1D60.30-Monkey Hunter


“Note: When shooting the gun, make sure to clear out the first 3-4 rows on the side of the room where the monkey is.  If the ball ricochets, it could seriously injure a student in the first few rows.”

Video: Watch this demo | Demo #2

Description: If a hunter aims his gun at the monkey, and the monkey lets go the instant the hunter fires, will the monkey be safe?
The monkey took physics 131 and says he can escape the bullet if he lets go the instant the hunter shoots.

If we neglect gravity, the bullet will travel in a straight path and hit the monkey as the laser shows.
Taking gravity into account, both the monkey and bullet fall with the same vertical acceleration for the same amount of time.
Thus, when they hit, both have fallen the same distance from the no gravity situation.

The other explanation (which makes this a useful demonstration for relativity) is that within the reference frame of a free-falling body,
the monkey will be stationary and the bullet will come toward it along a straight-line path. The reference frame of the earth will move upward,
but there is no difference in frame between the monkey and the bullet, so the monkey will get hit.


  • Stuffed monkey, with foam shield and target on the chest, and metal plate attached to the head.
  • Monkey gun (metal apparatus, laser sight, air chamber, trigger, long wire to electromagnet, wheeling cart)
  • Power supply capable of putting out 10 – 20V.
  • Air compressor.
  • Several ball-bearings of exactly the right size for the barrel of the gun.

Setup Procedure:

    1. Fully charge the air compressor BEFORE getting ready to move the setup into the class.
  1. Using banana plugs, connect the power supply to the monkey gun. Make sure both switches are in the “ON” position, and both lights come on when the gun is plugged in.
  2. Connect the fully charged air compressor to the gun chamber.
  3. Set the compressor gauge to the appropriate level. Room 20 may need 50 psi, rooms 124, 126, and 134 probably more like 35 – 40 psi, and a smaller room such as 138 only about 25 psi.
  4. Leave the valve to the air chamber open, but DO NOT load the gun!
  5. Connect the long wire from the electromagnet to the gun.
  6. Hang the electromagnet from the ceiling as far from the gun as possible.
  7. Turn on the laser sight and align the gun to be pointing at the bull’s-eye of the target.
  8. Turn off the laser once the gun is properly aligned.
  9. Instruct the professor on the firing procedure (see below), but DO NOT LOAD THE GUN. Always have the professor do this JUST before using the gun, so that it is never left loaded and unfired after the class is over.

Demonstration Procedure:

  1. CLEAR OUT THE FIRST FEW ROWS. Though this rarely happens, if the ball ricochets it could injure a student in the first few rows.
  2. Turn on the laser and make sure it is on the target.
  3. If air valve is closed (perpendicular to the hose) then open by turning it parallel to the hose.
  4. Leave the valve open for 5 seconds to fully charge the gun chamber.
  5. Close the air valve.
  6. Load the ball-bearing into the end of the gun barrel.
  7. Tamp down the ball bearing using a rod.
  8. To fire, press and HOLD the red button on the trigger. If you do not hold the trigger down, the ball will not fire cleanly.
  9. If you want to repeat, re-hang the monkey and go back to step 1.

Have students clear out of the first few rows near where the monkey is.
There is a chance the ball could ricochet off something.
The lecture prep staff will never arm the gun for you. This is something you must do right before you’re ready to shoot it.