Are the laws of physics different in pickup trucks?

It happens all the time: a pickup truck will go screaming past me on I-95 with its tailgate down, and nothing’s falling out of it. There might be garden tools in the bed of the truck, or trash bags, paint cans, bottles - hell, maybe even bowling balls - and they don’t fall out.

Meanwhile, inside my smooth-riding car, my windows safely closed against any stray gusts of wind, I can put my extra-wide specially made spill-resistant travel mug with its patented anti-slip rubber bottom lining on my nice flat dashboard, and if I so much as increase my speed by 1 mph, I’m wearing my coffee. I don’t get it.

Similarly, if I pack my groceries in the back of my SUV with anything more than a quarter inch of space between the parcels, I can count on even the gentlest of turns completely scrambling my cargo, allowing my screw-top wine to pulverize my Cheetos into a glowing (if still tasty) powder. It doesn’t seem fair.

Are pickup owners issued a get-out-of-jail-free card exempting them from the laws of gravity, inertia, momentum, and all that other stuff that never seems to bother Jackie Chan? Or am I paying some unknown Karmic debt by continually choosing vehicles that happen to also be evil vortices of exaggerated gravitational inconvenience?

Anyone? Bueller? Anyone?

I think it’s an application of the principle that allows drunk drivers to escape completely unhurt after their car slams into a propane truck next to a high-rise, causing an explosion that levels a city block.

Some people take such little care for their own well-being that somehow the universe steps in and takes care of it for them.

The truck accelerates to its present speed slowly enough that friction keeps most of the stuff in it from sliding completely out. It slides around, mind you; if the truck driver had 20 cups of coffee back there they would likely spill unless he was very careful. Once the truck gets to cruising speed everything in the truck is moving at the same speed. It moves around on turns and changes of speed just like stuff in your car, and stuff does likely spill out from time to time. You just notice stuff flying all over the interior of your car more than one notices stuff moving around in the back of the truck.

WAG - could have something to do with wind dynamics as it streams over the truck cab :slight_smile:

I think one reason is that automobiles experience more acceleration backwards (braking) and sideways (turns) than forward (accelerating). I often carry a lot of stuff on a station wagon and sometimes the luggage will fall against my seatback or the side, but usually not onto the tailgate.

Also, the coffee in your mug can oscillate, so it picks up energy over time like a pendulum.

While driving at a steady speed, object placed against the front of the bed will tend to stay there.

When the air passing over the top of the cab reaches the vertical back of the cab, it forms a vortex. If you look at it from the driver’s side, the vortex is going clockwise.

At about three or four feet above the bed, the air is flowing backward. But as you get closer to the bed, the air can actually be flowing forward (thus pushing objects against the front of the bed). Most of the time, it just means that air is moving along with the truck, and doesn’t push the objects at all.

The size and force of the vortex depends on the speed at which the truck is travelling. So, that doesn’t explain how the stuff stays in place when the truck starts up at a red light. I know that when I drove a pickup with light stuff in the back, I would often hear the sound of it slamming into the gate when I started moving.

Short answer: Get a really heavy coffee cup and see if that helps.

Apparently the Laws of Physics are different for pickup trucks, well so far as transporting of dogs is concerned. It stuns me that so many people here drive on the highway with their pets in the pickup’s bed.

I’ve lost lots of things out of the back of trucks, mostly light things but on occasion something heavy will fly out if I’m on a rough enough road. Usually though, everything stays inside because they are pretty heavy, it is surrounded by things that are very heavy, and the truck bed is pretty rough. Still, things slid around quite a bit, but no one cares if a box of nails slides a bit.

As a hijack, many folks leave the tailgate down thinking it gives them better gas milage, but in fact it gives them worse milage. The aerodynamics of modern trucks are designed to give the best airflow with the gate up.

A friend of mine who works at NASA’s Ames Research Center tells me of the time he stopped by one of the wind tunnels (they’ve got about half a dozen there) when they were testing pickups. When smoke was added to the airstream, it could be seen sliding neatly by with the gate up, but gate down, the smoke was scrambling all around inside the bed, disclosing an aerodynamic mess.

As for stuff sliding around the back of a pickup, sure, stuff moves around like crazy back there. The difference is you’re not going to care if a toolbox bounces around a bit. If you’re silly enough to put unrestrained sacks of groceries back there, you will arrive home with a smorgasboard back there.

Too true. A few bungee cords live in the bed of my truck to try to prevent such things. (Even the paper bags at our grocery store have handles.) If I’m careful, I have no more problems than we do in the trunk of my wife’s car, as inertia still exists in a closed trunk, too. :slight_smile:

Yes. You were not suppose to have figured this out. Now that you have started yapping, the Gods of Physics are going to have to kill you.

Hey, those guys practice that, the pros can do it with a home made flatbed.

      • A friend who used to live in desert southwest (and who knows a thing or two about building irrationally fast cars) said that if you drive a [big-motored] pickup truck over ~140-150 MPH+ with the tailgate down, the wind blows the tailgate back up and closes it.
  • I do know from experience that lighter stuff tends to tumble around right behind the cab, but doesn’t often blow out really. If you have empty plastic soda bottles back there and the rear cab window is open, you’ll get the bottles flying into the cab through the window much more often than you’ll get them flying off the back of the truck onto the highway.

it would take a lot of long-lasting acceleration to move a heavy paint can from the front of the bed to the back (6 ft.). Most guys driving in pickups don’t accelerate that much. I am sure that the cans will move 6 inches or so, but they are far more to hit the front of the bed from braking forces, as braking forces pull more G’s than accelerating most of the time. Coffee cups weigh much less and are moved much more easily.

Pickup Truck Physics Lesson:

Six guys in a truck. Three in the cab (including me) and three in the bed. One of the guys in the bed, Rob, starts goofing around and rocking the truck back and forth. The driver pumps the brakes–the guys in the bed fall over–we all laugh. Truck is moving forward again, Rob REALLY starts rocking the truck now. He is standing up in the middle of the bed, one hand on a bedrail on each side and throwing his weight left and right with all his might. The driver SLAMS on the brakes! Rob literally flies forward off his feet and headfirst into the steel toolbox. The truck stops, we all get out and look at Rob. He is unconscious (we think, we’re praying he’s not dead) and a bump the size of a golfball was swollen up on his forehead. Fortunately, Rob opened his eyes as we pulled up to the emergency room and he lived on to do even dumber things.

So the law of inertia still applies in the bed of a pickup truck.

I don’t know how modern a truck has to be for the tailgate to be streamlined, but my '84 Ranger definitely had less drag with the tailgate down. Every weekend I would take a 2 hour drive to the mountains, and if I left the gate up my poor truck would barely reach 70. With it down, I could go 75 or higher, and the truck ran noticably cooler.

Flying Monk , I have had exactly the opposite experience. My “scientific” experiment had the same results as the NASA wind-tunnel “test” mentioned earlier in the thread.

I bought a new full-size GMC in Arizona while living in New Jersey (many reasons for this, none important). After letting my dad dork around with the truck for a month I drove it to NJ. On the great open highways highways of New Mexico I conducted the tailgate up/down experiment. My results: with a brand-new truck I averaged almost 2 MPG LESS with the tailgate down. Having the tailgate up produced better gas mileage and less wind noise (actually noticeable at 75 mph).

As for the OP and stuff sliding around in back, all of the things posted so far are contributors. But truck beds also are not uniform surfaces like dashboards - even the bare metal has ridges in it, and the bed liners have rough surfaces to minimize the sliding around factor. And although three bungee cords have taken up permanent residence in my truck bed groceries can be safely stored next to the cab and arrive unscathed with halfway intelligent arranging.

“God reserves special providence for fools, drunks and the United States of America.”

Otto von Bismark

I can’t tell you how many times I have seen a plastic soda cup, soda can, or fast food bag sitting on the bumper of the truck driving in front of me. It amazes me to watch that can or cup stay there through all the bumps that spill my coffee. Anyone else notice this phenomenon. This ever happen to anyone? You buy a soda at the convenience store, put something in the bed of your truck, drive home. Get the thing from the bed and WOAH! My drink is STILL on the bumper! I know it’s the vortex of air keeping it there, but it’s still interesting.

There is not a scientific explaination. Apparently GOD (or something) protects those that don’t know better. I have wondered this myself on many occasions. It has been my experience that stuff will fall out or some other catastrophy will develop if I don’t secure my load.