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G force vs. gravity

  1. Jun 12, 2012 #1
    I will be filming a tv commercial for a well known car brand. In the commercial the featured car will be driving through a 90 degree banked wall at over 100 mph. During this process the driver will experience roughly 3+ Gs... It is the Gs that keep the car "pinned" sideways. In the commercial, the driver will drop a water bottle. What I need to know is where the bottle falls? Does it drop to the floor? Does it fall to the passenger side window?
  2. jcsd
  3. Jun 12, 2012 #2
    The car is itself sideways, such that the passenger side window is parallel and nearest to the ground?

    If so, then I expect the bottle will accelerate mostly toward the car's floor, but somewhat toward the passenger side window as well. Compared to the car itself, the bottle will experience both the 3g acceleration toward the car floor and the 1g acceleration toward the center of the Earth.

    The best way to picture this is that the bottle continues in a straight line but for deflecting down toward the center of the Earth, while the car follows the banking of the turn. It's actually the bank pushing the car off a straight path that makes the bottle seem to go toward the car's floor.
  4. Jun 12, 2012 #3


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    Once the ball is released and is no longer in contact with the car, it will go at the same speed and velocity it had the moment it detached from the car. Vertically, it will fall towards the ground due to gravity, horizontally it will keep going in a straight line with the same speed and in the same direction when they last were part of the car. It doesn't matter if the bottle is inside the car or outside. This is Newton's first law.

    Consider these cases:

    Imagine a train travelling due north ready to enter a turn. Smoke comes out of the stack and the train is going to end up travelling due west after it completes the turn. Once the smoke leaves the stack, the smoke will go due north at the speed of the train (assuming no air resistance), even though the train is turning. Once the smoke leaves the stack, whatever forces that were on the train no longer act on the smoke.

    When you have loose items in a car and you slam on the brakes, what happens? The car stops, but the items keep going forward. Why do they do that? Because they keep going at the same speed in the same direction they had when they were a part of the car. No longer a part of the car, they fly forward at thier last known speeds and directions until they are stopped by the windsheild or back of a seat.

    Also, if you have a merry go round nearby, have someone stand on the rim of the merry go round and drop objects as the merry go round spins. The person on the ground will see the dropped objects (no longer part of the merry go round) travel in a straight line, tangent to the point they were dropped with the same speed of the merry go around.

    You can also drop balls from a bike or skateboard and see the same thing.

    Once the bottle detaches from the car, there are no more g-forces on the bottle because the bottle is not part of the car anymore.

    Does this help?
  5. Jun 13, 2012 #4


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    We are mostly physics geeks around here, taught from an early age that centrifugal force is an evil concept that shall not be adopted. We have a tendency to use inertial frames and talk about what's "really" happening as the car accelerates and the dropped bottle does not.

    This can result in more obfuscation than neccessary, especially when talking to folks who are not completely physics-literate.

    Go ahead and adopt the frame of reference of the car. The camera is probably at rest in this frame.

    The driver is being pressed downward in his seat by roughly 3 G's of total force. Part of this is from gravity (pointing out the passenger window) and part from centrifugal force (pointing down at the floorboards). The resultant will be around 20 degrees off from the vertical. [This is the arc-tangent of 1/3] The exact angle depends on the details of those "roughly 3+ g's"

    The bottle will drop down at this angle so it'll land a little farther toward the passenger side than the point from which it was dropped.

    If the driver is not bracing himself against the seat, he will be leaning at about 20 degrees from the vertical out toward the driver's side window as well.

    It is likely that in this scenario we can ignore Coriolis acceleration -- the bottle will not fall in a completely straight line, but pretty close to it.
  6. Jun 14, 2012 #5


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    but we can put this in the accelerated frame of reference using something called d'Alembert's principle. everything gets a fictional force that is what most people would perceive as "centrifugal force" (the force that ostensibly pulls you out as you bank around a steep curve).
  7. Jun 14, 2012 #6


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    Absolutely. But I think the reason we got bashed when we used the term 'centrifugal force' was that it implied that things 'fly outwards', which is clearly wrong. I think that the choice to tell us that centrifugal force is verboten was probably the least worst - bearing in mind that they had to come down on one side or the other in elementary dynamics. What we were taught at School explains well what happens when conkers fly off strings (common experience). What goes on on fairground rides is another matter - but we probably wouldn't have spelled 'coriolis' right, anyway.
    otoh, I think they got it really wrong by introducing the concept of electron flow for elementary electrical stuff. It was never dished out to us that way, in the 50s/ 60s and I am eternally grateful.
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