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Don't get why this law works the way it does...

U

Ultima

Platinum Member
Say someone was driving a car on the moon with ultralight wheels and let's pretend there's no rolling resistance at all. The driver floors it. The top speed should be really high, right? How come acceleration isn't linear? Since there's no aerodynamic resistance and no rolling resistance shouldn't the car keep accelerating at a constant pace. Why does the acceleration rate slow as the car goes faster? Say the car was on a train going 200mph and the driver floored it (again pretending there's no rolling resistance at all). Would the car accelerate painfully slow as if it was already going 200mph or accelerate as normal. It would accelerate as normal, right? So how come the car on the ground next to it already doing 200mph (and with no resistance) accelerates much slower?

*physics newbie*
 
There IS no law of diminishing acceleration. It so happens that a car moves because it is driven by parts that move at a finite speed. When you approach that limit, the vehicle stops accelerating.
 
Say someone was driving a car on the moon with ultralight wheels and let's pretend there's no rolling resistance at all. The driver floors it. The top speed should be really high, right?
Click to expand...
The top speed would be right around 0 MPH since the combustion engine wouldn't work properly in the Moon's atmosphere. I guess you could use one of those newfangled electric cars though. 😉

As far as the car on the train...it depends on your frame of reference.
Assuming that the train is keeping a constant speed, then the car on the train would accelerate at the same rate as it could if it were on the ground.

Generally, in the normal frame of reference, the speed of something is determined by the difference between the traveling object and the surface it is moving on.

Of course, when your car is sitting in your driveway, you don't think of it as moving but it is still spinning around the earths pole as well as hurtling around the Sun. So, there are several ways that you could determine it's speed. That's what frame of reference is all about. 😉


Edit -- Wanted to add that if YOU were on the Moon (in a space suit, of course 😉) you would still not be able to run infinitely fast. It's just a physical limitation.
 
Originally posted by: Hayabusarider
There IS no law of diminishing acceleration. It so happens that a car moves because it is driven by parts that move at a finite speed. When you approach that limit, the vehicle stops accelerating.
Click to expand...

By jove I think he's got it.
 
Hay, the Law of Deminishing Returns refered to the fact that before the edit, the post was blank. Ultima was practicint ready, fire, aim.

The answer is that there can be no auto accelleration on the moon because there's no atmosphere.
 
What slows an acceleration curve on earth is wind resistance which increases witht the square.

You guys obviously don't know about the light speed piston.
 
hmm damn this car-testing program then.
Even when I set all mechanical losses to 0%, aerodynamic drag to 0 and rolling resistance to 0 the acceleration curve still, well, curves even with a flat engine response (same HP no matter the RPM).
0-40 in 4.5 but 0 - 80 in 10.9 not 9.0.

Uh, so basically if the car had no power losses then it would accelerate the same if I floored it at 3000000000mph as at 10mph? Assuming constant power, no losses, the mechanics can handle it, etc... this is just a physics question forget about those details 🙂
 
Originally posted by: Moonbeam
What slows an acceleration curve on earth is wind resistance which increases witht the square.

You guys obviously don't know about the light speed piston.
Click to expand...

Read up a bit on that and it looks like it increases with the cube :Q

No wonder I haven't seen any 300 or 400mph cars 😉
 
M, I saw the empty post too 😉

I was not referring to your post directly, but you summed up his unstated law by using the phrase "diminishing returns" I did what most college students do so well on the internet and plagerized 🙂 to define his conundrum. And Ultima, assuming a constant force, acceleration continues. Of your want to bring in wheels, escape velocities, near light speed effects, etc then all bets are off.
 
Hehe Hay,

Assuming a constant force, the accelleration is also constant and if you double your speed the wind resistance is four times as bad. That squaring.
 
0-40 in 4.5 but 0 - 80 in 10.9 not 9.0

That is gearing. In lower gears for a given rpm, the tires cover less distance than they do in higher gears. With no gravity or wind resistance, you start to accelerate faster as you move into higher gears.

Plus you still have to move the mass from a dead stop, which takes more energy than increasing velocity of an already moving object, if you trying to move it in the same direction it was going.

When you reach max rpm in your highest gear acceleration will stop.

If I can get this correct, your top speed will equal:
rpm x gear ratio (final ratio including trans and differential) x circumference of the tire x 60 (minutes) = inches per hour traveled.

Divide by 12 to get feet per hour and divide again by 5280 to get miles per hour.

I think I goofed that up, Somebody correct that....
 
Originally posted by: MrChicken
0-40 in 4.5 but 0 - 80 in 10.9 not 9.0

That is gearing. In lower gears for a given rpm, the tires cover less distance than they do in higher gears. With no gravity or wind resistance, you start to accelerate faster as you move into higher gears.

Plus you still have to move the mass from a dead stop, which takes more energy than increasing velocity of an already moving object, if you trying to move it in the same direction it was going.

When you reach max rpm in your highest gear acceleration will stop.

If I can get this correct, your top speed will equal:
rpm x gear ratio (final ratio including trans and differential) x circumference of the tire x 60 (minutes) = inches per hour traveled.

Divide by 12 to get feet per hour and divide again by 5280 to get miles per hour.

I think I goofed that up, Somebody correct that....
Click to expand...

No I think I got it now 🙂 Hayabusa and others cleared it up for me... also I assumed a flat power response which means gearing doesn't matter 😉

Also, 0 - 40 is mph not distance, so even if the distance to get to 80 is much much longer the time should only be double (just like it takes twice as much time to brake from 100 as it does from 50 although the stopping distance is 4 times as long). Anyways, yeah, that blank post was from me hitting enter too soon 😉
 
Originally posted by: Moonbeam
Hehe Hay,

Assuming a constant force, the accelleration is also constant and if you double your speed the wind resistance is four times as bad. That squaring.
Click to expand...

or cubing in the case of air resistance. go 2x as fast it's 8x as bad 🙁

What about tire resistance? Is that a square or linear?
 
"Also, 0 - 40 is mph not distance, so even if the distance to get to 80 is much much longer the time should only be double (just like it takes twice as much time to brake from 100 as it does from 50 although the stopping distance is 4 times as long). Anyways, yeah, that blank post was from me hitting enter too soon "

I know it is mph.

With lower gears you dont travel as far or fast as in higher gears. Without wind resistance, a car accelerates faster from 40-80mph than it did from 0-40, unless it is grossly under powered or over geared. But you need the lower gears to get the car moving without stalling the engine or breaking parts. I used to drag race, so i talking in terms of 1/4 mile drag cars.

Cars take longer to stop at higher speeds for a number reasons, mass x velocity, heating of brake components, and friction required at the contact patches of the tires to name a few.
 
Friction is usually represented by a coeficient, so it's a constant. It doesn't do anything, but doubling your speed does not make air resistance go up 8 times only four times. If you mean by 2x doubling your speed twice which is really 4x then 8 is correct, but it isn't ever a cube relationship. Air resistance increases with the square of the velocity.
 
Friction is usually represented by a coeficient, so it's a constant. It doesn't do anything, but doubling your speed does not make air resistance go up 8 times only four times. If you mean by 2x doubling your speed twice which is really 4x then 8 is correct, but it isn't ever a cube relationship. Air resistance increases with the square of the velocity.

Right, What I meant, but didnt explain was that as the speed rises so does the need for increased friction at the contact patches. Since that is a constant, what is more than enough at low speeds becomes less than enough at higher speeds, and that results in skidding. So even if your brakes were strong enough, there isnt enough friction available to handle higher speeds to stop the vehicle, you stop the tires, which skid, but the vehicle doesnt stop.
 
well for one thing there is no air resistance on the moon... ever put your hand out the window going 150 km/h or faster?

hard to keep it without flying back.. 😛
 
Originally posted by: MrChicken
"Also, 0 - 40 is mph not distance, so even if the distance to get to 80 is much much longer the time should only be double (just like it takes twice as much time to brake from 100 as it does from 50 although the stopping distance is 4 times as long). Anyways, yeah, that blank post was from me hitting enter too soon "

I know it is mph.

With lower gears you dont travel as far or fast as in higher gears. Without wind resistance, a car accelerates faster from 40-80mph than it did from 0-40, unless it is grossly under powered or over geared. But you need the lower gears to get the car moving without stalling the engine or breaking parts. I used to drag race, so i talking in terms of 1/4 mile drag cars.

Cars take longer to stop at higher speeds for a number reasons, mass x velocity, heating of brake components, and friction required at the contact patches of the tires to name a few.
Click to expand...

Yeah, yeah, I was just debating the physics though, not concentrating on details like what (which is why I said to ignore resistance, gearing, power losses etc 🙂)

Someone's gotta come up with a car with a aerodynamic co of 0.01 🙂
 
Originally posted by: MrChicken
Friction is usually represented by a coeficient, so it's a constant. It doesn't do anything, but doubling your speed does not make air resistance go up 8 times only four times. If you mean by 2x doubling your speed twice which is really 4x then 8 is correct, but it isn't ever a cube relationship. Air resistance increases with the square of the velocity.

Right, What I meant, but didnt explain was that as the speed rises so does the need for increased friction at the contact patches. Since that is a constant, what is more than enough at low speeds becomes less than enough at higher speeds, and that results in skidding. So even if your brakes were strong enough, there isnt enough friction available to handle higher speeds to stop the vehicle, you stop the tires, which skid, but the vehicle doesnt stop.
Click to expand...

Well I did a search on air resistance and came up with this formula:

Wind Drag Force (in Watts) = (Frontal Area in Sq. meters) * (CO of air drag) * (meters/second ^ 3) * (air density/2)
That last one is the air pressure. It's the velocity cubed thing that makes me think its a cube relationship.
 
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