Shoot the Monkey

Monkey released from platform is shot by simultaneously fired cannon.

What it shows:

This is a demonstration of the independence of the horizontal and vertical components of velocity of a projectile. Often referred to as "the monkey and hunter," the problem is the following. A hunter (at ground level) aims a gun at a monkey hanging from a branch high in a tree. The monkey, being very intelligent, does not want to be shot. It knows that light travels faster than bullets and reasons that, if it lets go of the branch the instant it sees the flash of the gun, it will fall down far enough before the bullet reaches its destination and thus miss its mark. What happens? Unhappily the monkey does not know that the gravitational acceleration is the same for all falling objects (whether they be bullets or monkeys) and thus the bullet falls down the same distance as the monkey, hitting it on the way down. The bullet's initial velocity, or the distance between hunter and monkey, is irrelevant to the final outcome and affects only the height above the ground where bullet meets monkey.

How it works:

The monkey (Curious George stuffed animal) is electromagnetically suspended from a hydraulic lift (HiJacker™). The gun is a spring-powered cannon that shoots a 1.5" (3.81cm) diameter steel ball. A laser beam is adjusted to precisely shine down the center of the cannon barrel and is used to aim the cannon at the monkey. As the cannon ball exits the barrel it passes through an infra-red photo gate which triggers a circuit that turns off the electromagnet and releases the monkey. The projectile follows a parabolic trajectory and strikes the monkey on its way down.

shoot the monkey

The brass cannon (mounted on a large bench) is a scaled up version of the one published in Meiners1 and the reader should consult that reference for construction details. It can be set between 12.5 and 22.5 degrees w.r.t. the horizontal and has a ±5cm accuracy of hitting the target at a distance of 8 meters. Adjustability of the initial speed of the projectile allows the collision to occur at any chosen height, including close to the floor. This is accomplished by setting the spring compression with the knurled knob at the rear of the cannon. Pulling back on the trigger handles releases the steel ball projectile. Being a strong spring, it takes some considerable force to compress it as well as release it (shooting), and the lecturer should practice the technique without moving the cannon. Also, note that once loaded, it's not possible to check the aim of the cannon because the steel ball blocks the laser beam.
photo gate
The steel ball passes through an infra-red photo gate as it exits the mouth of the cannon. This turns off the current to the electromagnet, releasing the monkey. The lecturer need only worry about the toggle switch on the control box. The switch must be in the LOAD position to load the cannon (this bypasses the switching transistor and the monkey won't be released). To enable the photo gate circuitry, the toggle switch is switched to the FIRE position. After interrupting the electromagnet current, the circuit automatically resets itself and reestablishes power in 1.5 seconds. Circuit details are given following the Comments.
target monkey
Our Curious George measures 45 cm from head to toes. A small piece of iron has been sewed onto his hat so that he can be suspended from an electromagnet. Although the use of Curious George adds the obvious theatrical appeal to the demonstration, he is sure to arouse some sentimental memories in the audience and, animal rights activists or not, these feelings are usually accompanied by hisses and boos. You can assure the audience that he will not be harmed because of the large (13cm × 19cm) metal shield strapped across his torso (in fact, the purpose of the shield is to provide for an audible "hit"...not all members of the audience have ideal viewing angles of the collision and sometimes it's hard to tell whether or not the target was hit - the loud sound of the steel ball striking the metal shield makes it entirely unambiguous!). Note that, given the inaccuracy of the cannon, the size of the shield also insures a "hit" every time.

The electromagnet is mounted on the hydraulic lift allowing for ease of height adjustment. Cannon and lift are positioned on opposite sides of the front of the lecture hall and the distance between them is pretty much dictated by the size of the hall. Rather than change the cannon angle for aiming purposes, it's typically left at maximum inclination (22.5 deg) and the target height is adjusted accordingly.

Setting it up:

Accurate laser alignment is absolutely essential and should not be left to the last minute! It's not difficult, but take your time to get it right. A clear plastic plug, with a cross-hair in the center, fits snugly inside the mouth of the cannon. Clear tape is stuck over the small hole in the rear of the cannon. Adjust the three screws near the front of the laser to center the beam in the rear of the cannon. Adjust the three screws near the rear of the laser to center the beam on the cross-hair in the front of the cannon. This will slightly misalign the first adjustment, so readjust the front laser screws, etc. It's an iterative process and three or four iterations should do the job.

Having chosen the location for the cannon, the main concern is that the cannon cart is not easily moved. It helps to push its back against the front row of seats. Being on casters, the front needs to be blocked up with a 2×4 to keep it from rolling. The hydraulic lift is positioned on the opposite side of the floor space and, as a precaution against a potentially harmful ricochet, as far away from the front row as practical. Caution: although the cannon has never misfired, it would be wise to not allow anyone to walk in front of the loaded cannon.

shoot the monkey

shoot the monkey

shoot the monkey

shoot the monkey


The demonstration is not only pedagogically important but also historically significant because it was Galileo who first perfected the subject of projectile motion and computed tables of ranges and heights of trajectories for the use of cannon and mortar. Until Galileo, the Aristotelian view of motion prevailed: projectiles first moved with violent motion (produced by the propelling force) and ended up falling down with natural motion (free fall), and underwent some kind of mixed motion in between. The Galilean concept of parabolic trajectories, which may be thought of as the superposition of two separate motions, combined and followed simultaneously by the projectile, is an abstract concept and the preconception of the beginning physics student is probably a more Aristotelian view. It may very well be that part of the amazement in this experiment lies in the dispelling of that view. Or maybe the demonstration just brings out the worst in us as we delight in killing the ape. Psychology aside, this is one of those classic demonstrations that is simply a "must." It's always a big hit.

shoot the monkey

Circuit Notes:

· The photo gate is an infrared emitter/detector pair (Radio Shack Cat. No. 276-142). The pair is mounted at the mouth of the cannon with a separation of 1.75" (4.4 cm) between the LED and the photo transistor. The photo transistor behaves as a variable resistor; the more light intensity, the less the collector/emitter resistance. The 15 kΩ resistor sets the sensitivity and the collector output is 0.3 volt with light illumination (at this separation) and 5.0 volt with the light blocked. Use the IR sensitive card to check the LED output.

· The voltage divider on the inverting input of the comparator (LM311) sets the threshold at 1.7 volts and the output is 0 and 5 volts for light and no light, respectively.

· The one-shot (74121) is a monostable multivibrator configured so that the input (B) is positive-going edge triggered and its output is a 1.5 sec long pulse (pulse length ≈ RC). The high impedance output is buffered by an FET.

· The N-channel Enhancement-Mode Vertical DMOS Power FET (VN0106N3) can handle a 0.5A continuous drain current (P=1W). Its gate is protected by a 1k resistor and a 15V zener. Normally off, the one-shot pulse turns the FET on for 1.5 sec so the drain is pulled down close to ground (= 0.02V, determined by the 4 Ω D/S on resistance in series with the 1k pull up resistor) for this period. This turns the power transistor off for 1.5 seconds.

· The power transistor is a TIP120 Darlington (Radio Shack Cat. No. 276-2068), capable of dissipating 65W with a continuous 5.0A collector current. Maximum collector-emitter voltage is 60V, which determines the resistor in parallel with the electromagnet. A switch on the control panel ties the collector to ground (in the LOAD position) so that the electromagnet current is not interrupted when the photo gate is triggered (in the process of loading the cannon).

· The dc resistance of the electromagnet is 1.07 Ω (plus 0.6 Ω leads) and its inductance is 6 mH (measured at 1 kHz). In the interest of collapsing the magnetic field as rapidly as possible, it is desirable to shunt it with a high resistance (current decay time const. = L/R). However, the breakdown voltage of the power transistor limits the allowable back emf and thus the shunt resistance. Typically only 1.5 A are necessary to suspend the monkey, but a worst case scenario is when the maximum current (3.6 A) must be dumped. In this case the breakdown voltage (60) equals the supply voltage (6) plus the IR voltage (I=3.6A) and R turns out to be 15 Ω. This gives a time constant of 0.4 ms, which is plenty fast to release the monkey.

· Although not shown, the 5 volt regulated and adjustable 0 to 6 volt unregulated power supplies are an integral part of the photo gate control box.

1 Physics Demonstration Experiments, Harry Meiners, editor, (Ronald Press, NY, 1970) Vol. I, p. 130. Most institutions have some version of this cannon, whether it be a compressed-air or rubber-band powered. Even references as early as Demonstration Experiments in Physics, Richard Sutton, editor (McGraw-Hill, NY, 1938) pp 43-44 give five different variations of this experiment.