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How does momentum, inertia and drag affect the motion of an object?

  1. May 17, 2013 #1
    How does momentum and inertia affect changes in speed, when considering acceleration from thrust, or from decelleration from drag?

    Say, for a fixed mass of 100kg, at a velocity of 10m/s, is a force of 10N is applied in the forwards direction, then F = ma -> a = F/m. a = 10/100, a = 0.1 m/s^2. But how does the object's inertia affect this? Or is that dealt with in that equation?

    Also, if the same object where no longer being forced forwards, how would the air resistance affect the motion of the object when considering its momentum and inertia?
     
  2. jcsd
  3. May 17, 2013 #2

    russ_watters

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    Staff: Mentor

    Inertia is the property of mass that resists acceleration, so it's in the equation. Air resistance is just a force. And momentum doesn't have any impact here.
     
  4. May 18, 2013 #3
    So the momentum has nothing to do with how fast an object slows down due to air resistance?

    Also, with inertia, how can you calculate the minimum force required to move a mass? And if you apply a force greater than the minimum, how will it affect the resulting acceleration?

    Thankyou for your time.
     
  5. May 18, 2013 #4
    Well of course momentum being mass×velocity, and air resistance (drag) being a product of velocity2, then yes, changing the momentum will change the air resistance. Mass and/or inertia don't appear in the drag equation, so objects with similar size and shape but different masses will have about the same amount of drag.

    As for minimum force - it seems you are asking about our environment here on the surface of a planet, where things are affected by gravity and friction. ANY force will move ANY mass, although perhaps by an imperceptible amount, under the right circumstances - out in deep space, for instance.
     
  6. May 18, 2013 #5
    Ok, so my confusion was from not taking into account friction, when trying to move a mass. So to move it, you must overcome the opposing forces, such as friction. And I see what you mean about momentum and inertia being proportional to mass, but not affect the slowing rate from drag.

    Thanks again everyone for their help.
     
  7. May 18, 2013 #6
    We can only answer qualitatively since we don’t have a specific problem that we’re working with.

    Momentum is the property of a body that is able to impart motion (and hence momentum) into other bodies. Inertia is a measure of the mass of a body and is what gives a body momentum. Drag is a force that is depending on the geometry of the body and acts in the direction opposite to the motion of the body. In general it’s a complicated function of the body’s speed.

    This statement is not worded precisely enough to answer your question. You didn’t say what the meaning of F is. Is it the total force on the body, i.e. the sum of all forces acting on the body? If so then F = dp/dt = ma. However if this is only the applied force, i.e. the force acting on the body by, say, an electric field E on a charged pith ball, charge q, then ma = qE – s(v) where s(v) is the expression for the drag force. This is an example of damped motion.
     
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