Unusual Physics: Understanding a Strange Truck Phenomenon in Reality

[Rogerio]

1. missing data: what is the exact grade of the track? even slight differences from level will throw off results.

2. never use physics when logic will do.

The truck is not going down a hill, or munck99999 would tell it.
We need use physics - logic alone won't solve this problem.

BTW, I think I know the answer. However, 10 km/h seems to be too high...

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Added:

In fact, I could build a toy with this behavior, but I don't believe a real truck could act the same.

So, I keep my initial opinion: I don't see how could the truck speed up about 8-10km/h .

 

[PBRMEASAP]

We need use physics - logic alone won't solve this problem.

Words to live by

BTW, I think I know the answer. However, 10 km/h seems to be too high...

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Added:

In fact, I could build a toy with this behavior, ...

By all means, enlighten us!

 

[Rogerio]

Well, as I don't believe in the real truck behavior, I won't explain the toy till the end of the thread.

 

[Gokul43201]

2. never use physics when logic will do.

3. Never use logic without understanding the physics.

In this case, you used an incorrect line of logic because of a failure to understand the physics of the problem.

 

[munky99999]

Well I believe it's a Ford F250 and the wheels that are standard on those trucks touch the wheel. The rubber is what gives the driving and braking, then there are like training wheels type wheels, they are max 15lbs each those little wheels.

There is no grade on tracks, the trains pulling huge weights could never pull them more then like 10 degrees; and that's the brand new huge ones. If you take a 10 year old locomotive it won't be going much more then 5 degree slopes.

So I'm pretty sure slope isn't a factor. I'm in ontario, very very little slope in this province also.

I thought this thread died, my instant notification stopped working oddly.

Like your going along. Your going a constant 50km/h. You hit the brakes with constant force. It also doesn't matter what type of brakes(air, anti-lock, and ABS(whatever that means) were tried, all same result) While keeping your foot on the brake. The first thing the truck will do is gain 8-10km/h ontop of the 50km/h so the odometer and inertia feeling, saying 60km/h or so. After that you start to actually brake and you loose speed. all the way to stop.

I'm pretty sure i understand the friction change to increase speed. I do believe it would explain very well as to why.

If you work for the railway it's illegal to step on the rail even in dry conditions. OTher then near around crossings which usually are not as slippy because of cars driving over them.

 

[PBRMEASAP]

Well, as I don't believe in the real truck behavior, I won't explain the toy till the end of the thread.

Oh, I get it. You were just pulling our legs. Sorry, I'm a little slow when it comes to message board sarcasm.

 

[Gokul43201]

I'm pretty sure i understand the friction change to increase speed. I do believe it would explain very well as to why.

But it does not ! It can only explain why the deceleration is not as great.

Unless your truck is being towed/pushed by something, the only way to gain speed is from the rotational KE of the wheels. And even then, stopping them will result in heating of the brake pads, so it doesn't seem like there's anything to be gained there either.


Moreover, you claim that the speedometer shows this increase in speed. The only way a speedometer will show a greater speed is if the wheel (shaft) is spinning faster. There's no way that this can happen when you hit the brakes...unless of course, your speedometer works differently than most.

I'd like to see a source or reference that makes this claim. Or is this personal experience ?

 

[Cliff_J]

Some vehicles gather the speed from one tire. If there was a traction difference between the tires and its a tire connected to the other by an open differential than it could result in one tire spinning faster. The traction difference could even be a braking difference where one drum brake's leading shoe is grabby because the drum surface is irregular. Remote possibility, but it doesn't sound like multiple controls and data points were gathered here.

If the test data couldn't be gathered using a radar gun or fifth wheel to eliminate system interactions then the data may not reflect reality at all but simply a measurement error.

Otherwise did the people in the truck get set back in their seat like someone had accelerated instead of braked? Probably not. A rock hanging from a string tied to the rear-view mirror would be an indication of how to draw the vector...

Cliff

 

[munky99999]

Well my dad and relatives work for CPR and have like a safety book thing for it. I didn't believe it at all. It didn't make sense.

Low Quality Pic of one.

This one shows better, the smaller truck is what I'm talking about.

Personally I have never actually been in the presence of such a thing, I've never actually drove one of them.

From the pics you can see that the wheels are much smaller then what a locomotice has.

 

[WORLD-HEN]

let the speed of the train after considering the retardation due to friction be X
let the speed of train withought considering the friction be x
let retardation due to friction be y

X = x - y

now, if the friction decreases by an amount m

X = x - (y - m)
= x - y + m

so the speed increases by m.

 

[gerben]

but how do you explain that "the" friction decreases when hiting the brakes?
(and which friction do you mean?)

 

[Gokul43201]

let the speed of the train after considering the retardation due to friction be X
let the speed of train withought considering the friction be x
let retardation due to friction be y

X = x - y

now, if the friction decreases by an amount m

X = x - (y - m)
= x - y + m

so the speed increases by m.

x, X are velocities; y is an acceleration; m is either a dimensionless number, or a force but you also claim it is a velocity.

You are now in a dimensional mess !

 

[dextercioby]

It's more of a logical one...Adding accelerations and velocities...

Daniel.

 

[Integral]

I envisioned a train, not a truck, I have seen these rigs, looks like a lot of fun( as long as nothing else BIG is using the tracks!) . With a full train behind you a lot of strange things can happen, but this is not a train.

I retract my overly quick guess.

More data is needed. External speed measurement or an accelerometer would be necessary to determine what is happening. It is not clear how a trucks speedometer could read anything with the rear wheels locked up.

 

[T.Roc]

maybe munky99999 wanted a formula to explain, but i thought a common sense explanation was what he sought. ??

(keep it simple, stupid)

now i realize no one here has experienced this, so you are viewing it differently than me. to me this is like someone wondering why it is harder to walk uphill (who has never walked up a hill).

on my feet (running from ground to ice, "locking" up legs), on skis (as mentioned), in full size pickup (regardless of 4WD & ABS), and in 13 ton tracked armored personnel carriers, i have experienced momentary increase of speed due to my momentum crossing over to a medium with much less friction (ice).

a geared wheel has both engine resistance and road friction (also resistance). when you lock up a wheel, you are bypassing the engines braking effect, as well as decreasing the surface area of the wheel used to resist. this lower resistance, small surface area will travel across the next distance of "slick" road/track at a faster rate than would have been allowed by the circumference of the wheel on "normal" conditions.

also, the odometer does not need to be connected to a drive wheel, allowing for correct speed indication even when drive wheels are locked up.

TRoc

 

[Gokul43201]

If the speed really increases, there is a forward acceleration, implying a net forward force.

Where does this force come from ?

 

[krab]

So if you are driving along in this vehicle, and a slight grade comes along, you needn't use extra fuel to make it up the grade? You can just hit the brakes hard enough to lock the wheels and so gain 8-10km/h, then let go the brakes, and sail right up the grade? I don't think so!

I think it is true that if you have parallel tracks, 2 trucks at 50km/h side-by-side, and one brakes normally, while the other locks the wheels, the latter truck can at some point be traveling 8-10km/h faster than the former.

 

[Kittel Knight]

the biggest ego...

It's more of a logical one...Adding accelerations and velocities...

Daniel.

You said nothing. This kind of contribuition is just ridiculous!

Then University of Toronto should start reading...phyics...

Daniel.

Certainly ur going to explain the phenomenon before someone does it, or...

should you start reading physics, too?

 

[munky99999]

my dad is still out of town in the safety class, so the books and stuff aren't available to me till like wednesday i think. I'll see what i can do.

 

[Integral]

also, the odometer does not need to be connected to a drive wheel, allowing for correct speed indication even when drive wheels are locked up.

I believe that speedometers and odometers make the basic assumption that the wheels are in rolling contact with the road surface. The actual sensor is at some point in the drive train but if the wheels are not rolling, any indication of speed is based on a incorrect assumption, and is therefore suspect. The only way to get correct speed data in this situation is to have either an external device (radar gun) or an on board accelerometer.

A big difference between one of these rigs an a normal car or truck is that the drivers hands and eyes are much freer to roam. Notice that the steering wheel is completely useless, there is no possibility of losing control and smashing into the ditch. All a driver has to do is control the speed, I would bet that it is very easy to lock the wheels up when braking, it is also a lot more likely that the driver will be watching the speedometer when braking, perhaps if someone where to do the same sort of thing in a car on the road you may see similar effects, but who, in a skidding car, pays a lot of attention to the speedometer?

The real question here is where is the speedometer sensor and how does it interact with the drive train. This is a more of question for a mechanic then a physicist.

 

[physicsisphirst]

There is no 'friction energy' part of the system. It is a dissipative power. Once energ is converted in this 'friction energy' (ie heat, sound etc.) it is gone. So having less friction doesn't speed you up, absence of friction doesn't add energy to the motion. As said before, this only works for an applied force, even when the breaks are on.

ok thanks for the explanation - it makes sense.
but wouldn't the absence of friction decrease the decelerating force and therefore create an acceleration?
what about a curling stone that you slide over a carpet that is on an ice rink. what will happen when the rock hits the edge of the carpet and slides onto the ice? will it speed up or not?
is this situation similar?

 

[marrsc]

The speedometer takes its reading from the transmission. Most automotive braking braking is done by the front wheels. These trucks have a high center of gravity. In the initial moments of braking the front wheels react first and unload the rear wheels breaking their traction and allowing them to spin faster than their true speed. Then the rear brakes must overcome the considerable rotating mass of the tires wheels axles and drive shaft transmission and engine before everything settles down and the speedometer again shows a true reading.
I hope I have stated my theory clearly enough. I am more of a mechanic than a physicist.

 

[Integral]

Marrsc.
We may be getting closer. Did you look at the pics of these rigs? The front wheels are jacked up, not in contact with anything but air! Looks like the only braking is from the rear.

 

[Gokul43201]

ok thanks for the explanation - it makes sense.
but wouldn't the absence of friction decrease the decelerating force and therefore create an acceleration?

No it does not. To have a forward acceleration you need a net forward force. This is Newton's Second Law.

By reducing the net backward force, you do not create a net forward force.

what about a curling stone that you slide over a carpet that is on an ice rink. what will happen when the rock hits the edge of the carpet and slides onto the ice? will it speed up or not?
is this situation similar?

This situation is similar, though not identical. But here too, there will not be any speeding up... only a reduction in the rate of slowing down.

 

[Gokul43201]

Marrsc.
We may be getting closer. Did you look at the pics of these rigs? The front wheels are jacked up, not in contact with anything but air! Looks like the only braking is from the rear.

I think the front wheels get jacked up only during maintenance, when the truck is stationary. Look at the pics of the moving trucks. It does look like they have their front wheels on the ground. If not, you'd need a rear wheel drive.

 

[marrsc]

I assumed that the trucks in the pictures with the front wheels in the air were in the process of being set up for the tracks. In some of the other photos they appear to be in contact. When I have seen these in action they always have their front wheels in contact with the rails. Probably so their front brakes will work. I am assuming that the cylinders that lower the guide wheels allow some travel, like a shock absorber. If they are rigid like a hydraulic cylinder then the shift of the center of gravity would be a lot less but still possible.

 

[munky99999]

there is 2 different Highrail systems, it just depends on if its front wheel drive or rear wheel. But CPR only uses all wheel drive which you can change to RWD, FWD, AWD

Also I believe it doesn't matter which you want.

Anyway. I still go along with the changing friction acceleration. Monday-tuesday I should be getting the thing.

 

[Integral]

Ahh... Yep, Now that you mention it, the ones that I have seen tooling down the tracks had all four wheels on the tracks. I think you have hit on the root cause.

 

[Gokul43201]

It certainly looks like a good plausibility argument.

I'm not entirely convinced about this part though.

...the front wheels react first and unload the rear wheels breaking their traction and allowing them to spin faster than their true speed.

Unless you have an increased torque on the rear wheels, they won't spin faster. Where does this torque come from ? Are the rear wheels coupled to something else ?

 

[physicsisphirst]

No it does not. To have a forward acceleration you need a net forward force. This is Newton's Second Law.

By reducing the net backward force, you do not create a net forward force.

This situation is similar, though not identical. But here too, there will not be any speeding up... only a reduction in the rate of slowing down.

oh dear
i think my physics needs more help than i realized if i have forgotten this sort of thing.
thank you for explaining all this, Gokul.