Microwave oven door physics

March 10, 2006 8:14 AM

Why does a microwave oven door sometimes bounce and sometimes close?

When you push a microwave oven door closed, sometimes the springs in the door resist enough to keep it from closing and it bounces back like a Windows error-message "bonk" sound. Sometimes you're a Zen master and your "follow-through" closes the door perfectly. I want to know whether there's some physics mumbo-jumbo that explains the difference between the first "bonk" motion failure and the second "chunk" yeah-that-sucka-is-closed success.

posted by cgc373 to science & nature (17 comments total)

I would guess the "speed" of the door would have to be greater than the "resistance" provided by the springs in the door latches.
posted by jon_kill at 8:19 AM on March 10, 2006

Many years have been spent studying this phenomena but unfortunately because of the microwaves involved we can only know the speed that the door is travelling at or the force needed to close the door but sadly not both.
posted by oh pollo! at 8:27 AM on March 10, 2006

I would guess the "speed" of the door would have to be greater than the "resistance" provided by the springs in the door latches.

Speed isn't really important and resistance has to do with electrical physics, not mechanical.

What's happening from a physical standpoint is that you need a certan amount of energy to overpower the springs and close the door. The kenetic energy of the door is m*v², but because the door is on a hinge it gets a little more complicated, since diffrent parts of the door move at diffrent speeds. Nevertheless, the amount of kenetic energy quadruples for every doubling of the speed.
posted by delmoi at 8:37 AM on March 10, 2006

How bizarre. I asked this same question in my head this morning. I reckoned it's because the force isn't quite enough to overcome the springs, but enough to push the springs so that they'll bounce the door back. After all, if you push with your hand, there's a ton of resistance and power there.. if you push it without following through, there's not.
posted by wackybrit at 8:38 AM on March 10, 2006

There's a threshold between 'hard enough to slide the pawl past the latch' and 'not hard enough to . . .'

When it bounces, it didn't have enough speed to clear the latch. (I assume that you're giving it an initial shove and expecting it to close without pushing it all the way closed.)
posted by Kirth Gerson at 8:38 AM on March 10, 2006

I think another factor is how much the door weighs. A heavier door won't need to move as fast as a lighter door to depress the same latch spring.
posted by benign at 8:40 AM on March 10, 2006

Ah delmoi has it. Let this be a lesson to me: always preview.
posted by benign at 8:42 AM on March 10, 2006

This is a good physical illustration of the chemical concept of activation energy.
posted by jellicle at 8:44 AM on March 10, 2006

Speed isn't really important

Yes it is, it's directly proportional to kinetic energy, since the mass of the door doesn't vary. You even say this.

resistance has to do with electrical physics, not mechanical.

Resistance, in physics, a force that tends to oppose motion.

So jon_kill has it.
posted by cillit bang at 8:50 AM on March 10, 2006

Kirth's right about the initial push, which probably makes a difference, since if you press the door closed, you can feel the resistance of the springs in the muscles in your arm. But the point of the question is about the difference between that satisfying feeling of "yes" when the door closes because you've done it "right"; and the "dammit" when you don't. When you mess it up, is it merely about a lack of force applied, or is there another element involved?
posted by cgc373 at 9:05 AM on March 10, 2006

It's like rolling a ball up a hill. Too little initial speed, and it comes rolling back down. Do it fast enough, and it rolls over the hill and down the other side.
posted by rxrfrx at 9:08 AM on March 10, 2006

I have never had the door bounce on any of the microwaves I've ever used. Are you people just giving the door a shove and letting it slam shut, or are you pushing it all the way until it clicks? The latter is the right way.
posted by kindall at 9:26 AM on March 10, 2006

This is not complicated at all. You need so much force (or energy, if you want) to overcome the inherent resistance in the the spring-loaded latch (that resistance is ultimately provided by the electromagnetic repulsion among the atoms and molecules of the spring-loaded latch, but that is neither here nor there: it's just some force X that must be overcome). The word-line of the door is simple if it closes -- it's more complicated if it bounces. The noise is made by the collision of the door and the latch setting up a vibration, which excites the air, which your ear sends to your brain, which turns it into sound for you.

If the door bounces, your force was less than X. If the door closes, your force was greater than or equal to X. Your force is directly related to the speed with which you push the door (velocity, really, but that's a quibble), and that speed came from the electrochemical energy stored in your muscles, which ultimately came from photosynthesis of some kind.

There's nothing more to it ;-). Take a physics class on classical mechanics, and you'll spend plenty of time making potential energy graphs that will make this type of phenomenon more obvious to you.
posted by teece at 10:23 AM on March 10, 2006

I guess I should add -- I am imagining that you are talking about pushing the door, and then letting it do its own thing, not continually guiding it with your hand. It won't bounce in the latter because your brain is getting continuous feed back, and will adjust the force in your hand until the door closes.

And the different sound simply come from the latch vibrating differently. In a failure, the latch is hit, and then left free, which makes a longer, more music-like note. If the door closes, you just get the short, ka-thunk sound of closure, as the door and latch are in contact, so the spring-loaded latch can't vibrate as long.
posted by teece at 10:28 AM on March 10, 2006

Thanks, teece, and everybody. (I'm trying really hard not to snark about any "right way" to close a microwave oven door. Trying and kinda failing.) I suspected there was little magic, but I was hoping for a little magic. Guess not.

. . . This time.
posted by cgc373 at 11:07 AM on March 10, 2006

Is the cat's tail completely inside?
posted by weapons-grade pandemonium at 11:14 AM on March 10, 2006

An analogy would be like trying to roll a ball up a hill. Visualize one of the Skee-Ball games. If you roll the ball too softly it will climb partly up the incline but then come rolling right back at you with all the speed that you threw it (minus a small amount for rolling friction.) However, if you roll it hard enough to make it past the crest it will "latch" and not come back to you.
posted by Rhomboid at 1:26 PM on March 10, 2006