If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Optimal turns at Indianapolis Motor Speedway with JR Hildebrand

Created by Sal Khan.

Want to join the conversation?

  • female robot grace style avatar for user Luke Bayler
    Fascinating discussion! I'm curious what the relative radii of these three circles around the track are.
    (41 votes)
    Default Khan Academy avatar avatar for user
    • starky ultimate style avatar for user Brandon
      The increased radius of the much larger circle really opened my eyes to the problem. Its like the driver is performing the best "trade-off" or "compromise" between centripetal force and the work required to complete the turn AS WELL AS setting up the car to then accelerate down the straight line as quickly as possible. Another instance of mathematics underlying everything that happens, even if we are unaware of it while we do it.
      (47 votes)
  • mr pink green style avatar for user Joshua
    Cool! But what about if you were running, what is faster?
    (9 votes)
    Default Khan Academy avatar avatar for user
    • piceratops ultimate style avatar for user SumukhBhagat
      I guess the Inner circle would be a better option then.
      Because when running, you aren't at much velocity. So you'll not face much Centripetal Force because in the formula,
       Centripetal Force is directly proportional to the Square of Velocity. 

      And also You can observe in Olympic and other races the players choose the Inner circle always.
      (44 votes)
  • blobby green style avatar for user Ara 13
    As the car rounds the turn, does it lose speed due to the extra work needed to maintain the turn including friction; and if so, though the pedal is to the metal, is the car losing speed though accelerating?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user dotcom
      Even at full throttle around the corner the car will lose speed due to tire drag as the inertia of the car would love to continue straight ahead, yet the input from the driver (steering wheel through to tire contact patch) is asking the car to turn. The result of these forces, if the driver is correct, will be that the car changes direction (success), but then also loses speed (a clear trade off) as the tires themselves twist and flex to balance the forces interested in continuing on said path (inertia) and the driver's interest in changing direction. The driver turns the steering wheel; this continues down through the metal steering rod, rack/pinion, and out to the steel wheels. Everything is a solid metallic connection. Then, you reach the tires. It is the tires that mediate between the massive momentum (and therefore inertia) and the driver's interest in turning. In order to make it all work, the tires must twist (slip angle) between what the driver is asking the car to do, and what the pavement and grip level of the tires will allow. I'm sure there are plenty of other discussions about slip angle; I'm not an engineer, but I am a supporter of Mr Hildebrand, whom I've known well over a decade and believe in immensely; good luck JR! -DMc.
      (8 votes)
  • blobby green style avatar for user 39067
    Can any other car take such sharp turns if they used the same tire?
    (1 vote)
    Default Khan Academy avatar avatar for user
  • duskpin ultimate style avatar for user Temur Rakhmatov
    I am a little confused weather the centripetal acceleration decreases the speed of car. Becouse in the previous videos Sal told about the constant speed the object has during circular motion.
    (4 votes)
    Default Khan Academy avatar avatar for user
    • duskpin ultimate style avatar for user briannanryan
      The centripetal acceleration does not decrease the speed of the car. The problem with having more centripetal acceleration is that you have to accelerate the car very quickly over an even shorter period of time and most cars just won't be able to make that acceleration fast enough. Hope this helps!
      (2 votes)
  • male robot hal style avatar for user WAJIH RIZVI
    Wait, so is a higher centripetal acceleation better for a race car driver?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Cameron Shahidi
    Would the best thing to do in this situation (as the driver) be finding the smallest radius possible without going over the max centripetal force the car can handle? I don't understand why the driver would want to take the route with the largest R if they don't need to.
    (1 vote)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Andrew M
      The objective is not to minimize distance or maximize speed of the car, it's to minimize the time it takes to complete the race. That requires some optimal combination of finding a short path while keeping as much speed as possible. If you have to slow down too much to take the smallest radius, it's better to take a bigger radius that lets you avoid slowing down. It's a lot more complicated than it looks!
      (6 votes)
  • spunky sam orange style avatar for user Ishan Shetty
    The thing thats nagging me[stupid thing really] is that why does the path he chooses matter cause if he chooses the shortest path i.e. the inner one then by mv^2/r he has more centripetal acceleration so less chances of overturning or skidding so isnt a small radius better than a larger one.
    (1 vote)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Andrew M
      For the inner one he doesn't HAVE more centripetal acceleration he NEEDS more centripetal acceleration. Since that acceleration comes from a frictional force applied the tires, his chance of overturning or skidding is HIGHER when he tries to take the tighter turn at the same speed.
      (5 votes)
  • leaf green style avatar for user Jack Howard
    Where does Centripetal Force of car travelling on a banked curve come from? It has Fgy and Normal Force in vertical direction, and Friction and Fgx in horizontal direction all cancelled out right?
    (2 votes)
    Default Khan Academy avatar avatar for user
  • aqualine tree style avatar for user Adi Mandavkar
    can anyone explain me the difference between centripetal and centrifugal force?
    (1 vote)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Andrew M
      Centripetal force is toward the center
      Centrifugal force is away from the center
      In circular motion, a centripetal force is required.
      Centrifugal force is somewhat of an illusion. When you go in a circle, it feels like you are being pushed outward, but really you are being accelerated inward by the centripetal force.
      Sal has vids about circular motion.
      (5 votes)

Video transcript

SAL KHAN: This is Sal here with famous Indy car driver-- smiling when I said famous-- JR Hildebrand. And since you're here, I thought I would ask a question that's always been on my mind. JR HILDEBRAND: Yeah. SAL KHAN: We have a picture here of the Indianapolis Motor Speedway. And I've always wondered how you-- it seems like turning is a very important part of the-- JR HILDEBRAND: It's absolutely an important part of what we're doing. SAL KHAN: --of the race. JR HILDEBRAND: People get fixated on the car going straight. But the turning part is pretty important. SAL KHAN: Turning seems to be the part where a lot of the skill comes into it. And I've always wondered, what is optimal? Do y'all try to minimize your distance and kind of take the turn as quickly or as in short of a distance as possible by really hugging the corner, by going like that? But when you do that, you have to turn more. There's more g-forces. There's more kind of centripetal force that your tires have to deal with, the human has to deal with. Versus taking the outside where you have to cover more distance, but the centripetal acceleration, the g-forces aren't going to be as dramatic. So how do you think about that? JR HILDEBRAND: Well, every track ends up being a little bit different. But when we take Indianapolis here as the example, if you're already on the inside-- it's like the 800 meter runner's kind of path. It's the shortest distance. You can kind of get from point A to point B. The lap is the same every time, so it doesn't actually depend on you running a specific distance or not. For us, in this example, the car actually just won't do that. If you think about being all the way on the inside, being all the way on the inside through the corner, and then exiting all the way on the inside, it's having to do the most work to follow that path. And in Indianapolis, we're approaching turn one at upwards of 240 miles per hour. And that turn one is not-- it's hardly banked. It looks quite flat in person. So as opposed to NASCAR running at Talladega or Daytona, these big, giant super speedways, the car is having to do quite a lot of work to get through the corner here. SAL KHAN: So how do you--? Do you take the outside or--? JR HILDEBRAND: So then you look at that. And I think if you noted the radius-- if you drew a full circle out of each of those arcs-- SAL KHAN: Let's do that. So let's say that this is the shortest distance path. This is kind of a circle that looks something like this. Let me scroll over a little bit so we can see a little bit better. So this would be a circle like this if you were to keep that arc. It would be a circle that looks something like this. JR HILDEBRAND: So that's a pretty small circle in the grand scheme of things here, yeah. SAL KHAN: That's a small circle. And for the larger one, the circle would look something like this. So you have a larger radius, a larger turning radius. So you would have to have less centripetal acceleration, inward acceleration, and fewer g-forces on this outside one, the larger the circle is. JR HILDEBRAND: Right. And a different way to look at it, if you looked at the car trying to just go around these two different circles, and it's going to be going the same speed on either one, it's doing a lot less work to get around this outside circle. And therefore the speed that you could carry around that, that sort of goes up. The car has a limited ability to stick to the racetrack. So opening that up definitely makes a difference. SAL KHAN: But that's an important point. At least in Indianapolis, you're full throttle the entire way. I mean, obviously, if you hit the brakes, the car could do a very small turning radius. But you're at full throttle. You're not going to have any chance if you at all let off the gas. JR HILDEBRAND: That's right. When you qualify at Indianapolis, you've got to put in four laps, four of your best laps of the season, of your career in Indianapolis to qualify. And that you are absolutely flat trap all the way around the racetrack. There's no lifting. There's no braking. SAL KHAN: And so that's why you're saying the car just wouldn't do that. If you're going all out, the car just wouldn't even be able to make this path. JR HILDEBRAND: Exactly. That's a good point. From the driver's perspective, you have to stay flat out if you're going to go fast. If you're going to set a lap time that's relevant, you have to be able to stay flat out. And so at that point, you're searching for the line around the race track that you can do that most efficiently. And so then, in this example, increasing that radius by going from our green circle out to the purple circle does that rather effectively. SAL KHAN: I see. We're going for the purple to the green back to-- so you're saying like this. JR HILDEBRAND: Well, yeah. And so then to find the actual optimal line, what we end up doing is starting out on the outside of the track, then bending the car into the inside of the track, and going back to the outside of the track, really using all of the road that's available to us. SAL KHAN: Right. So that's interesting. So when I posed the question, it was kind of like my brain was just looking at these two circles. But you realize there's a bigger circle that you could fit here, that there's an arc like this. And this would be, if you imagine, this would be a part of a circle that's way huger than even that purple circle that we're drawing. So that center of that circle is like here or something. So you have a lot less centripetal acceleration that you have to place, inward acceleration that you have to place on the car. JR HILDEBRAND: Exactly. And therefore, the car is able to carry a massively increased level of speed through the corner. And that's really what we're looking for. And you consider, I think it's a very interesting-- when I think about what I'm doing as the driver, I don't think I really am consciously thinking that much about the mathematics that go into finding this optimal racing line. You sort of instinctually just gravitate towards what the car feels like it wants to do. But when we look at it from this perspective, you've got the car going down the straight away here. It's at 240 miles per hour. That's almost as fast as the car is going to go. So it's just this sort of terminal velocity. The drag of the air hitting the car won't allow it to go much faster than that. SAL KHAN: The engine's giving all the power it can. JR HILDEBRAND: Yeah. You're absolutely flat out. SAL KHAN: And that's just offsetting the drag of that, so that you can't accelerate to that top speed. JR HILDEBRAND: Exactly. It's almost like you're hitting a wall of air at that point. You're not going to be able to accelerate any faster. And so what you're really trying to do is you're trying to-- in order to set that fastest lap time, which ends up equating to the highest average speed around the lap, that's what's the lowest number in terms of lap time perspective-- you're trying to get the car to most efficiently get through the corners so that you can allow it to accelerate down the straights as much as you can. You're getting it to diverge from this intended course that is going on here as efficiently as you can. And so by creating the largest radius around the corner, that's how we end up finding that optimal line. SAL KHAN: That's fascinating.