Goal of Problem Set #6: This assignment is meant to help you understand:
|
| You are on a road trip with three of your closest friends. You are sitting in the front passenger seat of a four-door sedan as it heads straight down the open road at the speed limit. All of the car's windows and its sunroof are closed. Suddenly, a truck in front of you drops a big sack of cotton puffs. The bag bursts and the puffs sail up into the air in front of your car. For a few seconds, you get to see the precise airstream around the car, highlighted by the moving cotton puffs. What you see it pretty simple: the airstream hits the front of the car and spreads out in all directions in order to flow around the vehicle. Some of this air flows around your side of the car and you pay particular attention to that portion of the airstream. As this air flows off the windshield, it bends inward toward your side window in order to follow your side of the car. The air continues to bend inward until it passes your window but manages to straighten out by the time it reaches the rear side window. The air flows straight past that window, parallel to its surface. The air then bends inward again toward the rear of the car, but doesn't get very far before it breaks up into a huge swirling air pocket behind the car. |
1. The cotton is long gone and you decide to open your side window to let in some air. The moment you open the window a little, the pressure inside the car drops below normal atmospheric pressure and your ears pop. Evidently the pressure just outside your side window is lower than atmospheric pressure. Explain briefly why that should be the case. |
2. You roll your window up and the air inside the car soon returns to atmospheric pressure. Now your friend in the rear seat opens the rear side window, but there is no sudden pressure drop. Why does opening the rear side window have so much less effect on the air pressure in the car than opening the front side window? |
3. The day is getting warmer and so you all open your side windows. Things are going great until you decided to toss your chewing gum out your window. You feel a good breeze drawing air out your window, so you flip your gum into that breeze and the gum goes out the window with it. Unfortunately, the gum then blows into the rear side window and startles your friend. How can air be flowing (or even accelerating) out of your side window and then flowing (or even accelerating) back into the rear side window? |
4. You find this funny pressure effect outside your window interesting, so you stick your hand out into the passing airstream. You expect the air to have a very low pressure. But with your palm turned to face the onrushing air, you feel a tremendous pressure on your palm, pushing your hand toward the rear of the car. Why does the air in your palm have such a high pressure? |
5. You turn the upper edge of your hand forward toward the onrushing air and now you feel your entire hand pushed upward, as well as backward. If something is pushing your hand upward, your hand must be pushing that something downward. What is being pushed downward and how does that downward push affect its motion? |
6. The windows are all closed again and you decide to open the car's sunroof. Once again, the air pressure inside the car suddenly drops below atmospheric pressure when you begin to open the sunroof. Evidently the air pressure on top of the car is less than atmospheric. (A) How does that reduced pressure affect the support force that the roadway exerts on the car? (B) How does that change in support force affect the car's traction - the maximum force of static friction that the wheels can exert on the roadway? |
7. To compensate for this traction problem, the car has a spoiler above the trunk and rear wheels. This wing-like surface deflects the passing airstream upward. Why does that action improve the traction for the rear wheels of the car? |
8. The huge air pocket that forms behind the car is not good news for the car's fuel efficiency. Why not? |