This has always haunted me. Any questions or want me to explain further I will. Thanks in advance for all that reply.
You asked and we answered nine months ago. Maybe check your theoretical car for carbon monoxide leaks?
fucking lol
Man I feel like I just wasted my time doing a napkin calculation now lol
Does the engine stay running and the throttle open to whatever degree it was before it was dropped? Because if not, the answer is practically nowhere. The mass of the wheels and tires is trivial compared to the rest of the mass of the car, and if the only energy you have in the system is the tires rotating without the power of the engine behind them I’d doubt it would be enough to move it more than a couple of feet. For instance my car weighs 3098 pounds and the wheels including tires are only something like 29 pounds each, so 119 pounds or so in total. They’re only 3.84% of the total weight of the car. (Yes, I’m deliberately conflating weight and mass here, because I can’t be arsed. Bite me.) You’d better slap it into neutral first as well, because otherwise without the engine running the freewheeling tires would have to fight the engine through the transmission and that’d rob you of basically all of the energy to begin with.
Otherwise, if you knew the mass and diameter of the wheels and the mass of the rest of the car, you could calculate this in a blithe sort of spherical-inelastic-cows sort of way. I think you’ll be disappointed by the results. A layman’s approximation without doing any fancy math on the matter would predict that you’d punt the car forwards in the above example for a single instant at the equivalent of about 3 miles per hour, after which it’d just be rolling under its own inertia.
However, you’d be quickly stymied in reality by the fact that dropping practically any car onto its wheels from 20 feet in the air would, at minimum, bottom out the suspension and possibly cause the tires to contact the insides of the wheel arches, stopping them dead. This is before we get into any other damage caused by such a drop. So you’d need a special suspensionless Solid Body Physics Car to test this with. I’m picturing something like an enormous pine box derby car.
However, you’d be quickly stymied in reality by the fact that dropping practically any car onto its wheels from 20 feet in the air would, at minimum, bottom out the suspension and possibly cause the tires to contact the insides of the wheel arches, stopping them dead.
OP’s question very much feels like a question an elementary school boy would dream up, so I pretty much imagine a kid playing with a toy car in this situation. In that case, the numbers and reality don’t matter, it’s all about the fantasy and built-up anticipation of the potential energy and the car going fast - the action is what matters, I don’t think it’s a serious question.
The speeddometer is reading one hundred mph. And the engine going 100 mph. Ok let me rephrase. Trying to put it best i can. If you got a car reved it up from a still posistion to 100mph and just let it travel down a straight strip. Would it reach anywhere to a one mile marker? Or would it travel half a smile or what?
It seems like what you’re actually asking is how far will it be able to coast assuming a starting speed of 100 MPH.
That will be hugely dependent on tons of variables, not least of which being rolling resistance of the tires on the pavement (itself affected not just by the tires but also temperature, and moisture, and diameter of the tires, and roughness of the asphalt…) plus a car’s drivetrain is chock-a-block full of frictional losses throughout oodles of bearings and the constant rub of the brake pads on the rotors even when the pedal isn’t depressed, and so on and so forth. So I don’t think anyone will be able to accurately calculate that for any particular car. A field test is probably your best bet. If you’d like to not get arrested, start from 50 MPH instead and double your result. I conjecture that the deceleration from road friction and other losses will be near as makes to difference to linear.
A car rolling in neutral on a flat surface can coast pretty far. With a lifetime of driving experience behind me, my gut feeling says a quarter mile is definitely possible, and half a mile probably is as well. Coasting a full mile feels like a bit of a stretch.
exactly
Almost nowhere is my assumption. Might roll slightly forward if we assume the wheels and axles survive being dropped.
So let’s do some shoddy calculations.
The amount of kinetic energy in a spinning wheel can be described as
KR = 1/2 × I × w^2
Where KR is rotational kinetic energy
I is rotational inertia
And w is angular velocity
Assuming the wheels are uniform cylinders their rotational inertia follows
I = 1/2mr^2
Where m is mass and r is radius
Let’s take a car, say the Toyota 86. It’s wheel radius is about 0.2m and it’s wheel weight is about 10 kg so it’s rotational inertia is about 0.2kgm^2
Therefore it’s KR at 100mph (45ish meters per second) is
1/2 × 0.2 × 45^2 = 202.5J
Multiply by the 4 wheels we get about 800J of rotational energy
Lets assume all of this rotational energy gets turned into kinetic energy punching the car forward. (This is beyond generous)
To find it’s velocity forward we can use
KE = 1/2mv^2
Where KE is kinetic energy
m is the mass of the car (about 1200kg for a Toyota 86)
And v is the velocity
Rearranging 800 = 1/2 × 1200 × v^2
Gives v to be 1.15m/s
Or about 2.6mph
You can sort of intuite how far a car will roll forward when it’s at 2.6mph and you’re not giving it any more gas, if not the answer is not very far, I’d guess maybe a meter or so. It should be noted that this is a gross oversimplification that glosses over details that would make the car move LESS far. Cars are driven forward by the power being applied constantly by the engine and not by the momentum in the rotating tyres. I’d love to hear others thoughts on this problem and if you agree or disagree with my methodology :)
I agree with your methodology and I’m further stoked to see that my assessment, which was largely a guess generated by inverting the relative speeds and masses of the body of the car and its wheels, arrived at a rather similar result. Neat.
(I used the weights from my Crosstrek, which is a bit heaver than a GT86 and mine at least has larger diameter wheels, but coincidentally uses an engine based around the same design. FA/FB gang, yo.)
20ft? Probably nowhere given the damage.
Agreed it wouldn’t go far. Even without the damage, the only stored energy is in the drivetrain. That’s the mass of the wheels, axle, transmission (moving parts only), and pistons, multiplied by how fast they’re moving (squared). Not something you could stop easily with your hands, but not much when it comes to moving the mass of a car.
Yeah the sudden force would tear the drivetrain apart, even if it wasn’t damaged by the fall.
Do you think so? It should be analogous to slamming on the brakes and stopping the wheels. Not great for a manual transmission but not like it doesn’t happen to new drivers all the time.
