Weight powered car

CraigD · 05-20-2009

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.

Turtle · 05-20-2009

Quote:

Originally Posted by CraigD

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!?.

So much for my cone axle as an advantage for this project.

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.

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.

arkain101 · 05-20-2009

Aerodynamics is also your friend!

modest · 05-20-2009

Quote:

Originally Posted by Turtle

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.

Quote:

Originally Posted by arkain101

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].

Quote:

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

GAHD · 05-20-2009

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

Turtle · 05-20-2009

Quote:

Originally Posted by modest

... 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.

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?

)

modest · 05-20-2009

Quote:

Originally Posted by Turtle

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

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.

Quote:

Originally Posted by Turtle

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?

)

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

~modest

Turtle · 05-20-2009

Quote:

Originally Posted by modest

...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.

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.

Quote:

Originally Posted by Modelest

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.

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