centripetal force

[MPGranger]

[QUOTE=pletby;261518]Fighter jets flying in circles to gain speed? Argh. Ignore whoever you 'heard' that from. They can gain speed by using gravity (diving) or increasing throttle, but certainly not from turning. That's an awesome way to scrub off speed and slow down!

I have a similar post on cleanMPG and someone there posted a nice explanation, with illustration, of how that works out. I probably failed to communicate the concept properly. I don't want to repost it, in fear of looking like I'm trying to take credit for something I did not think of.

[Rokeby]

FWIW, I'm the one who posted an answer at CleanMPG.
Here's what I said, I only wish that it were shorter:

In a simplistic way, what your friend said is correct about aircraft.
But I don't see it being applicable to road vehicles. Here's why:



In sustained level, constant velocity flight an aircraft's lift is the same as the aircraft's
weight. In a banked turn, what was the lift vector in level flight now has two
components; a vertical component, and a horizontal one called centripetal force.

In order to keep flying level in the turn, the pilot needs to increase the
vertical component
to equal the planes weight. One way to do this is to increase forward velocity which
increases lift and the vertical component. If the bank angle of the turn is increased,
more velocity/power is required to keep the vertical forces balanced and the plane in
level flight.

What your friend was probably trying to convey was the idea that where a long
straight run-up to increase velocity is not available, an aircraft flying in a circular
pattern and increasing the bank angle and increasing the forward velocity to provide
the necessary lift can achieve a similarly high velocity.

(While it is not strictly needed in this primer, as bank angle is increased, to make a
"coordinated turn," some rudder and elevator inputs are also needed.)

The problem with a car is that there is no lift vector to generate centripetal force.
Things get a little sticky here. Some folks would have us believe that in a turn there
is a thing named centrifugal force pulling the car and occupant outward. As I
understand it, the correct interpretation is that the turning wheels generate a inward
centripetal force but the momentum of the car and its occupant is in a straight line.
The car and the occupant appear to be pulled outward as the vehicle undergoes a
constant acceleration -- change in velocity or direction -- in the turn. But it is only the
difference between their tendency to want to continue in a straight line, and the car's
turning inward.

For a car, the inward centripetal force needed to successfully negotiate a turn can only
come from the friction of the contact of the tires with the road. This is a constantly
changing function of the:
* stickiness of the tires -- they get sticker as they heat up,
* suspension -- amount of compression, shock rebound/dampening rates,
* front/rear weight distribution -- it changes under acceleration and braking,
* individual tire loading -- for vertical load analysis purposes a four wheeled vehicle is
"indeterminate," which is to say very difficult to calculate with certainty,
* turn radius -- constant, increasing, decreasing,
* angles of departure between the line of travel and each tire's centerline,
* road surface -- texture, dry/wet, sand/gravel, flat/wavy
and probably some other stuff as well.

For any given degree of road surface banking on a roughly circular course that has a
constrained usable width -- a road, an off ramp -- there is only so fast you can go
before the tires start to loose traction and loose some or all the formerly generated
centripetal force that was holding the car in the turn. Sure, you can add more steering
angle when the front tires begin to scrub, or you can add power to get the powered
wheels spinning.

But these things would require using more fuel in the turn, not less.

And then, I could be wrong... again.

[euromodder]

Originally Posted by MPGranger

Also, it's apparent that I do not posses a physics degree. I was wrestling with centrifugal/centripetal conundrum; I used wikipedia, but alas I still don't get the difference.

The centripetal force is external, like gravity pulling at a satellite.
Or you pulling on the control ropes of a model airplane on a wire ; or swinging a lasso.

If you stop pulling on the rope, the plane or the lasso will shoot off due to the centrifugal force of the moving object which is no longer countered by the missing centripetal force.

[Rokeby]

Not to get too technical, but:

The forces on a turning car are complex. When you are in a car turning,
you feel like you're being pushed toward the outside of the turn. People refer
to this as the centrifugal force, a term that is a unfortunate because there
isn't any force pushing you outward. Centrifugal force is what physicists call a
pseudo or a fictitious force, because it doesn't really exist.

The force that makes a car turn is called the centripetal force. Centripetal
literally means "toward the center"...

A race car doesn't have a string making it turn. The tires have to do that job,
and they do it by exerting force toward the center of the turn. Engineers talk
about lateral force. The lateral force is perpendicular to the direction the car
is going at any moment.

The size of the centripetal force is given by multiplying the mass of the car
by the speed of the car squared, and then dividing by the radius of the turn.

Read more at StockCarScience

FWIW, when the centripetal force acting on a body traveling in a circular
path is "cut," goes to zero, the body continues in motion in a straight line
tangential to the former circular path, and not radially as might be guessed.

[pletby]

The way I see it it's pretty simple. Anything I do to take myself off a straight line is going to cause extra drag. If I happen to be going downhill enough in a turn I MAY gain some energy, but otherwise it's waste. I think Newton had a law to that effect. I'm to tired to look it up. Good night.

Nice explanation on the airplane too. I've been in a turn tight enough to require full throttle to maintain altitude. It's pretty fun. Especially when transitioning from turning one way to the other as practice for steep turns. Passengers begin turning green.

[euromodder]

Originally Posted by MPGranger

Anything I do to take myself off a straight line is going to cause extra drag.

Yup.
We steer through additional friction between the ground and the tires that are being turned.
It's causing more drag (lots of RR drag, and a bit of extra aero drag) - enough to be seen on an instantaneous fuel consumption meter.