The Projectile Motion Demo

P115/P116 Laboratory Demonstration #2

Summary:

* Shoot a 2 cm diameter projectile 2-4 meters.
* Measure the range and/or muzzle velocity.
* Predict the maximum height.
* Fire the projectile through the hoop

You can click on the image if you wish to download it to your own computer. It is in gif format.

Further information on Theory, Apparatus, Procedure, and Helpful Hints is available. Equipment is stored in Rockerfeller room 302 (cabinet B.) Also, if timing the projectile, you will need a Sophlab photogate timing set-up with a Sophlab PC found in Room 402 or 403.

The purpose is to introduce simultaneous motion in multiple dimensions as well as equations for dealing with the special case of projectile motion.

Theory

A simple projectile travels in two directions simultaneously, each of which can be represented by a separate vector. These vectors are both time-dependent, and their values can be related to one another at particular times to give position coordinates.

Apparatus

The equipment needed for this demo is:

* a computer
* two microphones that interface with the computer
* timing software that utilizes the microphones
* a projectile launcher
* a box half-full of wadded paper balls
* a projectile launcher
* a two-meter stick
* a loop about ten centimeters in diameter.

Procedure

A ball is fired from the projectile launcher across the room into a box. A microphone near the launcher starts a timing program in the computer, which clocks the flight until the impact of the ball on the back of the receiving box. Another microphone inside the box stops the computer.

The distance between the cannon's muzzle and the impact point in the box divided by the time gives the horizontal velocity. The horizontal velocity can then be used to relate a given time to a horizontal discplacement. It can also be related to the vertical discplacement at the same point in time (remembering to take into account the initial height of the cannon) by the initial vertical velocity V(y)=V(x)/tan(theta).

Now a loop can be clamped to a two-meter stick at the correct vertical displacement, and a plamb line hung from it to correct the horizontal alignment. The cannon can be fired again (the student holding the loop should be wearing safety glasses) and the ball should pass directly through the center of the loop on its flight.

Helpful Hints

1) Noise: The noise in the microphones needs to be muffled since the sounds involved are so loud. Place electrical tape over the crystal microphone cartridges and wrap the microphone up with paper or wrapping material.
2) Receiving Box: Before class, set the distance between the box and the projectile laucher so it will hit the back of the box each time. This way, you have a constant, set x-distance. Fill the box with wads of paper to keep the ball from bouncing out.
3) Launching: If the apparatus is mounted on something that jerks slightly, this will decrease the distance of the projectile. Tightening the mount or adding weight to make the mounting more stable will cause the projectile to fly farther. Set-up everything the same way every time.
4) Alignment: Put tape marks where each foot of the cannon's tripod should be placed. These can be used to realign everything after each shot.
5) Safety: Be careful. If the measurements are off just a little, the student holding the ring for the projectile to be fired through could end up with an injured finger. Wear safety goggles and be alert.

To return to the Mechanics Demo Catalogue .

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