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Modeling motion with air resistance (integral calculus)

  1. Mar 14, 2007 #1
    Air resistance is a force that acts in the direction opposite to the motion and increases in magnitude as velocity increases, let us assume at least initially that air resistance r is proportional to the velocity: r = pv, where p is a negative constant. suppose a ball of mass m is thrown upward from the ground. The net force f on the ball is F = r – mg (the direction of the force r is downward (negative) when the ball is traveling upward and the direction of r is upward when the ball is traveling upward.)

    1. Use the net force equation and the fact that F = ma to write a differential equation for the ball’s velocity.

    2. Assume m = .5kg and p = .1. Make a direction field for the differential equation and sketch a solution of the initial value problem v(0) = 50 m/s.

    3. Solve the initial value problem algebraically. Hint: be sure to take the constant of integration into account.

    4. Find an equation of the height of the ball at time t.

    5. When does the ball reach the apex of its trajectory? When does the ball land?

    6. Does it take the ball longer to come up or come down?

    This is what ive done so far.
    m(dv/dt) - pv - mg = 0 for the upward motion


    m(dv/dt) + pv - mg = 0 for the downward motion

    taking upward to be positive (and both p, g < 0).
  2. jcsd
  3. Mar 24, 2007 #2
    I think you're on the right track..., usually I see motion differential equations with dv/dt written as x with two dots above it, but it's basically the same.

    2 and 3. What you would need to do now is solve this differential equation... the initial value is used to solve the Cauchy problem (to get rid of that pesky constant of integration) and compute a velocity-time graph

    4 to 6 are mechanical

    Hope that helped :)
  4. Mar 24, 2007 #3


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