Physics and winter driving

This post is a bit of a rant disguised as an educational opportunity. Forgive me.

We got about 40 cm of snow on Friday. As you can imagine, it takes a while for all the side roads to be plowed. This is maybe the 5th or 6th snowfall of that magnitude this winter so folks really have no excuse for not knowing how to drive in it. And it isn’t like we never get snow in winter around here. But it seems that some folks have such a poor grasp of basic physics that they really shouldn’t be allowed out in this weather in charge of a heavy object that can move at speed.

This week’s example: Coming out of the gymnastics centre on Saturday the road goes uphill a little bit. Nothing crazy, mind you, but an incline. With snow on it. There are several cars going out of this dead end street. I’m behind someone in a mini-van. She must have taken 5 to 10 minutes to get up that incline. She’d get part way up and come to a stop, try to get started in the snow going uphill, fail, back down (I had the good sense to stay well back to give her room to manouevre though I did that because I could see someone wanted to get down the hill, and it wasn’t wide enough.), try again, rinse, repeat. As I watch her, she is slowing down as she goes up the hill. WTF? She completely loses any momentum that she has. She has only minimal grip on the road because of the snow. So she gets part way up this very small hill, comes to a stop, and can’t get started.

Newton’s first law of motion tells us that an object in motion wants to stay in motion unless some equivalent force acts against it (inertia). A heavy object on a hill with little friction (snow reduces friction) has the force of gravity pulling it down the hill. To go up you need a force greater than the force of gravity. Force is mass times velocity. So if you go a bit faster, you get some momentum that will overcome the force of gravity and get you up the hill. (This was not sheet ice, folks, just less traction than usual). By going more slowly than usual, this woman was actually making it harder for herself. She didn’t have enough momentum to get up the hill. And with the snow, she couldn’t get from stopped to enough speed to overcome gravity without spinning her wheels in the snow. Eventually she made it. I got up in one shot. As did the folks behind me as far as I know.

I suspect this woman was driving slower because there are circumstances when driving on snow when you want to slow down. Going around a corner, with reduced friction due to snow, inertia can do bad things if your speed is too high. I observed this during the last big snowfall (about 10 days previously). I turned right onto a 3 lane road to find a car in front of me perpendicular to the direction of traffic in the right hand lane (ish). Once he got himself sorted out, it appeared he wanted to be in the left hand lane. Clearly he had come around the corner a bit too quickly. The back end of his car wanted to keep going the same way it was going (inertia, again), so he fishtailed. His back end ends up on the left side of the road and the force starts him going in a circle. He stops himself when his front end is up against the right curb. Luckily it was a wide road. The car immediately behind him could get around him to the left and I could stop in time. Otherwise, it could have been a nasty pile up. By going slower around a corner, you can more easily control the effect of the desire of the back end of your car to keep going straight and keep the fishtailing to a minimum.

Momentum and inertia. No need to understand the formulae for calculating exactly how much force or how big a distance or anything else. Just a basic understanding of these basic principles of physics would make it so much easier for folks to drive safely in the white stuff. Sheesh.

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5 thoughts on “Physics and winter driving

  1. Hey, welcome! Glad to see you over here.
    We have trouble with the winter drivers up here too. …both kinds… the ones that forget momentum and the ones who forget inertia.

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  2. As a fellow northerner, I sympathize. There seems to be a mass amnesia regarding winter driving, and everyone has to learn all over again, every year.

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  3. I once gave a workshop on friction/motion/inertia to a group of second year game design students.

    The winter drivers that I find irritate me the most are the once who are driving summer speeds on snow covered roads and refuse to pull over into their lane when they meet a car.

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