Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Guinea and feather

Demonstration

American astronauts on the Moon repeated Galileo's classic experiment.

Apparatus and materials

Vacuum pump

Hoffman clip

Rubber pressure tubing, approx 30cm length

Rubber disc, small

Paper, small piece

Tubing, Perspex or glass, approximately 5cm in diameter, 60cm long

Safety screen

Health & Safety and Technical notes


Glass tubing should have ends which are carefully annealed to withstand small shocks. Before evacuating the tube, ensure that there are no cracks and the outside should be covered in a spiral of Sellotape to help if the tube should shatter. 

Use a safety screen between the demonstration and the class, and eye protection for the demonstrator. 

One end of the tube is sealed with a solid bung. The other has a one-hole bung with a glass tube through it. The glass is connected with pressure tubing to the vacuum pump.

 

Procedure


a Place the rubber disc and piece of paper in the tube, letting them fall onto the solid bung. Seal the tube and close the Hoffman clip on the pressure tubing. 

b Invert the tube rapidly and watch the motion of both paper and rubber disc. 
 
c Repeat he same procedure after evacuating the tube and compare the motions with those when the tube was full of air. 


Teaching notes


Pressure tubing apparatus

1 With air in the tube, the paper flutters down and arrives after the rubber disc. Once the tube has been evacuated, the paper and disc will arrive together. 

The conclusion: in the absence of air resistance, objects of quite different mass fall in exactly the same way. Acceleration does not depend on the mass of a falling body. For an explanation, see the Teaching notes to the experiment Falling objects
 
2 It is possible to 'hear' when the tube is evacuated, as the pump becomes more and more quiet in its action. 
 
If students ask for evidence that there is a vacuum in the tube, then once the experiment is completed, place the pressure tube under coloured water. As soon as you open the Hoffman clip water will rush into the tube, propelled by atmospheric pressure. The air left in the tube with the water is air that the pump was unable to remove. 
 
3 There is a fable that Galileo gave a wonderful demonstration from the Leaning Tower of Pisa. The story says that he dropped a little iron ball and a big cannon ball side by side. Everyone was astonished, and some even angry, to see that they arrived at the ground together. They expected the cannon ball, which weighed ten times as much, to fall ten times faster. For this is what Aristotle had taught: heavier objects fall faster. 
 
Galileo could not produce a vacuum to prove that the removal of air would allow objects of different mass to fall exactly together but he was sure it would be the case. 
 
Instead, he invented a thought experiment to prove Aristotle wrong. Galileo said: Suppose I let three equal bricks fall to the ground, starting together neck and neck. They are all the same. They all fall with the same motion and all arrive together. Followers of Aristotle agreed. 
 
Now suppose I repeat the experiment but first tie together two of the bricks with a light, invisible chain, so light that it isn't really there. Then I suppose I have a brick and a double brick. According to Aristotle, the double brick will fall twice as fast. Is that likely, just because there is a little chain there? 
 
Ah yes, one of Aristotle's followers might say, one of the pair of bricks gets a little ahead and drags the other one down faster than the single brick. 
 
Oh I see, Galileo would reply, one of the pair gets a little behind and drags the other backwards making it fall slower! 
 
Galileo had shown that Aristotle's thinking produced two different predictions. He made his opponents furious by making their arguments look foolish. 

This experiment was safety-checked in April 2005