George

Actually, that number tells a lot. Figuring that the car weighs something like 2600 pounds with driver. Decending at 9FPS means that the gravity is doing 2600*9=23400 ft-lbs of work on the car every second. Since 1HP=550 ft-lb/s, a little division:

23400/550=42HP.

Yep, that downgrade was adding 42HP to the power driving the car along!

George

p2filz

i conqur

djct_watt

George > djct_watt @ Math
Me = (respect) x (you)

It takes waaaay to much brain power for me to process that stuff to that degree. It's been WAY too long since I've applied this stuff. Haha! 42HP!!! That's awesome. My next mod: making my driveway steeper

djct_watt

Not to be a wise-butt, but. . .

conquor = to defeat or subdue by force, especially by force of arms.

conCUR = To be of the same opinion; agree

George

I twiddle concepts like this every day and make a valiant attempt to help others learn how to do it too! A lot of it is just recognizing a situation to which the basic tools of physics can be applied, something that comes easier with practice.

One of my christmas gifts from my wife is Richard Feynman's book "The Pleasure of Finding Things Out". The title kind of says it all!

Another way to develop a basic understanding of uphill/downhill power is to drive a VW bus. Any vehicle that weighs 4000 pounds and makes 50 HP will make the driver very aware of what hills do to the vehicle's performance. That hill on which Edlin was bustin' the ton downhill would have my Bus down to about 40MPH on the uphill!

djct_watt

The one I'm having trouble applying is: to which vector does the force apply? Is that all downwards force? I'm lost when it gets to that point. I'd start busting out some tip to tail drawings, but I'm extraordinarily lazy right now, since I just ate a ton of food.

Would it be the hypotenuse vector of 155ft/s and 9ft/s?

djct_watt

Ok, I busted out some calculations out of boredom. I'm no expert, so please correct me if I'm wrong.

-------------> Car
|
VFall

I think you were right about 42hp, but isn't that at a downward vector? I'm really lost, so I might be totally off track.

155^2 + 9^2 = 155.26^2

And that means the fall has increased the distance traveled by .26 (as opposed to 155 flat). Therefore
0.26*2600 = 676
676/550 =1.23hp?

Am I wrong or am I right? I have no idea!

George

You can think of it in two different ways. I was thinking of it strictly in terms of energy conseveration. The gravitational potential energy of the car was decreasing by 2600lbs*9ft/s or 23400 ft-lb/s. This energy had to go somewhere, and it went to overcoming the drag of the car. Since 550 ft-lb/s is commonly called a horsepower, I just did the division to get 42HP. Kind of spooky, as I was just watching the Hitchhiker's guide to the galaxy (BBC version) with my daughters...

The other way to think of it is in terms of the car's motion. The car is going 155ft/s downhill. The force vector in the direction of motion is the weight of the car times the sine of the angle that the road makes with the horizontal. We know the run and drop of the car (155&9) so we can calculate the angle as tan[theta]=opp/adj=9/155 or [theta]=3.4 degrees.

The force vector is therefore 2600*sin3.4 or 156 pounds.

Multiply this by the vehicles speed vector of 155m/s and you get 155ft/s*156lbs=24,000ft-lb/s. Divide this by 550ft-lb/s to get horsepower and you get 44HP. The difference between the values is due to rounding error because of the small angle, which is why I prefer the energy conservation approach.

George

whatmovesu

R U a physicists?

p2filz

ya i conqur with that too, its just easier this way

killerxromances

with mods, i've gone 118mph. But, who cares, top speed is boring. autox and road course.

Cya

mike51392

THATS FUNNY IV PUSHED MY BOX TO 110MPH AND THEN THE CAR SHUTOFF AND DROPED TO 70MPH I THINK

killerxromances

Thats funny that you use all caps to tell your story. And your car didn't shut off, the rev limiter isn't at 110mph, so you just let off the gas.

Cya

whatmovesu

RedneckwithanxB

yeah, i had the box up to 125 comin back from HIN miami.....

barried in the governer

but i kept up with a C class benz, a hemi C, and an S40

hotbox05

redneck you have what mods? because i dont believe you've gone 125.....

djct_watt

Ah yes! I get it now. Thanks for the explanation!

djct_watt

You do realize that you can keep up with a Formula 1 racer, if they aren't racing. . . so it's not exactly something to brag about.

A hemi C has 340hp. . . and is a LOT faster than an xB. . . even a turbo xB. Even if you somehow managed 200hp, you'd still have a tough time. I'd think that you'd need a MINIMUM of 250 to keep up. I think the 300C was just driving normally (or was in a semi-hurry).

RedneckwithanxB

Im runnin with a blower, pus i was not the first car in line, so i was picking up a lttle bit of a draft

hard2excribe

122... flat land ...tail wind....stock auto xb