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# Weight powered car

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Great idea Turtle, but I think it might be most-useful for a speed race. In that case you could lessen the torque at the start of the race so the wheels don't spin out then increase the torque.

I'm not sure how such a transmission could be used when you're going for distance. In that case you want the least amount of string used (or movement of the weight) per distance the car travels. This would mean having a large drive wheel and a small axle. There is a point where the torque is so small that the car won't move, or it will start moving, but stop at some point down the track. You want to get as close to that point as possible without reaching it. Perhaps Turtle's idea could be used to assure that the car is always just barely moving, but never picks up any real speed or never stops completely. Maybe starting with enough torque to get the car moving from a standstill then lowering the torque so that it's always just barely enough to keep it moving would be useful—which is how the spindle is depicted in Turtle's pic.

~modest

EDIT: here's a link: Mousetrap Car and Racer Propulsion Theory

Aha. but how fast will the car get moving? And, thus we ask, how far will it coast after the weight has stopped pulling?

I can smell a physics equation in here that has to do with

Force Momentum and Coefficient of friction. :)

Somewhere in there is a golden zone.

Speed is surely a key factor!

Yet, will a dangling weight start swinging if acceleration is too harsh? A swinging power source would be an all around no no. right right?

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A swinging power source would be an all around no no. right right?

That's one of the ideas I was trying to draw out. What if the weight did swing? Perhaps it is possible to put the horizontal movement to work, as well as the vertical dropping?

For this type of assignment, it's probably not a good idea but it's a fun idea to think about...:)

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Right! As long as the movement of the weight was perpendicular to the direction of travel you could gain some extra mmph moments. Good Call. Ah, but you would lose some mmph on the upper swings.. too bad so sad :)

I guess to keep this in perspective...

We have 1 force to do work for us here. And that is gravity. (as long as we dont start whipping the weight around in a circle, and create centrifugal force)

As we all know, you can't get more out than was put in. So, the max is 9.8newtons, for 10cm.

w = f x d (if i remember)

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The mechanics of this kind of problem – fundamentally, a work problem - are pretty simple.

The energy available in a 1 kg mass at a height of 0.1 m is about $W = 1 \,\mbox{kg} \cdot 9.8 \,\mbox{m/s/s} \cdot 0.1 \,\mbox{m} = 0.98 \,\mbox{J}$

This must equal the work of the car moving distance $d$ with a force of friction $f$, so $W = d \cdot f$.

Rearranging, $d = \frac{W}{f}$

So, if we measure the rolling resistance of the loaded car – say by pulling it with a scale – and the mechanical resistance of its various pulleys – a tougher measurement, but in principle doable with scales – add them together to get the total resistance $f$, and divide our 0.98 J energy by it, we get the distance it should travel.

For example, if the total resistance is 0.1 N, we expect it to travel about 9.8 m.

Notice that how fast it goes, or how much of the distance it coasts after the weight has dropped its full distance, doesn’t effect the total distance traveled, except that $f$ is likely to be higher for higher rolling speeds, and less when coasting (the pulleys etc. won’t be contributing as much drag then). It also doesn’t matter how the car converts the gravitational potential energy of the weight into work for moving the car, except that some schemes will have more or less friction than others, affecting $f$.

Also notice that, as $f$ won’t be truely constant, a more precise calculation would be more complicated. However, with most low-speed, every-day smooth-surface rolling machines, rolling resistance is nearly constant regardless of speed, so the simple calculation above is likely to be adequate.

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The mechanics of this kind of problem – fundamentally, a work problem - are pretty simple.

The energy available in a 1 kg mass at a height of 0.1 m is about $W = 1 \,\mbox{kg} \cdot 9.8 \,\mbox{m/s/s} \cdot 0.1 \,\mbox{m} = 0.98 \,\mbox{J}$

This must equal the work of the car moving distance $d$ with a force of friction $f$, so $W = d \cdot f$.

Rearranging, $d = \frac{W}{f}$

So, if we measure the rolling resistance of the loaded car – say by pulling it with a scale – and the mechanical resistance of its various pulleys – a tougher measurement, but in principle doable with scales – add them together to get the total resistance $f$, and divide our 0.98 J energy by it, we get the distance it should travel.

For example, if the total resistance is 0.1 N, we expect it to travel about 9.8 m.

Notice that how fast it goes, or how much of the distance it coasts after the weight has dropped its full distance, doesn’t effect the total distance traveled, except that $f$ is likely to be higher for higher rolling speeds, and less when coasting (the pulleys etc. won’t be contributing as much drag then). It also doesn’t matter how the car converts the gravitational potential energy of the weight into work for moving the car, except that some schemes will have more or less friction than others, affecting $f$.

Also notice that, as $f$ won’t be truely constant, a more precise calculation would be more complicated. However, with most low-speed, every-day smooth-surface rolling machines, rolling resistance is nearly constant regardless of speed, so the simple calculation above is likely to be adequate.

I recall the instructions the student posted here commented that the "best" cars go about 10 m; guess we shoulda got a clue eh!?. :hyper: So much for my cone axle as an advantage for this project. :eek: :hyper:

So we really have an engineering problem in the reduction of friction. Back to lube then, and in addition, care in precisely aligning the wheels on the axles and the axles on the vehicle so they run true and don't wobble around adding friction.

Maybe having the weight on a teeter-totter instead of using a pulley, and an axle string on the other end of the teeter-totter would attach to the drive axle and take out the friction in the system of a string on a pulley? Plus, a teeter-totter would remove the problem of the weight swinging. :steering:

One thing I'd like to see on these project threads, is a follow up from the student posters after the project is completed. What worked? What didn't work? Did you use any infomation other posters provided? What would you do different if you had it to do again?

That's a wrap.

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Here again I point out these specific instructions mention the car should be able to coast once the weight is down, so my thinking is to reach top speed just as the string comes off the drive cone and then you get the best coast distance.

Aha. but how fast will the car get moving? And, thus we ask, how far will it coast after the weight has stopped pulling?

That's what the link I posted speaks to. If you're going for distance it's less efficient to deliver the energy quickly gaining high speed then letting it coast than to deliver the power over a long time never gaining much speed and never 'coasting'.

The total energy is the same in either case, but at high speeds there's more air resistance and friction from the moving parts of the car. So, slow speed and low power for a long duration is conventional wisdom when going for speed [EDIT: distance].

Slow Moving vs. Fast Moving mouse trap vehicle Here are my thoughts on the ultimate distance vehicle. In sharing my thoughts with you please understand that I am not telling you how to build the perfect Distance Car but I am pointing out the application of physics as I applied it to my mousetrap powered vehicles. I tend to design my distance cars to travel extremely slow. One of my cars that travels 100 meters or more may take over 5 minutes to travel that distance. My idea is to reduce the power output to a minimum, only supplying enough energy to the vehicle to overcome the friction. By traveling slowly you will reduce the air resistance to a minimum vs. a fast traveling car that will have more air friction acting against it. Also, I feel that a quick accelerating car will create more heat energy during a quick acceleration than a slow accelerating vehicle which will reduce the energy needed to travel a great distance. Building a mouse trap car for distance means minimizing the wasted energy and converting more energy into displacement of the vehicle. With that in mind, I like to build cars that have very low frictional forces acting against them and move slowly. I try to find a harmonious balance between the movement of my vehicle and the length of the lever arm. My cars tend to have long lever arms and large wheels. If a lever arm is too long the vehicle will not travel the full distance because you must have enough torque to keep the car going and the torque changes with spring angle.

Your ultimate source for mousetrap powered cars and vehicles

~modest

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a little late for me to get in with the due date around the corner, but what *I* would do is make a trike, have the front wheel made out of a yo-yo, and attach the yoyo's string through the pulleys. Wrapping 95% of the "traveling" string around the yoyo should give a clean separation as the weight falls.

Further I'd have the car get "pulled forward" by the string via a pulley in the launch mechanism.

Let us know how it went and what you learned :D

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... If you're going for distance it's less efficient to deliver the energy quickly gaining high speed then than letting it coast then to deliver the power over a long time never gaining much speed and never 'coasting'.

The total energy is the same in either case, but at high speeds there's more air resistance and friction from the moving parts of the car. So, slow speed and low power for a long duration is conventional wisdom when going for speed [EDIT: distance].

~modest

Roger. I finally got that. :beer: :D As Craig showed so succinctly, no matter the type of drive or the speed, there is a theoretical limit to how far any vehicle can roll given a 1 kilo weight falling 10 cm onboard to power it.

Now you raise an interesting question though, and a different challenge which is how to keep the vehicle under power for the entire ~10 meter trip? I have some ideas, what about y'all? (might be a subject for a different thread? :shrug:)

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As Craig showed so succinctly, no matter the type of drive or the speed, there is a theoretical limit to how far any vehicle can roll given a 1 kilo weight falling 10 cm onboard to power it.

Yeah, if friction is constant then there is a limit :beer: I think all efforts should be geared toward lowering friction including the car's overall weight and the speed (which should both be as small as possible). I'm actually surprised the instructions explicitly say that it should coast when the weight is completely fallen considering the best performing vehicles will never go fast enough to coast.

Now you raise an interesting question though, and a different challenge which is how to keep the vehicle under power for the entire ~10 meter trip? I have some ideas, what about y'all? (might be a subject for a different thread? ) :D

Do you mean how to distribute the energy over a long distance? I should think making the axle small (the spool where the string attaches to the axle) and the wheels with a large diameter should do it. I'd personally use a 120 mm CD for the wheels then play with the axle size making it smaller if the car goes too fast and larger if it doesn't move.

The best-performing string would be another interesting issue. I'd think as thin as possible and as not-stretchy as possible. Maybe the lightest fishing line (6 lb?) or the E string on a guitar... maybe?

Any place where the string changes directions (there should just be one directly above the weight) would be well-served with a pulley. I've run into some small aluminum ones in model plane kits which were something like this:

ALUMINUM PULLEYS from Aircraft Spruce

Uh... what else... balsa or bass wood. 3 wheels are better than 4. There only needs to be 3 legs holding up the pulley which holds the wight (think of a Naive American teepee with only 3 poles).

Anyone up for an Hypography build off :shrug:

~modest

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...I'm actually surprised the instructions explicitly say that it should coast when the weight is completely fallen considering the best performing vehicles will never go fast enough to coast. ...
That is a little puzzling. :D I imagine this is some sort of "stock" project contained in a science curriculum, (who writes this stuff!!!??? ) but when the projects come up here at Hypog the students need something "cheap & fast" and go all white rabbit on us with no time to fill us in. :shrug:

Uh... what else... balsa or bass wood. 3 wheels are better than 4. There only needs to be 3 legs holding up the pulley which holds the wight (think of a Naive American teepee with only 3 poles).

Anyone up for an Hypography build off

~modest

Ready for a Hypog design off anyway. :hihi: So, why do you say 3 wheels better than 4? over....:beer:

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Ready for a Hypog design off anyway. :shrug: So, why do you say 3 wheels better than 4? over....:D

Less possibility for friction, perhaps? Heck, two wheels would be more ideal but then you have balance to deal with.

I might actually try to build one of these just for fun. :beer:

Then again, I might not...depends on my motivation levels in the up-coming days. :hihi:

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Anyone up for a build off?

I just might be turtle.

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So, why do you say 3 wheels better than 4?

Less weight, I should think. Page 8:

Keep in mind that 3 wheels work best for any design.

By the same reasoning the frame could be triangular as well as using only 3 legs for holding the weight. To fix a point in space requires a minimum of 3 points (e.g. Bucky's Tetrahedron), so the most efficient designs are often triangular. Are you thinking differently?

~modest

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Less weight, I should think. Page 8:

By the same reasoning the frame could be triangular as well as using only 3 legs for holding the weight. To fix a point in space requires a minimum of 3 points (e.g. Bucky's Tetrahedron), so the most efficient designs are often triangular. Are you thinking differently?

~modest

Erhm...no; I don't buy it yet. That page makes the claim

Keep in mind that 3 wheels work best for any design.
without any substantiation. :D Better how? Steering? What better? Where better? Why better? The weight of 3 wheels is only less if you are talking about using one less of 4 identical wheels. :beer:

But I was thinking differently yes, in that I was waiting to see if you were going to claim 3 wheels was less friction, because... (as if) I was going to counter that friction is independent of surface area. :shrug:

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I was going to counter that friction is independent of surface area. :beer:

How so? :D

(or were you joking and it went right over my head?)

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I was going to counter that friction is independent of surface area.

How so? :D

(or were you joking and it went right over my head?)

Not joking this time. :beer: Just one of those scientifical factoids stuck in my memory banks. Here's a bit on it.

Friction

...Leonardo da Vinci made the first experiments on friction using a rectangular block sliding on a dry, flat surface. His main observations are that:

1) the friction force is independent of the area of the surfaces in contact.

2) the friction force is proportional to the applied load.

These were framed as laws by Amontons in 1699 and researched by Coulomb in 1781, who also noted the distinction between static friction (force needed to start motion) and kinetic friction (force needed to maintain friction). ...

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