M-EngMom2: Conservation of momentum, example: ballistic pendulum.

PIRA: 1M40.40

Equipment: specially made ballistic pendulum, meter stick with sliding

index mounted horizontally on small stand, spherical projectile with spikes

on one side, spring and rod mounted on stand to act as gun for the

projectile, short rod as index for potential energy of gun. The equipment

is located in Set 7, Cabinet 3, Shelf 1.

Procedure: "The weird wooden ballistic pendulum is supported by two cords attached to cross bars clamped to ring stands. It is very important that the cords be parallel so that as the pendulum swings no angular momentum is imparted to it by the constraints. The horizontal meter stick is mounted to a very short vertical stand rod so that it just misses the lowest part of the pendulum when the pendulum is at the lowest position. The sliding index, which measures the farthest right position reached by the pendulum in its swing, is set an inch or two from the end of the pendulum when hanging quietly. (Care must be observed that the pendulum will not rise above the index and hence lose contact before the end of the swing). At the left a suitable rod is mounted horizontally to a stand. The driving spring, which should fit loosely and freely, is placed on the rod as far as it will go. Next, the projectile, which has a hole bored through the center, is placed on the rod. Lastly, a short rod is mounted above, parallel to, and as close to the rod as possible. This serves as an index for compression of the spring.

"In operating the equipment the projectile is pushed back with one hand while the gun is held firmly in place with the other, thus compressing the spring until the ball is stopped by this index rod. This insures that the same energy will be imparted to the projectile upon its release for each trial demonstrated. The projectile should be aimed so that it strikes the ballistic pendulum near the center of the near face as shown. The separation between gun and the face of the pendulum should be a bit more than illustrated, but it should be as close as possible and still insure freedom of the projectile when it bounces back from the pendulum. The demonstrator must be on the alert to prevent the pendulum on its backswing from striking the gun as this would upset the geometry for subsequent trials.

"The experiment should be performed first with the spikes of the projectile in front so that the projectile sticks with the pendulum upon striking. This insures inelastic impact. Then without changing a thing (even leaving the sliding index where the pendulum left it) repeat the experiment with the spikes to the rear. This will insure that the projectile bounces back upon striking the pendulum (though the impact is far from being perfectly elastic). The sliding index will be pushed a surprising distance farther than for the inelastic case. Caution: it is important that the pendulum be just as quiet as can be when the projectile is fired. This may require that the ventilating fans be shut down or a place found on the lecture table where air currents are at a minimum.

"To add interest I recommend that the procedure be described to the students in considerable detail before the operation is carried out, even to pointing out that in the first case the sticking of the projectile to the pendulum must occur while in the second case the projectile must rebound to some extent. Then ask the class, after at least a minute to think, to indicate by show of hands their response to the following questions:

1) How many think the pendulum will go farther when the projectile sticks?

2) How many think the pendulum will go farther when the projectile bounces?

3) How many think there is no difference?

"Then if the tally is recorded on the board, either in numbers or estimated

fraction of the class, is significantly short of the number present, ask:

4) How many don't think?

"My experience has been overwhelming support for (1). This I think may be

due to golf instructions to ‘follow through’ on a stroke." — Percy Carr

(1970).