Why, when a moving vehicle loses a wheel or a hub cap, it speeds by the moving vehicle? Are they not going the same speed? What causes the loose wheel / hubcap to accelerate?
That’s a damn good question.
Since the radius of the wheel cap is less than the radius of the wheel, the speed of the cap on the road [at the moment of impact on said road] should be less than the speed of the car, assuming a constant axle speed for car and wheel cap. Right?
The only thing I can think of is that aerodynamics might have something to do with it. But I don’t know how.
Bocephus, welcome!
Every hubcap I’ve ever seen come off of a straight moving vehicle has rolled initially as fast as the car was going, and almost immediately slowed down to a speed slower than the car. Hubcaps usually come off a car when the car is turning though. The hubcap will (if it rolls in an upright position) be “flung” off of the car with the same angular velocity that the car had at the time of release (roughly). From there it should slow down. It may appear as if the hubcap were going faster than the car because the car is probably slowing during the turn, and the car and hubcap are now going in different directions.
People usually don’t notice the loss of a hubcap while driving. Lost wheels, on the other hand, tend to be noticed by the driver. Most people’s reaction to the loss of a tire is to slow down and stop. When the wheel is rolling at a speed close to that of the car, and the driver hits the breaks, it could appear as if the tire were accelerating relative to the car.
That’s my 1.54221 cents, but I may be missing something.
Let’s see, the wheels are spinning on your car due to energy being applied to them by the engine. They are fixed to the axle and the weight of the vehicle rests upon them. The axle and the weight are sources of friction for the tire.
If a tire comes off it is no longer held to the axle and I believe more importantly, the weight of the car is removed. The tire still has potential energy and when released from restrictive forces they spin faster for a few moments and given the proper trajectory will exceed the speed of the wheels remaining on the car and the car itself.
Feynn, your example would explain an entire wheel, but not a cap. However…
I just recalled that the only time this happened to me was when I was driving in the mountains, and I braked for a corner. Both the front caps came off, simultaneously! As I was braking, of course they “overtook” the car. Steve-o must be right.
Think about a baseball pitcher releasing a baseball as he pitches it toward home plate. The ball, at its fastest point will be still touching some portion of the pitcher’s hand. The instant the pitcher looses contact with the ball, it begins to slow. The ball will not gain any speed due to being released from the force that accelerated it. Acceleration requires a force in the direction of the acceleration.
When a wheel is released from a car, there is no added force in the direction of “acceleration”.
Caveat – In my pitcher example, the ball actually could gain speed if the pitcher REALLY sucked, and the speed increase were in the direction of gravity’s pull. 
So, as an analogy, the wheel caps could overtake the car in sheer speed terms, but they’d have to be falling down the canyon the car just avoided by turning
Right, Steve?
Isn’t the answer obvious?
It goes faster because it’s FREE! IT’S FREE! The hubcap is rolling, and bouncing and running for joy!
There ya go! Coldfire solved it… So, the lesson learned from this is: whenever you see a hubcap accelerating after being released from a car, it’s time to pull the ripcord on you parachute.
snort
Hot damn, we’re a smart bunch!
OK, next one. What’s the deal about parking on a driveway and driving on a…
:: large objects are tossed at Coldfire ::
… What? WHAT??
The baseball analogy is not relavent. Once the ball leaves the pitchers hand, it does not make contact with another surface.
When we were young ( and stupid) we used to ‘bowl’ for trashcans. this involved driving down the street, holding a bowling ball out the window and dropping it when we came to a trash can. The ball always overtook the car. Naturally this puzzled the hell out of me so I ask my H.S. physics teacher for an explaination. Best I can remember he said the ball picks up extra inertia by going from not spinning to spinning in a very short period of time. Think of a backlash on a fishing reel.
Another possible reason. There is usually a reason the hubcap came off, and often it is because some sort of object(a curb was hit) the hitting of the object will probably cause both the the hubcap to come lose, and the tire to jump in the air. As the tire leaves the ground, it suddenly has no resistance, and the differential allow it to spin much faster,(at least until the limited-slip effect kicks in, if you got one). The tire is suddenly rotating at a much faster speed than the other wheels when it throws off the hubcap. The tire will hit the ground and quickly decelerate to the speed of the car, but the hubcap, spinning at high velocity, will hit the ground and takeoff in relation to the car, at least for a little while, until fricton slows it down, since its not under a common source of power.
warmgun and feynn, you guys are hallucinating.
If you drop a bowling ball from a car window and the car does not slow down, the ball cannot exceed the speed of the car and it must slow down if it starts spinning. This is because some of the energy the ball had in moving forward is converted into spinning energy, so the ball must slow down. The only way it could speed up is if it bounced off something at a funny angle (i.e. you didn’t drop it on level smooth ground) or you are driving down hill.
Feynn, you are using words that make sense individually but when you string them together they mean nothing.
Feynn writes:
What? The tire has what potential energy? The only potential energy I can see is that fact that the bottom of the tire is compressed a little due to the weight of the car. When the wheel is released suddenly, that compression would act like a spring and push the wheel up, but I don’t see how it could increase the horizontal speed of the tire. It might hop a bit, but I don’t see a reason it should speed up.
Steve-o is right with this - if you lose a wheel, you slow down, and the free wheel doesn’t slow down much, so it appears to go faster. But really, it is the car that is slowing down.
wolfman’s idea for a hubcap is somewhat plausible, but a little far fetched. I am still in the ‘you are imagining it’ camp.
But, did you ever see the Real TV type footage of the racecar that lost the tire?
Get this: Car enters banked turn. Tire comes off, and starts to roll up and away from the car. As the car rounds the turn, the tire rolls back down, and reattaches itself to the rim. No shit.
Thought that might give you a pleasant mental image, if not any help in this matter.
douglips -you’re guessing.
Now it’s been a long time ago and I wasn’t taking notes (maybe we did slow down a little?) but I know what I saw. Regardless of how it happened - it happened!
Anyone got a spare bowling ball laying around? I think a little field study is called for.
Warmgun, you say you “dropped” the ball. Whay exactly do you mean? Naturally the ball would exceed the speed of the car if you actually BOWLED it. If the car’s going 30 mph, and you heaved the ball, say, 10 mph, then the ball will go 40 mph (at least until friction slows it down).