Gizmodo Takes on Pinewood Racing

[B_Regal Racing]

Just have to know, when do you find the time to do your research? I love the analysis, but just with me reading, the Mrs. has reminded me I still have a family...

She told me to shut down the computer for tonight

 

[Papa V Racing]

Take care of the family... any way to get everyone involved may help out some... My kids go down and work on different things while I am working on a project...

 

[LightninBoy]

OK, I think I see the problem. The science guy is not too far off, but Shamwow has misspoken about what was done.

Shamwow -

 

[GravityX]

I'll take another stab at this later on when I get home tonight. I had it all typed in and ready to go last night and my browser crashed and there went all that thinking in one click of the mouse. At this point it was time for me to go to bed.

Who can tell me why the car slowed so much with the weight in front?

 

[Obsessedderbydad]

Who can tell me why the car slowed so much with the weight in front?[/QUOTE not as much potential energy?

I would agree with this. The weight in the front will get the car out early, but lose it down the stretch b/c it doesnt have as much PE as the cars with the weight in the rear.

 

[txchemist]

Why a little knowledge is dangerous.
OK, the equation is

If this was all you know, does it say a heavier car will go faster?
Remember we are just looking at this equation of how potential energy gets converted to kinetic energy

ALERT!! equation he shows is NOT correct-
PE=mgh
m=mass
g = acceleration due to gravity
h= height

so using the correct equation
PE=KE
mgh=1/2(m) v^2

does it predict a heavier car will be faster than a lighter car?

 

[ngyoung]

Who can tell me why the car slowed so much with the weight in front?[/QUOTE not as much potential energy?

I would agree with this. The weight in the front will get the car out early, but lose it down the stretch b/c it doesnt have as much PE as the cars with the weight in the rear.

Front weighted car wont have an advantage early. Adding the weight to the front just shortens the distance it falls in the slope. If all things being equal except CoG both cars will roll at the same speed in the slope. Just like dropping 2 objects off a building, both fall at the same rate until reaching terminal velocity.

Now if you have a front weighted 3 wheel rail rider it would cause both front wheels to be touching even if one was meant to be raised. Like shifting your weight on a chair with uneven legs.

 

[resullivan]

No because mass is factored out right?

 

[resullivan]

You just get gh = 1/2v^2

 

[resullivan]

nm

 

[txchemist]

Correct!- mass is factored out because it is the same on both sides of the equation.
The majority of folks that had high school physics ( and even most scientists) know this and so they overstate their data and assume that the small height difference between front and back weight placement is making all the speed difference.

Now when you actually test a few cars- weight of the car is a BIG deal. The data from S. Acton show it was the biggest deal, same with Doc Jobe- so the thing missing from almost all of these videos is WHY does the weight make so much difference- I have added some detail to the actual data from S. Acton badly interpreted by Shamwow

OK- what is going on???- now you can bring in all ideas

 

[resullivan]

Since the energy either translates directly into velocity or it lost in some other way I was leaning towards the answer having something to do with friction. I looked up the formula and friction = coefficient of friction X magnitude of normal force. Crud, no mass, but then I looked up the formula for the magnitude of normal friction = mass X gravity. In other words, the more mass the more friction we have. That being said this is my guess, because there is more friction there is more room to reduce that friction? In otherwords, polishing your axles reduces more friction with a heavier car than a lighter car?

 

[GravityX]

Let's try this again.

Forward weighted car will slow faster due to the amount of friction at the DFW. This added friction comes at the track surface. The added weight does not allow the DFW to slide/slip across the surface, but instead causes it to "dig-in" for traction. This additional traction drives the DFW into the rail harder, again adding more friction. This is not even considering the DFW axle-to-bore friction. All this friction added up slows the car quicker than a comparable rear weighted car.

Also, the forward weight car will not have as great a push off the hill as a rear weighted car. If there was a COM difference of 2 inches between the rear weighted and front weighted car, the rear weighted car will have 2 more inches of push off the hill. As the front weighted car is done accelerating off the hill, the rear weighted car is still accelerating, getting that extra 2" boost from the weight sitting higher up on the hill. Basically, in car speak, the rear weighted car has more hp than the front weighted car.

Whether or not this is correct, logically it makes sense. BUT! I've been known to be wrong before. Enlighten us with your wisdom John. We're ready.

Who can tell me why the car slowed so much with the weight in front?

 

[W&K Creations]

hey TxChemist

weight and mass are different correct? Weight is truly mass plus the effect of gravity on and thus can change depending on gravity or other forces acting on it while mass is set and standard to an object and doesn't change unless u manipulate said object....

as such the formula he used with weight on one end for PE and mass on the other end for KE would actually be essentially correct if his definition of "weight" was mass times the effect of gravity...

just curious LOL..

isn't the concept of two objects with different weights falling at the same rate really only apply in a vacuum....and the reason that in any other situation where your not in that vacuum, a heavier object does fall faster than a lighter object because of friction and resistance in the system which is part of the reason why a heavier car will be faster than a lighter car, the increased mass and effect of gravity on that mass allows it to overcome friction, resistance, MOI etc better......but only at least up to a certain point where significantly increased weight itself actually leads to increased friction which now overshadows its ability to generate more speed (and why we have seen that a 6 oz car is just as fast or faster than say a 12 oz car (I made those weights up just to illustrate the general concept that beyond a certain point more weight does not lead to a proportional increased speed forever and in fact goes the opposite way). If that is true then the concept he was stating is true....a heavier car is faster than a lighter car....its just the formula he used doesn't show that because its missing the effects of resistance and friction and MOI etc...

I still think the virtual use of blocks to describe the concept (not necessarily validate the scientific formula) was a good one for basic concepts....energy is neither created or destroyed in the system....just converted into other things....and going from blue all to green isn't reality as some blue goes to "red" which is energy going to friction/resistance which means less to green which is needed for speed which he tried to show....if any of the scouts next year ask me about this stuff the block idea is what I plan to use to show them....

 

[Obsessedderbydad]

Yes but isn't gravity constant?

 

[resullivan]

Ok, after thinking about it I have changed my answer. Is it because of inertia? A lighter car slows more quickly on the straight away?

 

[txchemist]

WOW! lots of smoke coming out of the ears as we think about this.
Yes weight = mg, but that sort of hides the deal about mass is on both sides and does not change speed. And, most friction does not involve mass either.
and Yes- I agree using the blocks is a super way to demo to get an understanding of the problem.

OK, we have some close hits, but not complete enough.

So a little side divergence- here is our cart-you have to imagine the wheel-
case A cart is empty,
case B it is full of milk in pails, but same dimensions so weight is 10x but air resistance is the same.

and the farmer has the same force to put the brake on in both cases- but does the heavy cart take a lot longer to stop?
Later on we can look at running the cars in a vacuum and see if the light car runs just as fast, but the above cart example will get a lot of us on the right track.
Now when you think that a big semi can not be stopped fast- the problem is the brakes overheat and fail- but IF the heat could be taken care of ( and heat is negligible on pinewood cars) the big rigs are not so hard to stop-
anyway- thoughts on stopping the cart A & B?

 

[resullivan]

Isn't that what I said right before your post?

 

[DerbyDad4Hire]

Why do we work so hard to keep the rear wheels off the rail? I would assume it would be the same problem if the weight was at the front with the dominant wheel riding the rail. Weighted wheels on rail = parking brake.

TX you might have already said this and I think Gravity X had it too.

My explanation is the non scientific common sense approach. I am at best 1/64 as smart as TX.

 

[IAE Racing]

My explanation is the non scientific common sense approach. I am at best 1/64 as smart as TX.

Heard it here first folks.