on top, excentric drive at the bottom). –  nd. Nov 18 '11 at 11:58
For those who want to Google it, it's "eccentric drive". –  Keith Thompson Nov 18 '11 at 21:17
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I've taken a phone apart to confirm the "imbalanced weight" coming off the shaft of a miniature motor. Which essentially creates a moment arm off-center of the shaft, once the motor begins to turn, this torque (force X distance) lets momentum (mass X velocity) move energy through the body of the motor, to the base of the motor, through the phone, and further dissipated through your body. Hence, felt vibration through your pocket.

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youtube.com/watch?v=POBzVAx9J80 –  Kostya Dec 1 '11 at 13:54
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It keeps its center of gravity constant. But since there is a rotating eccentric weight inside the body center moves in opposite fashion from the rotating weight making the phone appear to vibrate.

This is similar to recoil from a gun, or if you are on skates and try to spin a weight in the end of a rope it will make you skate back and forth. It would be fun to watch though!

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