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Instantaneous Deceleration

  1. Jun 11, 2010 #1
    If a top fuel dragster that can accelerate to 325mph in 5sec. in the quarter mile suddenly had its drive train precisely and instantaneously severed when the car reached 300mph would the momentum or the inertia of the rapid acceleration cause the car to exceed 300mph? If other words, would the negative acceleration be instantaneous or gradual?

    Thank you for your responses.

  2. jcsd
  3. Jun 11, 2010 #2


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    Staff: Mentor

    Force = mass * acceleration (F=ma)

    As soon as you remove the driving force, you no longer accelerate (change velocity).
  4. Jun 11, 2010 #3


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    I see what you are asking. Yes, acceleration can drop to zero arbitrarily fast.

    Pretend the dragster is able to accelerate so that its velocity is increasing by 1m/s every 1/100th of a second. (I just made this number up.)

    You are able to sever the drive train in less than 1/100th of a second.

    The dragster's acceleration will drop from (1/ms^-1)/(.01s) to 0 in that 1/100th of a second.
  5. Jun 12, 2010 #4
    It is hard to imagine that momentum would not push the heavy dragster past 300mph. If this was drawn on a graph would it show an abrupt (almost eye popping) change of acceleration and G-force?
  6. Jun 12, 2010 #5
    No. Momentum is simply given by the mass and velocity of an object, if the acceleration drops instantly, the velocity will become instantaneously constant.
  7. Jun 12, 2010 #6


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    Momentum does not push.
  8. Jun 12, 2010 #7


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    A change in acceleration from positive to zero does not cause any eye-popping. You are confusing a change in acceleration with a change in velocity.

    Yes, a graph of this dragster's motion would show an instant drop from positive acceleration to zero acceleration.
  9. Jun 14, 2010 #8
    Then there is something wrong with your internal model of what momentum is. The physical law is that constant motion continues unless acted on to change; it does not mean that all processes resist being changed. You said that you stop accelerating. That is taken at face value.
  10. Jun 14, 2010 #9
    I want it to be known that I do not approach this question with a background in physics, which is probably apparent to many here. I am now convinced that acceleration can cease in a millisecond regardless of the weight involved… and momentum is inapplicable. What is still hard to conceive is that this instant lack of G-force is not jarring to the occupant or the vehicle. Imagine this same scenario happening to the space shuttle with ten times the acceleration; wouldn’t it create a great deal of stress? Is not the instant absence of acceleration [G-force] the inverse, or equal to, an instant application of acceleration [G's]?

    Thanks again guys for the insightful responses.

  11. Jun 14, 2010 #10


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    Well, the drop in acceleration will travel through the vehicle as a wave. While it was under acceleration, the vehicle was compressed very slightly (since it is not perfectly rigid). When the acceleration drops to zero, the compression will rebound, the vehicle will expand to its unstressed length. This wave will travel back and forth along the length of the craft until it is dissipated.
  12. Jun 14, 2010 #11
    I think it would help to consider that removing the drive train or whatever from an accelerating car is not too different from just taking your foot off the gas pedal.

    The force you feel from acceleration is just the change in velocity. As soon as you stop accelerating velocity stops changing and so you feel nothing. This isn't the same as decelerating. Decelerating is also changing your velocity. If you were traveling at a high velocity and then slammed on the brakes, you would decelerate, and you would feel that also. When you let off the breaks you do not feel as if you are speeding up or anything, you just stop feeling a force.
  13. Jun 16, 2010 #12
    In a word, no. Removal of force is not equal to the application of force in the opposite direction.
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