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I was working on the Raisbeck Aviation test (from the test exchange), and while I made a couple stupid mistakes, I was puzzled by the key for a couple problems. Could anyone look at #19 and #20? I'm not quite sure why the answers would be negative, or how we could know whether to answer them as negative values.

404ic wrote:

reed303 wrote:

404ic wrote:Hi y'all,

Am I allowed to use alligator clips to wire my hovercraft? I remember some team getting chewed out last year for doing that...

S-O Battery policy says:
• insulate the battery terminals and attached wires when not connected;
• do not use bare wire at any time in the construction of your device;
• make sure that all wiring connections are properly insulated

https://www.soinc.org/sites/default/fil ... _18_17.pdf

We've soldered & shrink-tubed some connections, and used wire nuts for others.. You could also use crimp connectors for wire-to-wire splices. The connections to lugs on motors and switches were soldered.

Is it okay if I use electrical tape to insulate the wire connections?

That should probably work. I've seen people do it and be fine with it.

Is it ok to use a flexible mesh, or does it have to be rigid to cover the propellers? The rules say that a 3/8" dowel should not touch the prop, but does this mean that if I press it against a flexible shield and the shield flexes and touches the prop it will be not ok? Or is it enough that the dowel doesn't go through the hole and touch the prop?

cmurthy wrote:Is it ok to use a flexible mesh, or does it have to be rigid to cover the propellers? The rules say that a 3/8" dowel should not touch the prop, but does this mean that if I press it against a flexible shield and the shield flexes and touches the prop it will be not ok? Or is it enough that the dowel doesn't go through the hole and touch the prop?

A flexible shield is fine as long as the dowel can not directly touch the prop, like without pressing or moving anything.

Tesel wrote:I was working on the Raisbeck Aviation test (from the test exchange), and while I made a couple stupid mistakes, I was puzzled by the key for a couple problems. Could anyone look at #19 and #20? I'm not quite sure why the answers would be negative, or how we could know whether to answer them as negative values.

It seems to me that the test assumes that force acts in the opposite direction of the other force for both questions. Also, if you do the calculation the answer turns out to be negative. Furthermore, force is technically a vector quantity. But I understand your concern about a negative force value and I am a bit puzzled as well.

geniusjohn5 wrote:

Tesel wrote:I was working on the Raisbeck Aviation test (from the test exchange), and while I made a couple stupid mistakes, I was puzzled by the key for a couple problems. Could anyone look at #19 and #20? I'm not quite sure why the answers would be negative, or how we could know whether to answer them as negative values.

It seems to me that the test assumes that force acts in the opposite direction of the other force for both questions. Also, if you do the calculation the answer turns out to be negative. Furthermore, force is technically a vector quantity. But I understand your concern about a negative force value and I am a bit puzzled as well.

#19 doesn't seem to make sense with a negative number because the force is in the same direction as the velocity, so valid answers are +32 N or 32 N in the direction of the motion, but I think that for #20, the negative sign is meant to indicate that a direction is required, such as -3630 N or 3630 N in the opposite direction the car is going. However, the sigfigs seem to be off (for example, #19 should be 30 N or 3 * 10^1 N, and #20 should be -3600 N or -3.6 * 10^3 N).

EDIT: Typo with the answers

404ic wrote:

reed303 wrote:

404ic wrote:Hi y'all,

Am I allowed to use alligator clips to wire my hovercraft? I remember some team getting chewed out last year for doing that...

S-O Battery policy says:
• insulate the battery terminals and attached wires when not connected;
• do not use bare wire at any time in the construction of your device;
• make sure that all wiring connections are properly insulated

https://www.soinc.org/sites/default/fil ... _18_17.pdf

We've soldered & shrink-tubed some connections, and used wire nuts for others.. You could also use crimp connectors for wire-to-wire splices. The connections to lugs on motors and switches were soldered.

Is it okay if I use electrical tape to insulate the wire connections?

Yes, but most electrical tapes start to melt above 250°F, so make sure you don't have connection points that get hotter than that.

What are some effective ways to prevent your hovercraft from turning 90 degrees halfway through the run and getting stuck?

NilaiVemula wrote:What are some effective ways to prevent your hovercraft from turning 90 degrees halfway through the run and getting stuck?

Basic solution is to have the hovercraft hug the side rail of the track. More complex solution could be to use bearings on the front corners of the hovercraft to reduce turning. I'm not sure which would be more reliable as I haven't really tested bearings yet.

UTF-8 U+6211 U+662F wrote:

geniusjohn5 wrote:

Tesel wrote:I was working on the Raisbeck Aviation test (from the test exchange), and while I made a couple stupid mistakes, I was puzzled by the key for a couple problems. Could anyone look at #19 and #20? I'm not quite sure why the answers would be negative, or how we could know whether to answer them as negative values.

It seems to me that the test assumes that force acts in the opposite direction of the other force for both questions. Also, if you do the calculation the answer turns out to be negative. Furthermore, force is technically a vector quantity. But I understand your concern about a negative force value and I am a bit puzzled as well.

#19 doesn't seem to make sense with a negative number because the force is in the same direction as the velocity, so valid answers are +32 N or 32 N in the direction of the motion, but I think that for #20, the negative sign is meant to indicate that a direction is required, such as -3630 N or 3630 N in the opposite direction the car is going. However, the sigfigs seem to be off (for example, #19 should be 30 N or 3 * 10^1 N, and #20 should be -3600 N or -3.6 * 10^3 N).

EDIT: Typo with the answers

That makes sense to me... sometimes it's difficult to tell due to the wording, but I can't disagree with your interpretation. 20 just asks for average force, so obviously a negative force would be necessary. 19 asks the average force applied to the pillow, which I believe would still be positive, since it has opposite sign from the force applied to the ball.

As a newbie test-writer and fairly experienced competitor, I wouldn't say sig figs are super important in physics events... I wouldn't take off points for them, and the only tests that have very explicitly stated they would. I think there's a different mentality between chem and physics for sig figs.