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May the Force be with you (Posted on 2018-05-04) Difficulty: 4 of 5
With the recent resurgence of vinyl records you decide to dig your old turntable out of the closet. It actually still works, and you note it spins up nicely to exactly 33 rpm (revolutions per minute) once it is set upon a level surface. As it spins at that speed, a fly (mass = 12 milligrams) lands on the spinning portion, exactly 3 cm from the center axis, and begins to walk radially outward at 2.0 cm/sec.

Assuming there is enough friction between the fly and the turntable to prevent any slipping, describe all the external (i.e. non-inertial) forces acting on the fly, and identify in which direction each force acts. If any of these forces are a combination of two or more physical principles, also list those.

Bonus: The magnitudes of most of these forces can be calculated from the information provided, but some cannot. Which ones cannot be calculated and why?

  Submitted by Kenny M    
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Solution: (Hide)
Solution: There are 4 such forces that act on the fly.

1. Gravity -acts vertically downward towards the center of the earth.

2. Aerodynamic forces because the fly is moving through the air. One component is due to the turntable spinning and the force is perpendicular to the radius of the turntable, and acts opposite the motion of the fly as it spins. The second aerodynamic force is resisting the motion of the fly as it walks outward on the radius of the record, and this force always acts along the radial path the fly walks, pointing towards the center of rotation.

3. Friction – this one is trickier, as it is the friction that supplies the forces that make the fly accelerate as it moves with the turntable, and also stops it from sliding due to other forces acting on it. a) Friction forces equal and opposite the aerodynamics forces in 2). b) There is also a component of the friction force that causes the acceleration of the fly due to the turntable spinning motion that is a centripetal force (a force that acts on the fly towards the center of rotation of the turntable. c) In addition, because the fly is walking out radially away from the center of rotation, there is a Coriolis force that tries to push the fly sideways off it’s straight path. So, there is a component of the friction force that acts perpendicular to the radius of the turntable that opposes this Coriolis force It always points in the direction of rotation of the turntable.

4. Normal force – the force from the turntable that pushes back on the fly, counterbalancing its weight.

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  Subject Author Date
HintKenny M2018-05-05 18:40:22
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