Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A Hard Problem Need Help

  1. Mar 5, 2007 #1
    1. The problem statement, all variables and given/known data
    Question 20
    You are travelling 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.

    2. Relevant equations
    No Equations

    3. The attempt at a solution
    The correct answer is c, that it moves backwards. Can someone explain why?
  2. jcsd
  3. Mar 5, 2007 #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.
    Last edited: Mar 5, 2007
  4. Mar 6, 2007 #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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook