The 'apple and arrow' experiment
This lovely demonstration, which can be difficult to set up, shows that two objects which start to fall from the same height at the same time always have the same height as they fall. Also called Monkey and Hunter - see the free video at the National STEM Centre website.
Apparatus and materials
Dynamics trolley, spring loaded, with rod ‘trigger’ for spring release
Grooved launching ramp with fixed bulldog clips
Launching platform (a drawing board or other plank of wood)
Variable voltage supply
Coil (120 turns) and C-core for coil, or other electromagnet that will hold and quickly release the can
Clamp and stand
'Tin’ can (about 10 cm diameter)
Connecting leads, 2.5 m with crocodile clips
Aluminium foil, thin
Ammeter (0 - 1 amp), DC
Health & Safety and Technical notes
Beware of overheating of the wires - limit the current through the coil to 0.7 A.
Wear goggles in case of ricochets.
For those poor at aiming use a larger can.
It is possible to turn the can on its side and line the can with soft material so that the marble doesn't rebound when it is caught.
a Fix the launching ramp and dynamics trolley to the launching platform (a drawing board or other plank of wood). Place a small strip of thin aluminium foil between the bulldog clips. To ensure that the foil breaks, nick it so that it is almost cut across. Put the whole arrangement about 1 metre above the bench, fixed horizontally.
b The projectile is an ordinary marble. Place it in the groove of the launching ramp, level with the spring of the trolley. On release of the trigger, the spring hits the marble and fires it forward.
c The 'apple' is provided by the tin can. To hold it, use the C-core and 120-turn coil. Connect the coil to the variable voltage supply, with the aluminium foil as part of the circuit. The foil will act as a switch when it breaks.
d Set up the 'apple' about 2-3 metres in front of the launching ramp. Adjust its height so that the launching ramp is aiming directly at it.
1 The demonstration is meant to show that the arrow and the apple both fall equal amounts vertically in the same time. It is essential for students to see clearly beforehand that the arrow is aimed straight at the apple.
2 Do the experiment firing horizontally first. Afterwards you could do it with an inclined launching ramp.
3 You need to show that the apple falls when the circuit is broken, and that the circuit is broken when the marble bullet is fired.
4 It is a good idea to rehearse this experiment two or three times before demonstrating it to a class. The class will probably want to see the experiment repeated.
5 For an alternative firing mechanism, use a pea shooter (some equipment kits contain a pea shooter).
This experiment was safety-checked in March 2005