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Terminal Velocity formula

  1. Jan 18, 2010 #1
    We have the following formula for the velocity of an object moving in a fluid.
    v = vt -vte-kt/m.
    This formula shows that an object can never achieve terminal velocity but in the derivation of this formula given in "Fundamental of Physics" by H D Young and Freedman 10 edition we use the formula mg=kvt which shows that object achieves terminal velocity?
    Can any one please explain this seemingly contradiction?
     
  2. jcsd
  3. Jan 18, 2010 #2

    berkeman

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    Staff: Mentor

    What do you mean by "never"? In an exponential equation like that, you only have to go out a few time constants to get arbitrarily close to the final value...
     
  4. Jan 18, 2010 #3
    But sir vt=v only when time t = infinity.
    and hence v can never equal to vt.
    But while deriving it we have supposed that we have achieved vt.
     
  5. Jan 18, 2010 #4
    There are a number of laws in physics that have an exponential like this one. Radioactive decay is one. Another is the rate of loss of heat from a hot body. In both of these cases, the maths says that the value never reaches zero because t would need to be infinity. In that you are perfectly correct.
    However, this is the maths. In the real world, we find that we don't need to wait until infinity before the value falls to such a low level that it is so near to zero that the difference cannot be measured, or the difference is of no significance.
     
  6. Jan 18, 2010 #5
    Yes and just to add an example to what stronebridge is saying. Newton's law of heating and cooling which you may be familiar with or population modelling. These both have similar concepts behind them. If you put a steel ball in a 30F room. and The steel ball is at 0F, they will never become the same temperature except at infinity. This is because there is a point where the steel ball is 29.999999999999999999999999999999999999999999999999999999 F, and since the law of heating and cooling depends on difference in temperature, it will take forever for them to be equal.
     
  7. Jan 18, 2010 #6
    Thanks a lot Sir Stonebridge and Sir dacruick.
     
  8. Jan 21, 2010 #7
    The first formula is stating that if the object is released from a state of rest, and allowed to accelerate downward through a viscous fluid in a gravitational field, then its velocity at time t will be as specified. The second formula is stating that, AT terminal velocity, the downward force of gravity WOULD be equal and oppositely directed to the drag force (so that they sum to zero). But when the initial velocity is less than the terminal velocity, then the object never achieves a velocity equal to terminal velocity. Its velocity only approaches the terminal velocity asymptotically.
     
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