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Questions about potentiometers:

How would I know if a potentiometer will be burned out by my 5,000 mAh 8.4 V NiMH battery pack? I'm confused in terms of what spec to look at.

I'm looking at buying a multi-turn potentiometer with a knob to tell me the exact place that it's rotated. Is this level of precision a little overkill for Hovercraft, assuming the target times will be in whole seconds? Also, I've experienced only full seconds for invitationals/regionals, but what about nationals?

Also, if you're willing to share, what kind of experiences have you had with certain types of potentiometers?

Our current hovercraft is composed of a flat wooden base, then glued on the bottom of it, we have a narrow, flat foam border that goes around the edge of the wooden base. So when the lifting fan is off, the device sits on this "foam border". When turned on, it levitates by dissipating air pretty evenly around the entire border, and it has been working pretty well. However, we are having second thoughts on whether or not, this would satisfy rule 4j. The way it works, the upwards displacement of the vehicle is so little that is barely visible unless you are really looking for the levitation, even though it must be levitating by the laws of physics and the way it works. Does anybody have unofficial opinions or experiences with this type of design? Do you think this would pass the "push-down test", considering that it displaces itself such a small distance?

dhdarren wrote:Our current hovercraft is composed of a flat wooden base, then glued on the bottom of it, we have a narrow, flat foam border that goes around the edge of the wooden base. So when the lifting fan is off, the device sits on this "foam border". When turned on, it levitates by dissipating air pretty evenly around the entire border, and it has been working pretty well. However, we are having second thoughts on whether or not, this would satisfy rule 4j. The way it works, the upwards displacement of the vehicle is so little that is barely visible unless you are really looking for the levitation, even though it must be levitating by the laws of physics and the way it works. Does anybody have unofficial opinions or experiences with this type of design? Do you think this would pass the "push-down test", considering that it displaces itself such a small distance?

Both of our hovercrafts have a similar design, except our hovercraft has a flat foam base and a foam border instead of just a flat wooden base. Our hovercraft also hovers very low, at times barely a millimeter off the ground. But in competitions we've had no problem with this because when the lift fan is turned on the event supervisors can see that it is hovering as it moves without friction with the ground. So I think you're fine as long as you can show to the event supervisors that your hovercraft can move without friction with the ground.

ericlepanda wrote:

dhdarren wrote:Our current hovercraft is composed of a flat wooden base, then glued on the bottom of it, we have a narrow, flat foam border that goes around the edge of the wooden base. So when the lifting fan is off, the device sits on this "foam border". When turned on, it levitates by dissipating air pretty evenly around the entire border, and it has been working pretty well. However, we are having second thoughts on whether or not, this would satisfy rule 4j. The way it works, the upwards displacement of the vehicle is so little that is barely visible unless you are really looking for the levitation, even though it must be levitating by the laws of physics and the way it works. Does anybody have unofficial opinions or experiences with this type of design? Do you think this would pass the "push-down test", considering that it displaces itself such a small distance?

Both of our hovercrafts have a similar design, except our hovercraft has a flat foam base and a foam border instead of just a flat wooden base. Our hovercraft also hovers very low, at times barely a millimeter off the ground. But in competitions we've had no problem with this because when the lift fan is turned on the event supervisors can see that it is hovering as it moves without friction with the ground. So I think you're fine as long as you can show to the event supervisors that your hovercraft can move without friction with the ground.

^seconded, but be wary. I barely saved myself from a penalty regarding this at MIT. Ensure all 4 corners can demonstrate levitation

freed2003 wrote:Can someone help me with this question?
A long jumper leaves the ground with an initial velocity of 12 m/s at an angle of 28-degrees above the horizontal. Determine the time of flight, the horizontal distance, and the peak height of the long-jumper.

Ok first you need to separate the initial velocity into a vertical component and a horizontal component; the reason for this being that when the long jumper is in the air, the only force acting upon him is gravity. Gravity only acts upon the vertical component; the horizontal component of the initial velocity is not affected.

sin(28)*12m/s = 5.63m/s (vertical component)
cos(28)*12m/s = 10.60m/s (horizontal component)

To determine the time of flight we can use the equation Vf = Vi + at. For Vf we can use 0; this solves for the time it takes for the long jumper to reach maximum height. To find the time of the whole flight we multiply that value by two.

0 = 5.63m/s + -9.8m/s^2*t
Solving for t gets us 0.57 seconds. Multiplying that by 2 gets us 1.14 seconds. That's your answer for the travel time.

Now to find the distance. This is easy. Just take the horizontal component and multiply that by the time in flight.
1.14s*10.6 = 12.08m
holy heck that's a long jump. I feel like I did something wrong.

For the peak height we can use the equation Vf^2 = Vi^2+2ax. For Vf we use 0 because that's the velocity of the long jump guy when he hits MAXIMUM HEIGHT

0 = 5.63^2 + 2*-9.8*x
solving for x gets us 1.62 meters.

Opinions expressed on this site are not official; the only place for official rules changes and FAQs is soinc.org.

Hope this helps

By the way, I'm assuming that there is no air resistance or anything, that would mess this all up ok have a nice day byeeeeee.

Zioly wrote:Questions about potentiometers:

How would I know if a potentiometer will be burned out by my 5,000 mAh 8.4 V NiMH battery pack? I'm confused in terms of what spec to look at.

I'm looking at buying a multi-turn potentiometer with a knob to tell me the exact place that it's rotated. Is this level of precision a little overkill for Hovercraft, assuming the target times will be in whole seconds? Also, I've experienced only full seconds for invitationals/regionals, but what about nationals?

Also, if you're willing to share, what kind of experiences have you had with certain types of potentiometers?

Chose a potentiometer with a power rating (watts) that meets or exceeds the PEAK amount of power that it may be required to dissipate.

Calculating the power dissipated by your potentiometer (pot) will take some effort, but it is a good learning exercise. One approach is to use the P=V^2/R version of the power equation. The power dissipated (Ppot) will be a function of both Vpot and Rpot. In addition, the voltage across the pot (Vpot) will be a function of the resistance setting (Rpot) and the internal resistance of the motor (Rmot). The internal resistance of the battery also comes into play, but I would suggest that you can ignore this when selecting a pot. Since there are multiple values in play, you will understand this better if you build a spreadsheet to see Power dissipated as a function of Rmot, Rpot, and the V of the battery.

Here are your Hints:
- R in the power equation is the resistance the pot is currently set at. Call it Rpot.
- V in the power equation is the voltage drop across the pot at that particular R setting. Call it Vpot.
- The internal resistance of the motor (Rmot) can be measured between the leads of the disconnected motor. It will be a very small value, so you will need a decent quality VOM, and you will need to zero out (calibrate) your measurement.
- To find the Voltage across Rpot at a particular setting, research "voltage dividers" to find out how to calculate the fraction of your battery voltage that is dropped across Rpot. The motor, internal battery resistance and pot create a voltage divider. If you decide to use additional fixed resistors in series or parallel, adjust your calculations.
- Try all the possible values of Rpot, then find the peak power dissipation, and make sure your pot can handle it.

If you can set up a spreadsheet, you will learn alot about how power is being delivered to your motor versus your pot. It will help you chose the correct value of potentiometer, and select the necessary power rating.

Good Luck!

I am using a 7.2V, 3000 mAh battery to power my Dromida M370 motors. Yesterday I did a test for the first time, great run right after the run the motors started to smoke and then stopped working . We then tried today putting resisters in and the new motors we have (M370 again) wont even spin with the resisters i but will if hooked up to the battery. so if anybody has any ideas on how to fix it that would be great, And any thing u might think will be better. And maybe if u have reconmedations on better motors and battery. Thank You

KILLERpanda35 wrote:I am using a 7.2V, 3000 mAh battery to power my Dromida M370 motors. Yesterday I did a test for the first time, great run right after the run the motors started to smoke and then stopped working . We then tried today putting resisters in and the new motors we have (M370 again) wont even spin with the resisters i but will if hooked up to the battery. so if anybody has any ideas on how to fix it that would be great, And any thing u might think will be better. And maybe if u have reconmedations on better motors and battery. Thank You

I'm just throwing out ideas off the top of my head. Obviously, 3,000 mAh is a bit too much for your motors, which is why they burned out, so I wouldn't recommend trying to hook them up directly again. I'm assuming this is why you hooked up a resistor, so your motor wouldn't burn out. It's probably not spinning the motor because the resistance is too high.

We've discussed quite thoroughly some very good batteries, fans, and accessories previously on this thread. Feel free to take a look around, as it will answer many questions.

Zioly wrote:

KILLERpanda35 wrote:I am using a 7.2V, 3000 mAh battery to power my Dromida M370 motors. Yesterday I did a test for the first time, great run right after the run the motors started to smoke and then stopped working . We then tried today putting resisters in and the new motors we have (M370 again) wont even spin with the resisters i but will if hooked up to the battery. so if anybody has any ideas on how to fix it that would be great, And any thing u might think will be better. And maybe if u have reconmedations on better motors and battery. Thank You

I'm just throwing out ideas off the top of my head. Obviously, 3,000 mAh is a bit too much for your motors, which is why they burned out, so I wouldn't recommend trying to hook them up directly again. I'm assuming this is why you hooked up a resistor, so your motor wouldn't burn out. It's probably not spinning the motor because the resistance is too high.

We've discussed quite thoroughly some very good batteries, fans, and accessories previously on this thread. Feel free to take a look around, as it will answer many questions.

I could be wrong, but aren't mah just a measure of capacity? It would be the voltage that burns things out from what I understand. My friend has a 7.4V 5900 mah battery and it works fine

magic_carp wrote:

Zioly wrote:

KILLERpanda35 wrote:I am using a 7.2V, 3000 mAh battery to power my Dromida M370 motors. Yesterday I did a test for the first time, great run right after the run the motors started to smoke and then stopped working . We then tried today putting resisters in and the new motors we have (M370 again) wont even spin with the resisters i but will if hooked up to the battery. so if anybody has any ideas on how to fix it that would be great, And any thing u might think will be better. And maybe if u have reconmedations on better motors and battery. Thank You

I'm just throwing out ideas off the top of my head. Obviously, 3,000 mAh is a bit too much for your motors, which is why they burned out, so I wouldn't recommend trying to hook them up directly again. I'm assuming this is why you hooked up a resistor, so your motor wouldn't burn out. It's probably not spinning the motor because the resistance is too high.

We've discussed quite thoroughly some very good batteries, fans, and accessories previously on this thread. Feel free to take a look around, as it will answer many questions.

I could be wrong, but aren't mah just a measure of capacity? It would be the voltage that burns things out from what I understand. My friend has a 7.4V 5900 mah battery and it works fine

You are correct