Unusual Physics: Understanding a Strange Truck Phenomenon in Reality

[ohwilleke]

This is set in reality.

You have a truck, set with high wheels(railroad wheels). Your driving on the railroadtracks going 50km/h. It's completely dry. You stop easily and quickly.

The weird thing. If its raining and you try to stop from 50km/h, you gain 8-10km/h on average.

How is this explained?

How do you observe your speed? Are you using the speedometer (which really is a proxy for wheel RPMs) or are you using a radar gun?

If the former, the reduced friction should increase RPMs without necessarily producing a like increase in radar gun speed.

 

[ohwilleke]

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.

Sounds to me like you are using the truck's speedometer to measure speed. Problem solved.

 

[Cliff_J]

A meta comment on what wrong with a forum mixed with a personal attack?

I've only gone on ice and spun my rear tires at 160MPH (2160 RPM at the wheels) and grabbed the handbrake and yet experienced no forward acceleration. Must just be some repeatable isolated personal incident where your physics don't apply.

 

[Bystander]

Do you understand the mechanism by which the final velocity of a vehicle with locked wheels can be greater than that of the same vehicle with wheels rolling synchronously along a track with no change of energy, or not?

That was the question posed by Gokul and echoed by you. No friction, no magic in differentials, no intuition --- just old-fashioned, smash-mouth physics.

 

[CronoSpark]

This is set in reality.

You have a truck, set with high wheels(railroad wheels). Your driving on the railroadtracks going 50km/h. It's completely dry. You stop easily and quickly.

The weird thing. If its raining and you try to stop from 50km/h, you gain 8-10km/h on average.

How is this explained?

The rotational energy is transferred to the truck and not to the ground.
Since it is raining, there is a small (and probably negligable) amount of that energy that is loss due to friction.

 

[Kittel Knight]

The rotational energy is transferred to the truck and not to the ground.
Since it is raining, there is a small (and probably negligable) amount of that energy that is loss due to friction.

It would be ok, if the truck was rolling...

 

[Cliff_J]

Nope.

A wheel sliding on a track offers less friction than a roller bearing? If so, as Fliption said on page 1, why even bother with rolling wheels when a McCoy oiler dripping water in front of a steel skid would be superior and cheaper?

Torque applied to the axle would cause angular acceleration - this is rotational and it would then be transferred to the vehicle. That is similar to the classic statics problem of a bridge with a fixed end pivot and the other end supported vertically but not laterally. Even if we deform the structure or apply thermal coefficients of expansion to the bridge, its net lateral movement as a structure is near zero. And even on the free end, only a few inches.

At best the combined torques would serve to rotate the entire vehicle and compress the front springs/expand the rear springs. So if we measured from the top of the vehicle there might be a small lateral displacement of that point. But if the measuring point were as low from the center of rotation as the previous was high, the results would negate themselves to measurement errors.

Where is the lateral acceleration coming from? If this is basic physics, please provide the simple homework problem where the simple structure experiences an acceleration from the translation of angular to lateral.

 

[CronoSpark]

oh... what i was trying to say was that the force of friction acting on the wheels is negligable compared to the force produced by the wheels onto the truck when it is raining, which results in that acceleration when braking.

i guess a catapult design might be useful to illustrate this idea...