February 4, 2009

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Workout of the Day

Make up Day:  Make up a missed workout.  Make up your own. Or try this:

Deadlift
5-5-5-5-5

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Showing 24 comments
  • Bethanie
    Reply

    Somebody write something to get people going…I’m no good for that and I need a distraction from my reading.

  • Stephanie E
    Reply

    Bethanie- we’re going to Fleetwood Mac (yay!)
    I got my muscle up yesterday. Hope your boredom is cured.

  • Charlie_CFATL
    Reply

    “Imagine a plane is sitting on a massive conveyor belt, as wide and as long as a runway. The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction. Can the plane take off?
    Be careful what you wish for.

  • Bethanie
    Reply

    Charlie, I’m going to need more information about the airplane before I answer that question…
    Stephanie: Am I supposed to be surprised that you got your muscle up? No, I am not. Am I excited for you? Very! Congrats!
    Fleetwood Mac is going to be an awesome time, call me when we need to buy tickets!

  • David Hodges
    Reply

    Charlie, yes the plane will take off, and without any problem, provided the friction coefficient for the airplane’s axles is sufficiently small. In most cases (if not all) this would not be an issue.

  • VAS
    Reply

    Good job Stephanie. Those 18 years of gymnastics and diving finally pay off.

  • VAS
    Reply

    No
    The plane takes off when the air density above its wing is sufficiently less dense than the air below the wing. This difference in densities occurs due to air moving past the wings. In your example the air is stationary relative to the wings.

  • Rob M
    Reply

    Made up Kelly today: 26:28 as Rx’d. The running was a bit brisk.
    Congrats Steph on the muscle-up (and the repeat performance today), and most likely what you got on Cindy… you were hauling.

  • Jonathan H CFATL
    Reply

    Had to work late tonight, and also my shoulders feel as if I were tied to a dock, a ski rope was bolted to my hands, and then a speedboat took off at about mach 5. Also, I’m starting to get sick.
    Looking forward to not feeling like that anymore.
    Congrats Stephanie and good job Rob, I’m going to need to make up Kelly soon.

  • Bethanie
    Reply

    I need to make up Kelly too…tomorrow or Friday will be best…

  • RIEP Crossfit Atlanta
    Reply

    Did a workout Mike put on the board yesterday.
    Four rounds of 5 deadlifts at 140 kg, 30 pushups and 200 meter row. 90 second rest between rounds. Total time was 15:40 or so.

  • kba_cfatl
    Reply

    did
    3 rounds of 20 squats, 15 kb swings (1.5pood), 10 pullups
    7:42
    w/ the “angels” group: Amy J, Julie, Laura & Elizabeth
    I am glad we did 3 not 4 rounds.
    Congrats Stephanie on the muscle up.

  • Dan CFATL
    Reply

    Vas got the answer to Charlie’s question. The plane is not moving relative to the ground; therefore, no airflow over the wing, no lift.

  • David Hodges
    Reply

    The plane does in fact move! The conveyor belt underneath the plane does not prevent the jet engines from creating thrust. The conveyor belt actually does nothing since the forward thrust of the plane comes from the engines which are independent of the wheels.

  • David Hodges
    Reply

    “In your example the air is stationary relative to the wings.”
    Incorrect. The plane is not stationary. The conveyor belt moves in the opposite direction of the wheels, but the forward thrust of the engines will push the plane regardless of how fast the conveyor belt moves. If the friction coefficient on the axles is negligible, then the conveyor belt could move at an infinite speed and not affect the plane one bit. The thrust from the engines is what is going to push the plane forward, not the turning of the wheels. An airplane is not a car; the force that accomplishes forward motion is applied to the wings, not to the wheels. The wheels are irrelevant in this question.

  • David Hodges
    Reply

    “The plane is not moving relative to the ground; therefore, no airflow over the wing, no lift.”
    Notice in the problem that nothing is stated about the motion of the airplane; it is only stated that the conveyor belt moves in the opposite direction. Nowhere does it say that the plane is not moving. So if the jet engines are producing thrust, the plane will move forward.

  • Dan CFATL
    Reply

    Thrust is not the issue. An aircraft can fly without thrust. Lift is the issue. An aircraft cannot fly without lift.
    If, as stated in the problem, the conveyor belt under the plane is moving backwards exactly as fast as the plane moves forward, then the plane will not move relative to the ground as long as it remains on the conveyor belt. No matter how fast it was going on the conveyor, no matter what the thrust, it would be stationary relative to the ground.
    However, we have not considered the air being moved by the conveyor belt, and what lift effects it might have.
    So I guess Charlie will have to settle the dispute.

  • Dan CFATL
    Reply

    As I suspected this problem was not made up by Charlie. A lot of propeller heads are spending endless hours debating the answer at this website:
    http://kottke.org/06/02/plane-conveyor-belt
    Those that believe the plane will move forward relative to the ground, despite being on the conveyor, say that it takes off.
    Those who believe that the problem posits a situation in which the plane does not move relative to the ground say it will not take off.
    Both are right, based on their assumptions about whether the plane moves relative to the ground.

  • David Hodges
    Reply

    But the problem, as Charlie stated it, was only that the conveyor belt moves backwards at a speed equal to the wheels’ speed. It said nothing about the plane. The conveyor belt could move at an infinite speed and if thrust is applied to the wings, then the plane will move forward. If the plane moves forward, then it will product lift. If it produces sufficient lift, then it will take off.
    “Thrust is not the issue. An aircraft can fly without thrust. Lift is the issue. An aircraft cannot fly without lift.”
    Thrust is the issue. If thrust is produced, then the plane will move forward and produce lift. Why? Because the conveyor belt CANNOT effect a force in the opposite direction if the friction coefficient is sufficiently small.
    Sum your forces in the direction of the plane’s angle of attack, and you will see that they look like this:
    ΣFx = T – 0 = T
    Therefore, if the thrust is the only force acting on the plane, it will move forward. Forward motion creates lift.
    If you can show me a way that T – 0 = 0 for all values of T, then I will believe you.

  • David Hodges
    Reply

    “the conveyor belt under the plane is moving backwards exactly as fast as the plane moves forward, then the plane will not move relative to the ground as long as it remains on the conveyor belt.”
    The problem says nothing about the plane’s velocity; it talks about the speed of the *wheels*. But we don’t care about the speed of the wheels since their speed is irrelevant to the summation of forces acting on the plane. Could you slow down a plane in flight by spinning its wheels really fast? No.
    Just do the math.

  • Dan CFATL
    Reply

    I am squarely in the camp that believes the plane doesn’t move relative to the ground while it’s on the belt.
    Let me ask you this. When you are running as fast as you can on a treadmill, how much forward progress are you making relative to the ground? The plane is no different.

  • David Hodges
    Reply

    Wrong! The plane is quite different. I produce forward motion by the contact force betwixt my foot and the treadmill, thus my velocity in the global reference frame stays constant no matter how fast I run in my local reference frame.
    The plane’s motion is independent of how fast its wheels spin. The wheels do not affect how much thrust the plane produces, nor can they slow the plane down by any significant amount. A plane is not a car: it produces thrust by use of a jet engine, not by putting torque on a drive train. A car (or a human) moves forward using the friction created between the ground and the tire (or the sole of his shoe). An airplane moves forward because its engine produces thrust. How fast the wheels spin irrelevant to the forward motion of the aircraft.
    I need you to sum the forces here and keep in mind that the sum of the forces will tell you the direction in which the plane moves. Let’s think back to high school physics here:
    F = M * a
    The sum of all forces on the plane in the direction of its angle of attack can be expressed by the thrust, the drag, and any other force. The thrust is as good as the engines, and the drag is much smaller than the thrust. No other force is being applied to the aircraft. None. I don’t care if you spin the wheels backwards at 1,000,000 metres per second. That does not create a reverse force on the aircraft. And even if it did, it still would not prevent the engines from overpowering that force.
    There is a reason that the question involves an aircraft and not a car. For a car, you would be right, because the motion of the wheels on a car is inexorably tied to its motion. Not so with the airplane.

  • David Hodges
    Reply

    Somebody else says that I am right (for the same reasons I gave):
    http://www.straightdope.com/columns/read/2638/an-airplane-taxies-in-one-direction-on-a-moving-conveyor-belt-going-the-opposite-direction-can-the-plane-take-off
    Also, note this great paragraph:
    “A thought experiment commonly cited in discussions of this question is to imagine you’re standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you’ll have to overcome some initial friction tugging you backward, in short order you’ll be able to pull yourself forward easily.”

  • David Hodges
    Reply

    And in case you STILL don’t believe me, here’s proof:
    http://www.youtube.com/watch?v=IbRcg3ji_Pc
    Yeah, summing forces is all you had to do, but a little physical proof never hurts either.

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