Tuesday, 10 June 2014 14:02


Additional Info

  • ID Code: C4-32
  • Purpose: Demonstrate that bodies of extremely different densities fall with equal acceleration in the absence of air friction.
  • Description:
    This demonstration consists principally of a long glass tube containing a heavy disc and a brightly coloured feather. A nozzle and valve at one end of the glass tube allows the air to be removed from the tube using a vacuum pump. This allows the objects to fall with or without air resistance.
    • • Turn the tube vertically while still filled with air; show that the disc drops rapidly to the bottom end, and the feather flutters down slowly.
    • • Invite students to predict how this behaviour will change when the air is removed.
    • • Connect the pump and pump out most of the air. There will be an audible change in pitch when the tube is sufficiently evacuated, after 1-2 minutes.
    • • Turn the tube vertically again, and let the students see that both now fall at the same rate.
    • • CAUTION: The tube is thick glass; please handle with care.

    The key physics in play here is twofold. Absent other forces, the two objects undergo the exact same acceleration in free fall, and so will fall at the same rate. With no air in the tube, the only force acting on them is gravity, which pulls downward on each object proportional to its mass.

    However, when air is in the tube, there is a second force involved: air resistance.

    The force of air resistance pushes upwards on the falling objects. It depends on two factors: the surface area of the falling object, and its velocity. So the faster they fall, the more resistance they face from the air. But recall that the force of gravity is proportional to the mass of the object, and the net acceleration of an object is the result of the sum of the forces acting on it. So if two objects have similar surface area, but one has a higher mass, then the higher mass one experiences a larger downward force than the other, while air resistance will exert close to the same upward force on both, and so the heavier object then has a greater acceleration. And that’s what we see when the tube is full of air – the more massive disc falls faster than the less massive feather. Take away the air and the force of air resistance, and they fall together!

    Also see demonstration C4-33 a similar but larger demonstration for use in lecture halls.
  • Availability: Available
  • Loc codes: C4, FS1
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