What makes a drawing robot vibrate?
July 25, 2010 3:58 PM   Subscribe

You have a motor. You attach a weight to the shaft, and now you have an eccentric motor. Why does it vibrate?

I'm specifically interested in how the size of the weight and the distance of the weight from the shaft affect the degree of vibration. I need to make a diagram that will make sense to a 10-year old explaining terms like "restoring force" and "angular momentum" to show why a drawing robot vibrates.

posted by puckish to Grab Bag (8 answers total)
Conservation of momentum, that's why.

When the shaft turns, the weight moves. There has to be a counterbalancing movement the other way by the motor in order for momentum to be conserved.

(If the motor is firmly fixed to the ground and doesn't move, then it's because the Earth is moving. The Earth being so huge, the distance moved is infinitesimal, but it is still moving.)
posted by Chocolate Pickle at 4:07 PM on July 25, 2010 [1 favorite]

Give the kid a five-pound weight. Have him hold it in front of but close to his chest, and move it side to side. Then have him do the same with the weight at arm's length. Have the kid notice that as he moves the weight side to side, his body will move the other way.
posted by notsnot at 4:09 PM on July 25, 2010

Tie something to piece of string. Have the child swing the thing around and let go of the string. No repeat with something heavier, so that they feel the thing pulling back against them (obviously you want to do this somewhere so that if/when they let it go, it doesn't hit the picture window, the dog, aunt May or what have you).

The next part is to convince them the difference between wobble and vibrate is one of speed.
posted by Kid Charlemagne at 4:11 PM on July 25, 2010 [2 favorites]

I always thought it vibrated because it rotates round it's common center of gravity. Determining the center of gravity would be a matter of positioning the shaft and the weight in their respective positions and finding the the "balance point" (as if they were on a teeter-totter).

However, IFHSP (I failed high school physics) and I'm not certain why I feel compelled to try answering these types of questions.
posted by bonobothegreat at 4:43 PM on July 25, 2010 [2 favorites]

bonobothegreat, relax. (At least this time) You're right.

posted by IAmBroom at 5:28 PM on July 25, 2010

It's centripetal force, right? The force radially inward on the weight has to be equal to mv^2/r (m being mass, r being radius from the shaft) for the thing to continue to travel in it's circular path. (As opposed to flying off in a tangential path.) So, if the thing goes faster, much more force is needed to keep in in a circular path. If it gets heavier, or the radius gets shorter, a linearly proportional amount of force increase is needed. If you're holding onto the shaft, you are essentially providing this force, get the period fast enough, and it's a vibration.
posted by Horizontally a Champion at 5:38 PM on July 25, 2010 [1 favorite]


In your FACE Mr. Minchin!
posted by bonobothegreat at 8:02 PM on July 25, 2010

Theory: The thing's center of mass wants to stay in one place— and by "wants to" I mean "will do so unless a force is applied to it" (this is Newton's first law of motion). If you attach an offcenter weight to the motor, the center of mass of the motor+weight is offcenter, and the whole assemblage will try to move such that the center of mass stays in place (or rather, moves uniformly).

Practice: If you mount your vibrating motor very loosely, or float it on water, or something, so that it's free to move, you can easily see there's a point on the weight that stays still as the motor+weight gyrate around it. Demonstrating that this point is the center of mass is a further exercise for the student.

A better question for a ten year old might be: when you don't put an eccentric weight on the motor, why doesn't it vibrate?
posted by hattifattener at 8:16 PM on July 25, 2010

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