Acceleration, falling and drag

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What this model shows

The drop tube: a steel ball 2.4 cm across and a feather fall 1.00 m, with g = 9.8 m/s² everywhere in the tube. Air resistance grows with the square of the speed. The feather’s terminal velocity in air is 0.8 m/s and the ball’s about 60 m/s, so over 1 m the air resistance on the ball stays tiny. The pump leaves a thousandth of the air, so then both fall freely. A real feather flutters and turns as it falls; this one falls steadily, tilted so that you can see it.

Each drop is shown 5 times slower than real so that you can watch it; the strobe flashes (every 0.1 s), the graph and the times are real.

The glycerol tube: a 6 mm steel ball, drawn larger so that you can see it, reaches its terminal velocity of 0.070 m/s within a few hundredths of a second, long before the top band. At that speed the drag and the upthrust of the glycerol together balance its weight. It sinks in real time.

In a school the vacuum tube is a teacher demonstration: a strong tube, held firmly, behind a safety screen. This is simulated apparatus.

Reduce motion

With reduced motion each drop is drawn complete at once, and the simulated timer times the ball in the glycerol.

Simulated timer (it presses the stop-clock for you)Equipment

Select equipment to read its description in the card.

steel ball

feather

weight

air resistance

100

kPa

0.20 m

0.40 m

0.60 m

0.80 m

0.00

Speed–time graph · air in the tube

air in the tube: 100 kPa

Step 1 · Free fall and drag

Ball and feather in air

A steel ball and a feather wait in the catch near the top of a tall tube full of air. When the catch opens, both fall 1.00 m to the floor of the tube. A strobe lamp flashes 10 times a second, and each flash leaves a faint image of each object. Which reaches the floor first?

Equipment

Strobe lamp

It flashes 10 times a second while the objects fall. Each flash leaves a faint image of each object, as on a long-exposure photograph, so the gaps show how far each one fell in 0.1 s.