Driving Safely and Wind Direction ```Name: Katie H. Status: student Age: 18 Location: N/A Country: N/A Date: 2000-2001 ``` Question: Does the speed a car is going affect how much the wind pushes it off course? Is it safer to drive slowly if the wind is blowing perpendicular to your car? Why or why not? It is driving me nuts! No one can decide on an answer or give me a good reason for either one. Replies: Katie, Let us begin by assuming you mean pushed sideways off the road and/or tipped over as well. The factors that keep the car on the road are the weight of the car, the friction between the tires and the road surface, and the shape of the car. Shape (streamlining) is a factor because it relates to the area of the car against which the wind is pushing. The larger the surface area facing the wind, the greater the pushing force on the car. And, too, small car of a given weight would be harder to tip over than a taller vehicle of similar shape and equal same weight. The height of the car is a factor because the taller it is, (assuming the same wheel-base), the easier it will be to tip it over. How the frictional factor is affected by the wind is difficult to pin down. Is there anything being borne in the wind stream that could affect friction -- debris, sand etc.? How fast is the car moving down the road? If barely creeping along, you would have more time to react and possibly correct deviations from your intended course. Flying along at full throttle would not offer much time to correct a deviation and would, thus, be foolhardy. Assuming we may be talking about a Wizard of Oz wind, it would probably be best to not drive at all when the wind speed is dangerous. Regards, ProfHoff Katie - I hope a mathematician will give you an answer in numbers, but let me try to do it conceptually. Both the wind speed and your car speed are vector quantities. Do you know how to add vectors? The total of two vector quantities can be done graphically by drawing two arrows - one representing each vector - tail to tail. These will form two sides of a parallelogram. Complete the parallelogram and draw a diagonal from your first corner. If you drew each of the first vectors to scale, the diagonal will be the total of the two forces... in the same scale. You will see that the faster you are moving the less the change in the angle, but your total speed will increase. I would guess that there is a speed a which the effect of the wind would be least. That is, the speed at which you would go the least distance off the original path of the car in a given time. If I were trying to make sense of this, I would draw a graph of the crosswind speed vs the distance off course per unit time. Hope that makes sense. It is not really an answer, but perhaps a way to find the answer. Yours is an intriguing question. Have fun working it out. Larry Krengel It is safer to drive more slowly in a high wind, but not because of being blown off course. The reliability of the tires (skidding) is the major concern. Whether you are blown off course depends on how well your tires grip the ground. Moving faster just means you will go a greater distance before the wind takes its toll. If you keep your wheels from skidding, you won't slide sideways. Keep control of your steering wheel. The wind provides the greatest danger when your tires do the most work: turning corners and stopping. Turning into the wind requires greater push, greater force between tires and road, than the same turn without wind. Turning at a greater speed requires greater force. The two increases combined could push your tires beyond their limit, resulting in a skid. When stopping, the tires push against the road forward. Pushing sideways to resist the wind as well can push the tires beyond their limit, resulting in a skid. When your car skids, you lose control. the car is sliding rather than rolling. If the wind is strong enough to blow you into a skid when you are hardly moving, it can do the same at a high speed. Kenneth Mellendorf Sometimes it helps to consider the extreme cases in order to answer questions like this. If the car is sitting still (i.e., driving VERY slowly) the wind will not blow it off course at all. If it is moving at highway speeds you know that it can push it off course. So the answer to your question is that the car's speed does affect how much the wind pushes it off course. A safe driving speed will depend on the wind speed and road conditions. The reason this is true is that the wind applies a force to the side of the car which must be compensated for by adjusting the direction of the wheels slightly into the wind. It takes some time for a driver to react to a gust of wind and adjust the steering wheel to compensate for the additional force of the wind. During this reaction time the car is being blown off-course. If the car is going fast it goes further off course than if it is moving slowly. Note that in my response I have assumed that the wind is gusting. If it is a steady wind the driver can compensate for it when he/she starts driving and does not have to continually adjust for the wind forces as the windspeed changes. Greg Bradburn Click here to return to the Physics Archives

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