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Basic Physics: Force and Friction

Friction is a force, and from physics, you know that forces can change an object's speed or direction. The force of friction from dry pavement on your car's tires is much greater than the force of friction from snow or ice. Why? Because ice produces much less friction with your car's tires than the dry pavement does.

But how does this lack of friction affect how your car drives? Basically, changes in speed and direction take much more time and distance on ice or snow than on dry pavement. The most obvious example of this is slowing to a stop at a traffic light. In icy conditions, this stopping distance can increase by a factor of four or more because of the reduced force of friction. Changing speed isn't limited to slowing down, either. You may have noticed that speeding up is also harder in icy conditions.

What about going around a curve? A car that's going around a curve is changing direction, and forces are necessary to cause this change. Usually, the only significant force available for taking a car around a curve is the force of friction. So if you reduce the force of friction by making the road icy, you can't change the direction of your car as fast. You must drive your car more slowly around the curve or your car will skid out.

So far, the other forces on the car have been ignored. For example, the force of gravity can affect the motion of your car on a hill. If you're going downhill, a portion of gravity tends to speed you up. If you're going uphill, a portion of gravity tends to slow you down. In both cases, you need the force of friction to counteract these effects. Sometimes icy roads don't have enough of this friction, which is why getting a vehicle up a steep hill or controlling the car's speed going downhill in these conditions can be difficult.

Forces don't only affect the motion of your car on an icy road. From the motion of a soccer ball through the air to the motions of planets around stars, forces affect all kinds of motion. By studying physics, you learn a lot more about the relationship between force and motion. And if you understand the relationship between force and motion, you'll understand a lot more about the universe you live in.

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