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Loop de loop answer part 1

Figuring out the minimum speed at the top of the loop de loop to stay on the track. Created by Sal Khan.

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  • duskpin tree style avatar for user Shashank
    At , why is that we take acceleration at the topmost point to be equal to gravity? Why is the normal reaction taken as zero?
    (68 votes)
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    • blobby green style avatar for user Vicky O
      To keep the car in a circular path, the centripetal acceleration (v^2/r) has to be greater than or equal to the acceleration due to gravity (g = 9.81 m/s^2). We set a = 9.81 because this gives us the minimum speed the car must have to stay in a circular path. As soon as the car goes slower than this, g will be greater than the centripetal acceleration, so the car will fall off the track.

      At the top of the loop:
      Fnet = ma
      Fgravity + Fnormal = ma, and because a = centripetal acceleration = v^2/r, then
      Fgravity + Fnormal = mv^2/r
      So at the minimum speed for the car to stay in a circular path, Fnormal = 0. As the car's speed increases, however, so does Fnormal (because Fgravity stays constant).
      (61 votes)
  • leaf green style avatar for user Rayvon Hartzog
    According to the video on fifth-gear website a Professor at Cambridge University calculated that he had to be going 36 miles per hour. Was he just building in a safety factor?
    (37 votes)
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    • blobby green style avatar for user jimdetry
      I think the 36 mph is how fast you'd have to be going when you hit the ramp, assuming you weren't able to keep the engine running with enough traction on the wheels to keep the speed constant as you climbed up. So if you hit the ramp at 36 mph, the car will slow down but still be going faster than the minimum required (16 mph) when it gets to the top.
      (54 votes)
  • blobby green style avatar for user Ben Young
    It might sound stupid to ask this question, but I've got this question in mind that if the direction of the centripedal force is toward the center of the loop de loop, when the car was at the top, the centripedal force and the gravity force have the same direction, so the car will be pulled down with the two forces! What is the force that exert the car out of the center?
    (43 votes)
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    • piceratops ultimate style avatar for user Andrei-Lucian Șerb
      For the car to not fall, it needs to stay in a circular path. Every object that moves in a circular path, with a constant speed, experiences a centripetal acceleration. So if something is moving along a circle it's experiencing centripetal acceleration, or the other way around, if something is experiencing centripetal acceleration, then it has to move in a circular path. So if we want to make sure that the car does not fall, we have to maintain it's circular motion, which means that all the forces on the car have to sum up to a centripetal force. For a minimal speed, the only force that the car is experiencing (at the top) is the force of gravity, so by simply saying that the force of gravity is a centripetal force, the car has to stay in a circular motion.

      The centripetal force always points towards the center, but that does not mean that the car will go towards the center. Remember that the car also has forward velocity, so the centripetal force affects the direction of the forward velocity in such a way that it makes the car go in a circle. That's literally the definition of centripetal acceleration. If the car did not go in a circular path, then it would not experience centripetal acceleration.
      (36 votes)
  • male robot hal style avatar for user BrentWassell
    I'm having trouble with an initial concept Sal makes around - Why does AC = 9.81m/s^2? The 9.81 is from the force to due gravity, not centripetal acceleration. The 9.81 is in effect on the car at all points around the circle, not just at the top.
    (8 votes)
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  • marcimus orange style avatar for user S Chung
    I've had to revisit this topic because I realised that I didn't really understand it all!

    I understand that at the top of the loop de loop, centripetal acceleration is attributed by gravity and normal force, but what EXACTLY IS PREVENTING the car from falling off? Gravity and normal force are pointed towards the centre of the loop de loop, and if centripetal acceleration needs to be equal to or greater than these combined forces, shouldn't the car just fall off the track because there is an overall net force towards the centre?

    The only "logical deductions" that I've come up with are:

    1) The vehicle experiences inertia in that it would otherwise travel on the tangential if it wasn't for the inward force attributed by centripetal acceleration

    2) As per Newton's third law, the vehicle pushes on the track with a force that is equal in magnitude, but opposite in direction to the which the track pushes on the vehicle

    I still, however, cannot grasp how the vehicle stays on the track in spite of the above mentioned factors. Any help would be greatly appreciated! :)
    (7 votes)
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  • leaf green style avatar for user Tommy Badowski
    So does the weight of the car not matter? I mean even if the car would weight 5 tons it would still make and loop de loop?
    (4 votes)
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  • aqualine ultimate style avatar for user Kartik kamath
    My text book derives the formula for critical speed as root(5gr) ..... If i calculate it by that formula i get a different answer.. And yes the derivation is very convincing :p
    (2 votes)
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    • piceratops ultimate style avatar for user quantum cheng
      sqrt(5gr) is the speed required at the BASE of the loop de loop.
      So the kinetic energy at the base is 1/2 mv^2 = 5/2 mgr
      Using the conservation of mechanical energy,
      we get 5/2 mgr -2mgr = 1/2 mgr at the TOP of the loop. (this is kinetic energy)
      and again using Ek = 1/2 mv^2,
      The speed at the TOP is sqrt(2gr), as Sal has mentioned.
      (6 votes)
  • leaf green style avatar for user Peter Barke
    How much harder do these kinds of problems become if the track has the more general shape of an ellipse?
    In other words, how would you calculate this exact same problem if the width and height were different, and not perfectly equal as they are for a circle?

    (Of course, a circle is a type of ellipse)
    (3 votes)
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    • female robot ada style avatar for user Rodrigo Campos
      I found your question very interesting!
      Let's work with an ellipse of semi major axis a, semi minor axis b and b is parallel to the gravity.
      First, think about what type of movement is naturally elliptical. The best example, I believe, is the planetary motion. So, from that we can withdraw the following results:
      At a given point, the planet feels a central force directed towards the focus of the type
      (GM).m/r² , where G is the gravitational constant and M is the star's mass;
      The planet's speed v = sqrt[GM.(2/r - 1/a)] ------ This is a planet's speed equation as a function of the distance from the star;
      So, writing out the constant GM, we can obtain
      F = mv²/a
      That's what happens for a planet in elliptical motion
      We also need the following fact:
      For any body running on a specified path, the perpendicular force acting upon it equals mv²/R , where R is the curvature radius of the path at that point.
      So, what is the ellipse's curvature radius at the top? Here we use the planetary motion's result. The perpendicular force is F.(b/a) , since (b/a) is the cosine of the angle formed between the force directed towards the focus and the line perpendicular to the ellipse's surface at the top (which is vertical). Thus, mv²b/a² = mv²/R , so R = a²/b at the top of the ellipse.
      Knowing the curvature radius R, we can easily find P + N, because of the fact mentioned above: "For any body running on a specified path, the perpendicular force acting upon it equals mv²/R , where R is the curvature radius of the path at that point" . Since the perpendicular force equals P + N, then P + N = mv²/(a²/b) , for minimum required speed N = 0, so v = a.sqrt(g/b)
      Notice that this result coincides for what Sal found if a = b (a.k.a. circle)


      EDIT : I changed my last comment because I believe the explanation with analogy is unsufficient, one must also include the curvature radius, making the result much more general (the car can have any value for its acceleration parallel to the surface) and mathematically accurate.
      (2 votes)
  • male robot johnny style avatar for user Avinash
    At , Sal says something about the traction of the road. What does he mean by traction?
    (2 votes)
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  • starky tree style avatar for user Shreyas
    Why doesn't the car fall down on the highest point of the loop? Irrespective of its linear velocity at that point, isn't the net force on the car only it's weight acting downwards? How does the speed affect the car being able to stay in the loop?
    (1 vote)
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Video transcript

What I want to do now is figure out, what's the minimum speed that the car has to be at the top of this loop de loop in order to stay on the track? In order to stay in a circular motion. In order to not fall down like this. And I think we can all appreciate that is the most difficult part of the loop de loop, at least in the bottom half right over here. The track itself is actually what's providing the centripetal force to keep it going in a circle. But when you get to the top, you now have gravity that is pulling down on the car, almost completely. And the car will have to maintain some minimum speed in order to stay in this circular path. So let's figure out what that minimum speed is. And to help figure that out, we have to figure out what the radius of this loop de loop actually is. And it actually does not look like a perfect circle, based on this little screen shot that I got here. It looks a little bit elliptical. But it looks like the radius of curvature right over here is actually smaller than the radius of the curvature of the entire loop de loop. That if you made this into a circle, it would actually be maybe even a slightly smaller circle. But let's just assume, for the sake of our arguments right over here, that this thing is a perfect circle. And it was a perfect circle, let's think about what that minimum velocity would have to be up here at the top of the loop de loop. So we know that the magnitude of your centripetal acceleration is going to be equal to your speed squared divided by the radius of the circle that you are going around. Now at this point right over here, at the top, which is going to be the hardest point, the magnitude of our acceleration, this is going to be 9.81 meters per second squared. And the radius, we can estimate-- I copied and pasted the car, and it looks like I can get it to stack on itself four times to get the radius of this circle right over here. And I looked it up on the web, and a car about this size is going to be about 1.5 meters high from the bottom of the tires to the top of the car. And so it looks like-- just eyeballing it based on these copying and pasting of the cars, that the radius of this loop de loop right over here is 6 meters. So this right over here is 6 meters. So you multiply both sides by 6 meters. Or actually, we could keep it just in the variables. So let me just rewrite it-- just to manipulate it so we can solve for v. We have v squared over r is equal to a. And then you multiply both sides by r. You get v squared is equal to a times r. And then you take the principal square root of both sides. You get v is equal to the principal square root of a times r. And then if we plug in these numbers, this velocity that we have to have in order to stay in the circle is going to be the square root of 9.81 meters per second squared, times 6 meters. And you can verify that these units work out. Meters times meters is meter squared, per second squared. You take the square root of that, you're going to get meters per second. But let's get our calculator out to actually calculate this. So we are going to get the principal square root of 9.81 times 6 meters. It gives us-- now here's our drum roll-- 7.67. I'll just round to three significant digits, 7.67 meters per second squared. And significant digits is a whole conversation, because this is just a very, very rough approximation. I'm not able to measure this that accurately at all. But I get roughly 7 point-- I'll just round, 7.7 meters per second. So this is approximately 7.7 meters per second. And just to give a sense of how that translates into units that we're used to when we're driving cars, we can convert 7.7 meters per second. If we want to say how many meters we go to an hour, well, there's 3,600 seconds in an hour. And then if you want to convert that into kilometers-- this will be in meters-- you divide by 1,000. One kilometer is equal to 1,000 meters. And you see here, the units cancel out. You have meters, meters, seconds, seconds. You're left with kilometers per hour. So let's actually calculate this. And so we get our previous answer. We want to multiply it times 3,600 to figure out how many meters in an hour. And then you divide by 1,000 to convert it to kilometers per hour. So you divide by 1,000. And we get 27.6 kilometers per hour. So this is equal to 27.6 kilometers per hour, which is surprisingly slow. I would have thought it would have to be much, much, much faster. But it turns out, it does not have to be much, much faster. Only 27.6 kilometers per hour. Now the important thing to keep in mind is this is just fast enough, at this point, to maintain the circular motion. But if this were a perfect circle right over here, and you were going at exactly 27.6 kilometers per hour, you would not have much traction with the road. And if you don't have much traction with the road, the car might slip and might not be able to actually maintain its speed. So you definitely want your speed to be a good bit larger than this in order to keep a nice margin of safety-- in order to especially have traction with the actual loop de loop, and to be able to maintain your speed. Now what I want to do in the next video is actually time the car to figure out how long does it take it to do this loop de loop. And we're going to assume that it's a circle. And we're going to figure it out. And we're going to figure out how fast it's actual average velocity was over the course of this loop de loop.