Potential Energy

[Kgpride]

Hello. Are our participants allowed to attach weight to their arm, as long as they don't overcome the launch arm resting on the ground and remain in a neutral position? We saw in the provided video that weights can't overcome the launch arm, but it wasn't explicitly mentioned whether smaller weights are okay to bring it to neutral. Thanks!

[knightmoves]

Hello. Are our participants allowed to attach weight to their arm, as long as they don't overcome the launch arm resting on the ground and remain in a neutral position? We saw in the provided video that weights can't overcome the launch arm, but it wasn't explicitly mentioned whether smaller weights are okay to bring it to neutral. Thanks!

If the rules don't say you can't do something, then you can do it.

In this case, the rules say that the device, without the counterweight and projectile, must not contribute energy to the launch.

Adding weight as you suggest to move the center of gravity of your arm close to the fulcrum does not do this, so long as the center of gravity remains on the projectile side. The rules do not say that your "launch arm" can't be made out of a big bit of wood plus some taped-on weights. It can me made however you like.

[Crtomir]

I want to bring up an issue with the recent FAQ:

CAN GRAVITATIONAL POTENTIAL ENERGY BE SUPPLIED BY THE PROJECTILE ITSELF OR MUST THE PROJECTILE BEGIN AT GROUND LEVEL BEFORE TRIGGERING?

No, the projectile may not be a source of gravitational potential energy. Please review the "Examples of Energy Violations" document on the Storm the Castle event page at soinc.org"

The well-intentioned event supervisors at a recent tournament disqualified our kids' whipper trebuchet because they claimed the ball initially started from above the lever arm. Since the ball started with initial potential energy (higher than the floor) and dropped down during the launch, it somehow added that potential energy to the launch of the ball.

First of all, physics! If the ball is released from a position higher than where it started, there is no possible way that the ball's own gravitational potential energy can add to the kinetic energy of the launch. A ball cannot roll out of it's own bowl unless additional energy is added to it. The ball does fall down, but then is released from a position way above it's initial starting point. Therefore, none of the initial potential energy of the ball could possibly be contributing to the launch velocity (kinetic energy) of the ball.

Second of all, this was an FAQ, not a rules change or rules clarification. I would call your attention to the rules (3.d) where it says, "The device, without the counterweight and projectile, must not contribute energy to the launch." So how can the FAQ so blatantly contradict the rules when the rules clearly state that the ball is not to be considered when determining if the device adds energy to the launch outside of the event supervisor-provided counterweight.

[knightmoves]

I want to bring up an issue with the recent FAQ:

CAN GRAVITATIONAL POTENTIAL ENERGY BE SUPPLIED BY THE PROJECTILE ITSELF OR MUST THE PROJECTILE BEGIN AT GROUND LEVEL BEFORE TRIGGERING?

No, the projectile may not be a source of gravitational potential energy. Please review the "Examples of Energy Violations" document on the Storm the Castle event page at soinc.org"

The well-intentioned event supervisors at a recent tournament disqualified our kids' whipper trebuchet because they claimed the ball initially started from above the lever arm. Since the ball started with initial potential energy (higher than the floor) and dropped down during the launch, it somehow added that potential energy to the launch of the ball.

You've had bad luck with event supervisors, haven't you?

I agree with your analysis of the rules and the physics. I think there's a general problem with the FAQ system, which is that people often seem to submit questions which are either unclear, or contain two questions conflated in to one. And then the rules committee answers the entirety of the question, but people often read the answer as applying to just part of the question.

In this case, "Can gravitational potential energy be supplied by the projectile?" and "Must the projectile start at ground level?" are different questions.

You are obviously correct that the relevant measurements for "can gravitational potential energy be supplied by the projectile" are the height of the projectile in the ready-to-launch position, and the height of the projectile in the launch position. About the only example of a launcher I can think of where the projectile supplies potential energy is some kind of ramp where the projectile runs down a ramp then off a ski-jump.

jander14indoor made a similar point on the Roller Coaster forum, with respect to another FAQ. The person asked a specific, complicated question about enclosed transparent tracks in loops. The answer to the FAQ is probably intended to answer the specific question asked (abut loops only), but it's easy to read the answer as being a general statement about all tracks on the device.

I don't know how to improve the FAQ system, without asking the rules committee to answer each question with several pages of close-typed legal argument, which seems like an unreasonable burden.

[jgrischow1]

I want to bring up an issue with the recent FAQ:

CAN GRAVITATIONAL POTENTIAL ENERGY BE SUPPLIED BY THE PROJECTILE ITSELF OR MUST THE PROJECTILE BEGIN AT GROUND LEVEL BEFORE TRIGGERING?

No, the projectile may not be a source of gravitational potential energy. Please review the "Examples of Energy Violations" document on the Storm the Castle event page at soinc.org"

The well-intentioned event supervisors at a recent tournament disqualified our kids' whipper trebuchet because they claimed the ball initially started from above the lever arm. Since the ball started with initial potential energy (higher than the floor) and dropped down during the launch, it somehow added that potential energy to the launch of the ball.

First of all, physics! If the ball is released from a position higher than where it started, there is no possible way that the ball's own gravitational potential energy can add to the kinetic energy of the launch. A ball cannot roll out of it's own bowl unless additional energy is added to it. The ball does fall down, but then is released from a position way above it's initial starting point. Therefore, none of the initial potential energy of the ball could possibly be contributing to the launch velocity (kinetic energy) of the ball.

Second of all, this was an FAQ, not a rules change or rules clarification. I would call your attention to the rules (3.d) where it says, "The device, without the counterweight and projectile, must not contribute energy to the launch." So how can the FAQ so blatantly contradict the rules when the rules clearly state that the ball is not to be considered when determining if the device adds energy to the launch outside of the event supervisor-provided counterweight.

I was there as well. I saw the chaos, and I agree with you. One further point...even if the trebuchet was conventional or rolling arm, how many projectiles started at ground level? Every single one had at least a sling between the ground and the ball. Most had at least sheet metal. Some had 1/4 plywood. Some 1/2. Some 5/8. Some had a PVC chute. What if I raise the chute with a shim? etc. etc.