Movement of a Ball Bearing within a cavity inside a Projectile

[Gil Alard]

Summary:: Question concerning the behavior of a ball bearing inside a projectile fired straight up or at an arc.

Within a projectile is a 1-inch cylindrical cavity, inside of which is a steel ball bearing that can freely roll along the length of the cavity. When the projectile is fired, inertia keeps the ball at the back of the cavity. Other than a quick deceleration (such as the projectile striking a surface), is there any situation in which the ball moves to the front of the cavity during flight? I'm specifically puzzled as to the ball's position if the projectile is fired straight up. Does the ball move to the front of the cavity as the projectile slows, turns, and then accelerates to terminal velocity? What about a very steep arc, like 80°?

 

[jbriggs444]

Summary:: Question concerning the behavior of a ball bearing inside a projectile fired straight up or at an arc.

Within a projectile is a 1-inch cylindrical cavity, inside of which is a steel ball bearing that can freely roll along the length of the cavity. When the projectile is fired, inertia keeps the ball at the back of the cavity. Other than a quick deceleration (such as the projectile striking a surface), is there any situation in which the ball moves to the front of the cavity during flight? I'm specifically puzzled as to the ball's position if the projectile is fired straight up. Does the ball move to the front of the cavity as the projectile slows, turns, and then accelerates to terminal velocity? What about a very steep arc, like 80°?

Have you considered the effect of air resistance on the projectile?

In the absence of air resistance, why should the angle matter at all?

Edit:

In addition to air resistance (which is the elephant in the room), consider what happens if the ball and the projectile are less than perfectly rigid. While it is in the barrel of the gun, the ball is held strongly to the back of the cavity. This places a stress on the material and results in some minor deflection. Release the stress and there can be a rebound.

 

[Baluncore]

Last time I calculated the exterior ballistics of a projectile, the deceleration due to air resistance was 70 times the magnitude of the acceleration due to gravity. ( 70G ).

 

[joetheragman1775]

Last time I calculated the exterior ballistics of a projectile, the deceleration due to air resistance was 70 times the magnitude of the acceleration due to gravity. ( 70G ).

So there would be a shotgun effect. No different than a single projectile but in mass (normal shotgun in a wadding) with others, if you don’t count the marring of the shot surfaces. It could go just about anywhere or could it be controlled?

 

[Baluncore]

So there would be a shotgun effect. No different than a single projectile but in mass (normal shotgun in a wadding) with others, if you don’t count the marring of the shot surfaces. It could go just about anywhere or could it be controlled?

Welcome to PF.
The ball remains trapped within the closed projectile. The projectile trajectory and ball position is predictable.

Within a projectile is a 1-inch cylindrical cavity, inside of which is a steel ball bearing that can freely roll along the length of the cavity. When the projectile is fired, inertia keeps the ball at the back of the cavity. Other than a quick deceleration (such as the projectile striking a surface), is there any situation in which the ball moves to the front of the cavity during flight?

The ball will be at the rear of the cavity while accelerating in the barrel. Immediately the projectile departs the barrel, it begins to decelerate due to air drag, at a significantly greater rate than one g. The ball will therefore be forced against the front of the cavity and will remain there for the period of deceleration greater than 1 g.

If fired directly upwards, the projectile and ball will become unstable as the projectile begins to fall back. The aerodynamic shape of the projectile will decide how it falls, while the ball will sit at the lowest end of the cavity.