The ‘monkey-and-hunter’ experiment

Class demonstration

This demonstration shows that a projectile fired horizontally and an object dropped vertically fall at the same rate. If the monkey had known that, it might have acted differently.

Apparatus and Materials

For the demonstration...

• Tin (steel) can, decorated to indicate the monkey
• Tube and small ball (to act as gun and bullet)
• Electromagnet (e.g. coil and C-core)
• 2 crocodile clips
• Strip of thin aluminium foil
• Low voltage power supply (0-12 V d.c.)
• Long connecting wires
• Clamps and stands

Health & Safety and Technical Notes

Read our standard health & safety guidance

Ensure that students are well clear of the area between the gun and the monkey, so that they can see clearly and are unlikely to be hit by a misfired or deflected ‘bullet’. Notes on the equipment:

• The aluminium foil strip must break readily as the bullet passes through. It can help to cut the strip and press the two free ends together so that they overlap slightly. When the circuit is complete and the current flows, the magnetic field produced will hold the two strips together.
• To avoid any delay in the dropping of the monkey, reduce the voltage of the supply until the monkey/can is just held in place.
• The ‘bullet’ should be sufficiently heavy that air resistance does not affect its motion.

Procedure

1. Set up the electromagnet at a high point at one side of the room.
2. Connect it in a series circuit with its power supply and the strip of foil. The long connecting wires are needed because the ‘gun’ will be at the opposite side of the room.
3. Mount the foil strip across the mouth of the tube which acts as a gun.
4. Ensure that the tube is aligned so that it is pointing directly at the monkey. Ask a student to confirm this alignment.
5. Switch on the power supply and hang the monkey from the electromagnet. Turn down the current until the monkey just adheres to the magnet. (The monkey must drop as soon as the circuit is broken; any residual magnetism in the coil will delay its release.)
6. Fire the ‘bullet’ from the gun. This will break the foil, stopping the current to the electromagnet and causing the monkey to drop.

Teaching Notes

• The key to understanding this demonstration is the idea that both the bullet and the monkey start to fall at the same time. This happens because they both start moving freely under gravity at the same instant. They have the same vertical acceleration g ; the horizontal motion of the bullet is irrelevant.
• In the film above, the gun is shown pointing horizontally at the monkey. This is not essential, but it is a good way to start. The hunter could be down on the ground while the monkey is on a high branch. Imagine a straight line from the gun to the monkey; as the bullet flies through the air, its downward acceleration pulls it down below this line.

• After the demonstration, ask the students to suggest how a wiser monkey might have acted.
• This demonstration is often used as a popular exhibit at open days and school recruitment sessions.
• Here is a suggested teaching sequence which deals with projectile motion, including the maths of projectiles:

  Projectile Motion

• The ‘apple-and-arrow’ is a similar demonstration with further information about how to carry it out.

  The apple and arrow experiment