A Hard Problem Need Help

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In summary, when a car abruptly decelerates, the air inside the car tends to maintain the same speed and creates a pressure gradient, causing the balloon to move backwards due to the higher pressure in the front of the car. This is also why a pendulum would move forward in the same situation.
  • #1

Homework Statement

Question 20
You are traveling at constant speed in an airtight car with a
balloon floating motionless next to you. Suddenly, you slam
on the brakes so as to stop the car quickly. During decceleration,
with respect to the car the balloon
(a) moves forward; (b) remains motionless;
(c) moves backward; (d) can move forward or backward.

Homework Equations

No Equations

The Attempt at a Solution

The correct answer is c, that it moves backwards. Can someone explain why?
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  • #2
Great problem. I don't know why. I can do some handwaving and say the air in the car, relative to the ground, has the same average speed as the car, as of course does the balloon. So when the car abruptly decelerates you get a gradient of pressure from highest to lowest from the windshield backwards, and this pressure change develops rapidly.

But this is one answer I might need to empirically validate.
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  • #3
Yes, it is because of a pressure gradient, inside the car the front part is in higher pressure, because as the car is decelerating the air tends to maintain the same speed so the majority of the air goes to the front part of the car, like there was some gravity attracting the air to there. If the pressure in the front part of the balloon is greater than the pressure behind he goes backwards. But if you have a pendulum, it would move forward.

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