Help With Science Fair; Calculating Force

[IsaacFigNewton]

Calculating Force; A Nut Pushing Down on a Screw

Hello, I'm new to Physics Forums, and this question is not really about schoolwork/coursework, however because it is somewhat school related, I was afraid to post it anywhere else.

Anyway, I am doing a science fair project where I am designing/building a machine, and to know if it will work/be efficient, I have to calculate a force.

Homework Statement

If a nut, affixed to external pillars in a way that would prohibit it from spinning, but allow it to move vertically, is exerting 9.8 N (its mass is 1 kg) downward onto a screw with a thread of 45°, with how much force would the screw spin? If you need any further information/variables, just ask and I will quickly provide.


2. The attempt at a solution

Even though I am a very advanced student (I skipped 7th grade), I am only in 8th grade, and my school doesn't offer physics of this level, so I don't even know where to start.

If you do this problem for me, would you please explain the process of solving it, if it isn't a problem? Also, if you don't want to outright do this problem for me, could you at least point me to some sources that would show me how to solve the problem? I fully understand middle school physics, I am currently completing Algebra 1, and I have a basic understanding of trigonometry.


Thank you for even taking the time to read this, and I would be very grateful if you would help me in any way.

 

[tms]

This is the correct area to post such a question (although you might want to try the engineering section if you have no luck here), but as a matter of policy people will help you, but they will not do your work for you.

From your description, it sounds like you want the weight of a nut to turn a screw. Have you actually tried it? I'm skeptical that it would work, since, even with the threads at 45 degrees, half of the force will just push the screw down onto whatever is holding it in place. Friction will be your enemy here: between nut and screw, between screw and its supports, and even between nut and it supports. Do everything you can to reduce it. In the real world, screw gears always (as far as I know) work the other way around: the screw turns and moves the nut (or a gear).

 

[IsaacFigNewton]

From your description, it sounds like you want the weight of a nut to turn a screw. Have you actually tried it? I'm skeptical that it would work, since, even with the threads at 45 degrees, half of the force will just push the screw down onto whatever is holding it in place. Friction will be your enemy here: between nut and screw, between screw and its supports, and even between nut and it supports. Do everything you can to reduce it. In the real world, screw gears always (as far as I know) work the other way around: the screw turns and moves the nut (or a gear).

Yeah, that's been my main concern, it's why I set the thread to 45°. But how can I test it without actually building it? To build it I need to spend money, and before I spend any money I want to know if there is even a shot at it working, that's why I'm asking about it. By the way, The screw is attached to a generator. Also, can you think of any other gear or system that allow me to achieve my goal?

 

[IsaacFigNewton]

Calculating a Force; A Nut Pushing Down on a Screw; Science Fair

Hello, I'm new to Physics Forums, and I am doing a science fair project where I am designing/building a machine, and to know if it will work/be efficient, I have to calculate a force.

Homework Statement

If a nut, affixed to external pillars in a way that would prohibit it from spinning, but allow it to move vertically, is exerting 9.8 N (its mass is 1 kg) downward onto a screw with a thread of 45°, with how much force would the screw spin? If you need any further information/variables, just ask and I will quickly provide.


2. The attempt at a solution

Even though I am a very advanced student (I skipped 7th grade), I am only in 8th grade, and my school doesn't offer physics of this level, so I don't even know where to start.

If you do this problem for me, would you please explain the process of solving it, if it isn't a problem? Also, if you don't want to outright do this problem for me, could you at least point me to some sources that would show me how to solve the problem? I fully understand middle school physics, I am currently completing Algebra 1, and I have a basic understanding of trigonometry.

P.S. If you are skeptical of the screw gear setup working (like I am), can you think of another gear or system that would allow me to achieve my goal?

Thank you for even taking the time to read this, and I would be very grateful if you would help me in any way.

This thread is reposted from Homework/Coursework, to view the original thread discussion go here --> https://www.physicsforums.com/showthread.php?p=4244298&posted=1#post4244298

 

[tms]

If you're trying to drive a generator, then I don't think it will be a great success even if it works perfectly, because the screw is of finite length, and when the nut gets to the end the generator will stop. Perhaps you'd better explain just what you're trying to accomplish.

 

[IsaacFigNewton]

Well I was planning on having the nut's thread composed of two half circles that would be attached to servos that would release the screw at the end of a run, and then the nut would be raised back to its original position by a system of pulleys, at which point the servos would grab the screw again.

 

[tms]

What is your machine supposed to do?

 

[IsaacFigNewton]

Produce energy.

 

[tms]

That doesn't give anyone much to go on.

 

[Simon Bridge]

You have a vertical screw threaded 45deg, and a nut sits on top.
If the nut can rotate, then it's weight alone may make it drop down the screw.
That the idea?

You handle it like a mass sliding down a slope with friction... only there's a rotation involved. Thus, the effect of gravity and the reaction from the thread will create a torque.

You can also do it in terms of mechanical advantage - google for "physics of screws".

 

[IsaacFigNewton]

You have a vertical screw threaded 45deg, and a nut sits on top.
If the nut can rotate, then it's weight alone may make it drop down the screw.
That the idea?

You've got the idea until the second sentence. The nut can't rotate, and the screw can. The goal is to make the nut's weight spin the screw.

 

[Simon Bridge]

Same physics :)

 

[IsaacFigNewton]

Well the goal is to have a nut, connected to these rods that wouldn't allow it to spin, pushing down on a screw attached to a generator, making the screw turn and power the generator.

 

[phinds]

Well the goal is to have a nut, connected to these rods that wouldn't allow it to spin, pushing down on a screw attached to a generator, making the screw turn and power the generator.

OK, I get that, but you say that the generator is to then raise the nut so the process can begin again, yes? If so, that's a perpetual motion machine, which I assume you know to be impossible, and if it is not so, what am I missing about your description?

 

[berkeman]

Two threads merged into one in the HH forum.

 

[Simon Bridge]

Well I was planning on having the nut's thread composed of two half circles that would be attached to servos that would release the screw at the end of a run, and then the nut would be raised back to its original position by a system of pulleys, at which point the servos would grab the screw again.

The standard approach would be to tie string to the screw and nut, wind the other end around an axle, use the axle to run a generator, drop the weight, axle turns, generates power.

To return the weight, just crank the axle the other way with something.

In your case, only half the stored gravitational potential energy would be available to turn the generator. I think the static friction will stop it from working anyway - unless you are going for some fancy low-friction materials and you say you are on a budget?

 

[IsaacFigNewton]

OK, I get that, but you say that the generator is to then raise the nut so the process can begin again, yes? If so, that's a perpetual motion machine, which I assume you know to be impossible, and if it is not so, what am I missing about your description?

Well, I did originally plan to have the generator power servos that would raise the weight through a set of pulleys, but then I quickly realized that was obviously impossible. So now I've been thinking to use human input to raise them. What I was thinking was having the whole assembly attached to an axle, which would then go through a stand, and on the other side of the stand there would be handles attached to the axle, so a person could flip the whole assembly after the weight got to the end of he screw.

 

[IsaacFigNewton]

The standard approach would be to tie string to the screw and nut, wind the other end around an axle, use the axle to run a generator, drop the weight, axle turns, generates power.

I don't really see where the screw comes in...

 

[Simon Bridge]

I don't really see where the screw comes in...

It's heavy.

 

[IsaacFigNewton]

It's heavy.

Well at that point I could just have the weight.

 

[Simon Bridge]

Well at that point I could just have the weight.

Yes.
The point being that the screw is more useful for your purpose (generating energy) as a dead weight than it is as a screw.

This is a Science Fair project right?
What is the problem that your screw is solving, or scientific principle your screw-method is demonstrating, that is not demonstrated/solved by the falling-weight approach? That would lead to the judges saying, "Hey, this guy knows his science!"?

Lots of people make this sort of mistake - sometimes the judges do too though.
It's a science fair - where's the science?

But at least you spotted the perpetual motion aspect before you got bogged down into trying to make it work.

 

[IsaacFigNewton]

This is a Science Fair project right?
What is the problem that your screw is solving, or scientific principle your screw-method is demonstrating, that is not demonstrated/solved by the falling-weight approach? That would lead to the judges saying, "Hey, this guy knows his science!"?

Lots of people make this sort of mistake - sometimes the judges do too though.
It's a science fair - where's the science?

But at least you spotted the perpetual motion aspect before you got bogged down into trying to make it work.

Yes, the title of the project is, "Is Gravity Viable as an Alternative Energy Resource?"

I am testing different methods of creating energy from gravity, comparing them, and then concluding in answer to my original question.

And I kind of did get bogged down trying to make the perpetual motion work...for about a month or so...

 

[Simon Bridge]

And I kind of did get bogged down trying to make the perpetual motion work...for about a month or so...

I think everyone gets caught up in something like this as part of coming to an understanding about how the World works. Some people never break out of it and it's a life-time obsession with conspiracy theories and the works. They seem to be attracted to forums like this.

All methods of extracting energy from gravity are the same basic structure - you drop something, and intercept it on the way - slowing it down a bit as it does some work.

The best methods are those where nature works to bring the whateveritwas back to the top - which is how water-wheels work (they are actually a kind of solar power).

With your screw - you are looking at the general class of machines that spin as they fall. There are many ways of doing it - but what is the point of doing that? Why consider it in the first place? See what I mean? For each class of machine you are considering - see if you can figure out some advantage each has that is not supplied by another kind.

I think with a project like yours is that you have to limit it - otherwise the project is very big and you don't get anywhere.
Next year's science fair you can advance your project :)

 

[IsaacFigNewton]

I think everyone gets caught up in something like this as part of coming to an understanding about how the World works. Some people never break out of it and it's a life-time obsession with conspiracy theories and the works. They seem to be attracted to forums like this.

All methods of extracting energy from gravity are the same basic structure - you drop something, and intercept it on the way - slowing it down a bit as it does some work.

The best methods are those where nature works to bring the whateveritwas back to the top - which is how water-wheels work (they are actually a kind of solar power).

With your screw - you are looking at the general class of machines that spin as they fall. There are many ways of doing it - but what is the point of doing that? Why consider it in the first place? See what I mean? For each class of machine you are considering - see if you can figure out some advantage each has that is not supplied by another kind.

I think with a project like yours is that you have to limit it - otherwise the project is very big and you don't get anywhere.
Next year's science fair you can advance your project :)

Well the only reason I am even doing this project in the first place is because you aren't allowed to use biological materials until high school...I was originally planning on doing a project on how we can advance our fight against cancer by creating drugs that target the telomeres in cancerous cells, but that's a completely different discussion for next year .

Anyway, with my project I am comparing different ways of getting energy from gravity, and Hydroelectricity is one of them. Actually, my 6th grade science fair project (last year's because I skipped 7th grade :tongue:) was titled "Can I Improve Portable Hydroelectricity?" and I ended up building a small hydroelectric turbine housed in a pretzel container that can be put in a stream to create power. And so thinking of furthering that project for this year is what led me to this yer's project.

 

[IsaacFigNewton]

...the perpetual motion aspect...

Though what I don't understand is, if a pulley reduces the force necessary to lift an object by half, if you use enough pulleys, wouldn't you be able to reduce the force necessary to reset the weight to a level that the generator could achieve by itself?

 

[Simon Bridge]

Though what I don't understand is, if a pulley reduces the force necessary to lift an object by half, if you use enough pulleys, wouldn't you be able to reduce the force necessary to reset the weight to a level that the generator could achieve by itself?

Yes - it would reduce the force. But it still takes the same amount of energy - at best.

If you reduced the needed force by a half, then it would take twice as long to reset (strictly: you have to haul in twice as much rope).
And that is with magic lossless pulleys - each pulley will add an extra loss to the system so less of the applied power goes into resetting the system.

Have you seen:
http://www.lhup.edu/~dsimanek/museum/unwork.htm
... it's a good place to learn about these kinds of traps.

 

[tms]

It isn't a question of force, but of energy. Raising an object to a certain height takes a certain amount of energy (at least), regardless of how you do it.

In addition to that, each pulley loses energy to friction, and the more pulleys you have the more friction there is. Plus, since the pulley have mass, they require energy to spin.

There ain't no such thing as a free lunch.

 

[OmCheeto]

The closest I can come to a ready made device is a pair of lockwire pliers:

https://www.youtube.com/watch?v=CU-NAX7vwfY

If your experiment fails to perform as expected, you will at least have something you can nicely twist wires with.