Learning objective: acceleration of gravity is independent of horizontal movement. An object traveling sideways while falling will fall at the same rate as an object dropped straight down.

The equipment made for this experiment can also be used for other physics demonstrations. For example, the ballistics pendulum, which proves conservation of momentum. That's another instructable.

This is a classic physics experiment. The "hunter" aims his gun barrel directly at a monkey in a tree. (there are no elevated sights) When he pulls the trigger, the gun goes off. At the EXACT same moment, the monkey drops from the tree. Being a long range shot, there is a significant delay before the projectile reaches the monkey. The bullet "falls" in a parabolic arc while the monkey falls straight down. Because they fall for the same period of time, the monkey gets hit.

I suggest printing these directions so you can reference them while watching the video. Use "pause" feature as needed.

Making the Gun: (I'll include directions for a simpler version for non metal workers at the end)
1) Find a piece of tubing that fits a tennis ball or other projectile. Tubing from a basement support jack worked for me. If no snug fit can be found, a projectile can be made to size from hot glue, a potato, etc.

2) Cut a hole in an air tank with a hole saw. Other materials can be used for an air tank, but I no longer recommend PVC or soda bottles. They are too likely to burst.

3) Weld a pipe flange to air tank. If you do not already weld, I do NOT recommend this as your first project. I'll also write up instructions for non-metalworkers.

4)Making the welds airtight is difficult. Covering leaks with brazing and application of pipe dope (from plumbing supply) will allow you to make the seal complete. Pipe dope is applied from inside the tank. The pressure will push it into any tiny holes.

5) Connect airtank to sprinkler valve with standard plumbing nipple, using 4 turns of teflon tape + pipe dope.

6) Connect Valve to gun barrel. My particular barrel was NEARLY a snug fit to a bell reducer (standard plumbing fitting) Electrical tape applied outside the reducer made it snug. This connection doesn't have to be 100% leakproof. Lots of air flows into the barrel VERY  quickly, so losing a bit from the base of the barrel isn't a big deal. If your barrel is standard plumbing parts, it'll be more airtight. I, however wanted to use the tube which tennis balls fit.

7)The sprinkler valve will go in an ammo box. Cut holes in a steel ammo box with a hole saw. These accomodate the pipes into and out of the valve. Also drill a bunch of small holes in the box. If the sprinkler valve should fail, you want the air to escape, but not the pieces of valve. A proper pneumatic valve could be used in place of the sprinkler valve. If you want to spend $100 on this, you can get a metal valve which is rated for air. Sprinkler valves are not rated for use with compressed air, so the metal ammo can allows for added peace of mind.

8) All of the plumbing is now connected. Using measurements from this assembly, make wood cradles to support the components. you don't want strain on the plumbing connections.

9) Screw all the cradles to a plank and secure the tank, valve and barrel to the cradles with plumber's tape (sheet metal strip with holes)

For adjusting the aim, I Set it on a toolbox and tablesaw outfeed support. You could simply adjust it on sawhorses. 

Making the electromagnet
1) Find a bolt, 2 nuts and 2 fender washers. Cover the washers and bolt with plastic tape. This will prevent the coils shorting out if cracks develop in the magnet wire's varinsh insulation. Assemble bolt, nuts and washers to make something that looks like a spool.

2) Mount your supply of magnet wire on an axle so it can freely turn. Secure the bolt assembly in a drill. I used a cordless. Wind the wire onto your spool assembly.  A drill press or lathe could also be used. If you don't have any of these, a coil can be turned by hand, it will just take time.

3) Tape over the coil to keep it from unwinding. 2 leads should be accessible. scrape/sandpaper/burn the insulation from these leads. Solder some heavier wire to these leads, and secure the heavy wire to the coil with tape. This will prevent strain from being put directly on the delicate magnet wire.

4) Test to see if the electromagnet is strong enough to hold up the can.

The "Monkey" Target
1) Cut both ends from a tin can.

2) Stretch panty hose over 1 end to catch the ball. Tissue paper over the other end will give a surface that a face or target can be drawn on.

3) My magnet was not strong enough to hold the can up until I drilled a hole in the can and put a bolt through this. A piece of plumbers tape bent into a bracket and secured to this bolt keeps the can from rotating while suspended from the magnet.

Experiment Setup
1) The electromagnet is suspended fairly high. I used extension cord to connect it. It is heavy gauge, and long enough for the purpose. Do not apply house current to the magnet. An 18 V battery is enough, and 110 V would likey burn up the magnet. 

2) Another piece of magnet wire will be stretched across the muzzle of the gun. (use tape to insulate this FROM the barrel if the barrel is metal) When the short piece of wire is broken by the ball being launched, the electromagnet will drop the can. Therefore, magnet, battery and the short wire are connected in series. The same battery, or another one can be used to launch the ball. Using a mirror is a good way to sight along the barrel to the target.

Simpler Gun for Non Metal Workers (I'll post pics in another instructable)
Projectile: 9/16" ball bearing.
Barrel: 1/2" copper tubing, _Type M_.   Type L has a thicker wall and won't fit the ball bearing.
Hardware stores will cut pipe to length. 2 to 3 feet will work. Tubing cutters leave a burr inside the pipe. Deburr the top end. Having a burr at the bottom is actually good, since it will keep the ball from falling out the bottom end.
Mouthpiece: 5/8" ID clear flexible tubing. Since this is flexible, you won't mess up the aim. It's a tight fit over the bottom end of the copper pipe.
Barrel Supports: 2 Test tube stands borrowed from the chemistry department.  The type that clamps down on a test tube and is adjustable for height and angle.
Just as with the large gun, the connections for the wire at the end of the barrel need to be insulated from the barrel. 2 insulated alligator clips do this nicely.



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I'm certainly no expert in this, but If I remember high school science correctly this would have to be in a vacuum to work perfectly. the larger area of the can would cause it to drop slower than the ball because of wind drag.
Actuallly, I would like more pictorial detail on the gun. I want to make a gun to launch a rope into a tree to assist with felling the tree acurately.

True about vacuum, but sometimes it's a matter of "close enough" You'd probably start seeing air resistance problems with greater than 20 feet.

Are there specific parts of the gun you'd like more pics of? I'll see what I can do.

How high up the tree do you need to place a line?

I guess if I could get a rope 20 to 30 feet up it would meet most any situation. I was thinking of a potato gun type thing with a fishing reel attached to pay out line.
I guess in re reading this whole thing, the operation is fairly simple. tank-valve-barrel. I was just having a hard time following what you were doing without the step by step pictures.
I like that you used metal instead of plastic, and a tennis ball seems like a great projectile for my purposes.

In my experiment, I'm pretty sure I used a water filled tennis ball. Weight is good. Hypodermic needle gets it filled. Squeezing air out of ball, then submerging it seems to work quickest unless you have a big horse syringe.

For your purposes (not real high or great accuracy needed) you might try simply swinging the ball by the rope then releasing it.

In "monkey hunter" I simply laid out line on the ground such that it would not be tangled. For other ways to play out line, also look at my -ible, "harpoon/grappling hook" or there's a guy on site who uses the rim of a bucket on his bow for archery fishing. This acts as kind of a 14 inch dia fish reel:
You could put a blunt tip and tennis ball over the tip to protect the neighbors.

I'm always lookin for the simplest quickest solution unless I want complex for art-entertainment-science.