M-K4: Independence of Free Fall and Initial Velocity

PIRA: 1D60.30

These demonstrations are based on the old "monkey and the hunter" problem, the one in which the monkey conveniently drops just as the hunter fires a gun aimed at it, so that the bullet (however fast or slow) and the monkey drop the same distances as the bullet approaches the monkey. The demonstration, however, is less bloody!

There is a nice segment on the Mechanical Universe videodisk in which a plastic dart is fired at a cloth monkey. It is located on videodisk 6, beginning at frame 42375, and lasts a total of only 60 seconds, counting the explanation of the setup. The flight of the dart begins at frame 43882. It is good to show the flight in slow motion using the SLOW bar on the player or by manually pressing the STEP bar at whatever rate you want. Press the STILL bar at any time to stop the motion completely. This is more dramatic in slow motion, and the parabolic arc of the dart’s flight is readily seen.

Equipment: Spring gun with projectile, target card, electromagnetic target holder, DC power supply (wooden box type). The gun and targets are in Set 10 Cabinet 3 Shelf 4. The board is kept in hallway between rooms 3 and 5. (Recommended board for replacement is 3/4-inch thick, 5.5-inch wide, 6-foot long soft pine or white pint or smooth pine with no knots.)

Setup: Connect the leads from the apparatus to the power supply. Orient the blackboard in front of which the target falls so that its surface is at right angles to the plane of the intended flight path of the projectile. Close the trigger switch at the muzzle of the gun and hang the target card from the magnet by the smaller iron clip on the card. (If held with the other end up, the target is likely to fall with a falling leaf pattern.)

Adjust the voltage of the power supply so that the magnet holds the target consistently but with very little excess force. Check the trigger switch to see that it operates smoothly and does not bind. Cock the gun as follows. Hold down the trigger and twist the handle on the knob at the breach counterclockwise (as seen from the rear) so that it clears the notch in the barrel which holds it, pull back as far as it will go, push it forward to the limit, and twist the handle clockwise till it is firmly held in the notch again. While holding the gun horizontally, load it by carefully inserting projectile at the muzzle and tilting the gun slightly upward so that the projectile slides gently down the barrel and comes to rest against the cocked spring inside. Establish by test firing whether the sight should be fine, coarse or medium. Disconnect the power source. Never leave the gun cocked or loaded!

Procedure: Reconnect power source and demonstrate the fall of the target by opening the trigger switch. Also, show by holding the projectile at the muzzle of the gun that the trigger switch is opened by the front end of the projectile at the moment its rear end leaves the support of the gun barrel, hence both projectile and target start falling freely at the same instant. Close the trigger switch, hang the target, cock the gun, and load the projectile. YOU now have a dangerous weapon, do not point it where damage could occur if the gun is fired.

Here it is good psychology to select a student to do the actual firing. Participation by a student increases the interest of the whole group and lends assurance that there is no prestidigitation. Choose one preferably from the front or second row, or from an aisle seat, since less disturbance will be caused by their coming to the front. If seating is not assigned, students in front are often the more interested ones. Select one who is not nervous or jittery.

Before handing the student the gun (cocked and loaded), tell them: 1) whether the sight is fine, coarse or medium, 2) that the outside trigger operates with very little pressure so keep the trigger finger outside the guard until they are ready to fire, and 3) THE GUN IS NEVER TO BE AIMED AT THE AUDIENCE AT ANY TIME. If the student is tall, suggest a medium range for them to target; if they are short, or elects to shoot from a low position, choose a range near the limit set by the length of the

connecting wires. This policy will aid in having the projectile reach the target in a horizontal position and hence increase the probability of pinning the target to the backboard. Usually the student will succeed in striking the falling target and often in pinning it to the board.

Immediately after the demonstration disconnect the power source, because the holding magnet can get very hot, and place the projectile where a student cannot get it after class without your knowing it. If the gun is left cocked by a student who examines the gun, put in the projectile and fire into the board. It is not wise to leave the gun cocked, nor is it wise to fire the gun without the projectile.

Of this older version of this demonstration, Percy Carr wrote in 1970: "This is the best piece of equipment for the purpose I have seen. It was designed by L.B. Spinney, a member of the ISU Physics staff starting in 1893 and head of the department from 1897 to 1930."

Equipment: The newer setup is kept in Set 8, Cabinet 3, Shelf 3. It consists of a launcher and photogate which is aimed at an elevation of 15 degrees, a control box, and a drop box and target to be mounted about 12 feet away on a ring stand at a height of about 7 feet three inches.

Procedure: Sight through the launcher to line up on the target. Use the ramrod to load the ball to the long range position. Switch the control box to ARM. Pull the yellow string straight up to launch the ball.