Friday, December 11, 2009

Physics 101: A brief review before the front moves in

Here's the situation:

Forecasters have been talking about it all week, and now it appears that a wintry mix of precipitation -- either freezing rain, snow or sleet, or perhaps all three -- will start moving up the Willamette Valley late Friday afternoon.

"We are keeping a close eye on this system and it appears there is a pretty good shot of getting this wintry mix," said Shawn Weagle, a forecaster for the National Weather Service in Portland. "Predicting any kind of frozen precipitation for this area is a huge challenge."

Weagle said an approaching weather system aimed mostly at California will throw off a spoke of energy that will rotate up from the south, striking Eugene first, and then steadily moving north through Salem and into the Portland metropolitan area, probably around 10 p.m. Up to 1 inch of snow or sleet could accumulate, forecasters said.

A winter storm watch issued Thursday will remain in effect for most of northwest Oregon and southwest Washington through Friday night.

Now, a quick refresher on Newton's Three Laws of Motion, for those of you who slept through high school physics the first time around:

1. Objects in uniform motion tend to stay in motion until acted upon by another force.

2. The greater the mass of an object, the greater the force required to move it a given distance.

3. For every action, there is an equal and opposite reaction.

Neglecting the coefficient of friction for ice and assuming all vehicles are traveling at uniform speeds the same direction, here's what this means for local drivers between now and Saturday evening:

1. An SUV equipped with huge tires, 4-wheel drive, and Bluetooth, tailgating a second vehicle on a level, ice-covered street, whose driver attempts to brake within the same stopping distance as if it were a warm day in August, will rear-end the second vehicle at the 4-way stop at the end of the block.

2. The SUV will push the second vehicle farther into the intersection than a Cooper Mini would, because the SUV has greater mass. (Note that its greater mass does not improve the SUV's stopping distance on ice. It simply means that it will hit the other vehicle that much harder when the collision happens.)

3. When the second vehicle slides through the intersection and sideswipes a parked vehicle, the two vehicles will push each other in opposite directions, the parked vehicle bouncing off the curb (and into the middle of the street) while the second vehicle skids into yet another parked vehicle on the opposite side of the street.

So much for the theory. Now let's see what it looks like in practice (note that the station ID says KING 5 in Seattle, but the footage was taken in the Goose Hollow neighborhood in Portland):

As I look out my south-facing window, the sun is still out and the sky is mostly clear, but I've already made my grocery run and don't plan on going out again, unless it's on foot, for a couple of days. For the rest of you, consider the next 24 hours to be an open-book test.

Pencils ready? Begin.

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