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Most people can ride 10 km on their bike. However, running 10 km is a lot harder to do. Why?

According to the law of conservation of energy, bicycling should be more intensive because you have to move a higher mass, requiring more kinetic energy to reach a certain speed. But the opposite is true.

So, to fulfill this law, running must generate more heat. Why does it?

Some things I can think of as (partial) answers:

  • You use more muscles to run.
  • While running, you have more friction with the ground; continuously pouncing it dissipates energy to it.
  • While you move your body at a slow speed, you need to move your arms and legs alternately at higher and lower speeds.
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    $\begingroup$ Simplistically speaking, cycling puts more energy, including your own vertical movement, into horizontal movement, while running has that irritating waste of pushing energy into the ground vertically $\endgroup$ – RhysW Apr 15 '13 at 13:03
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    $\begingroup$ also important to note, your question assumes a flat surface. One can run further up an incline from a standing start than someone who cycles up the same incline from a standing start $\endgroup$ – RhysW Apr 15 '13 at 13:08
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    $\begingroup$ Don't forget the mechanical advantage a bicycle's drivetrain offers. $\endgroup$ – Dean Brundage Apr 15 '13 at 14:00
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    $\begingroup$ It is a common misconception that energy is required to keep mass moving. There is not such law of physics. There is an opposite law, which says that mass keeps it velocity unless you (de)accelerate it with force (and energy). It is friction which dissipiates energy and bycicle may only add to it. Otherwise, there is a much stronger question: why do you get tired when keep heavy weight in your hands? According to the laws of physics, table does not loose any energy when you put the weight on it (despite another misconception). $\endgroup$ – Val Apr 15 '13 at 20:32
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    $\begingroup$ I don't think you understand kinetic energy. You do not have to continuously supply energy to an object to maintain its kinetic energy (except to replace what is lost by drag or friction). $\endgroup$ – Kaz Apr 15 '13 at 22:16

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One word: inertia. When you're riding a bike on a level gradient you just need to give it a push to get going, then you can coast for quite a while before friction and air resistance slow you down. The human body doesn't have wheels that can store kinetic energy, so while running you have to give a good kick to get going, and then another kick to keep going on the next step, and so on. When hills are involved the difference is even more pronounced, since we run downhill the same way we do on the level, by continually pushing ourselves forward; whereas on a bicycle you can take advantage of the slope and just coast down it.

I suspect that raising and lowering your centre of mass isn't as inefficient as the other answers have suggested. This is because your legs are springy, so at least to some extent you're just converting energy back and forth between gravitational potential and the spring force in your legs. Humans are possibly the most efficient long-distance runners in the animal kingdom. There is a school of thought that says the reason we are bipeds is that we evolved as endurance hunters, chasing our prey until it collapsed from exhaustion rather than trying to outrun it over short distances. Whether that's true or not, we probably wouldn't do all that bouncing up and down if there wasn't a good reason for it.

You might ask why, if using wheels is so much more efficient, didn't we evolve that instead? I don't know, but it seems no animal has been able to evolve wheeled locomotion.

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    $\begingroup$ No animal ever evolved wheel locomotion because there was never an environment where wheeled movement would be beneficial. Nature lacks a specific important aspect that benefits wheeled movement. Roads / flat surfaces. $\endgroup$ – RhysW Apr 15 '13 at 13:02
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    $\begingroup$ I know that there are several theories on why wheeled locomotion hasn't evolved. My money's on a combination of the "incomplete form" argument @Gugg mentioned, together with the difficulty in connecting up blood vessels, nerves etc. around a continuously rotating joint. (I think the "lack of roads" argument isn't a great one, because wheels have their uses offroad as well.) $\endgroup$ – Nathaniel Apr 15 '13 at 13:57
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    $\begingroup$ VSauce on YouTube has a great video on why there are no animals that have wheels. $\endgroup$ – Daniel A.A. Pelsmaeker Apr 15 '13 at 15:28
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    $\begingroup$ Rubbish, there are tribes which traditionally hunt by outrunning animals on the plains and it's a battle of endurance. The beast collapses from exhaustion. $\endgroup$ – wim Apr 15 '13 at 16:40
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    $\begingroup$ @Cleonis - you should do a little more research on this. Most other animals are much better sprinters, but it is definitely true that humans are one of the best long-distance running animals, and there's a lot of research to suggest that being bipedal helps with this. There's also no evidence that species before Homo Sapiens had tools more advanced than sharp sticks, such as the snares and nets you mentioned. The popular theory is exactly as Nathaniel suggested, that we evolved as persistence hunters. $\endgroup$ – Joe K Apr 15 '13 at 17:27
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Many of us have ridden bicycles at some time in our lives. and in fact this mode of transportation has become markedly more popular recently as a result of the energy shortage. Each morning at my own university, Duke, people can be seen riding machines with masses of $10$ to $20$ kilograms and struggling to reach one of the campus entrances at the top of a long, steep hill. As in many other aspects of animal locomotion, there is a paradox here. Why should people encumber themselves with such heavy apparatus, particularly while going uphill? Ask a rider this question, and the response is usually: "It's easier than walking" or "It's faster than walking." But why should it be?

A number of incorrect explanations are offered: "A bicycle has gears." Shifting gears allows the rider to vary the speed at which the feet move; but even if the foot speeds of a cyclist and a pedestrian are matched, the cyclist still goes farther and in less time on a given amount of energy than the pedestrian. "Your weight is supported by the seat." But if you pedal standing up, biking still is faster and less costly of energy than going on foot. "Your center of gravity doesn't go up and down." But it does if you pedal standing up. Why, then, is bicycling easier than walking or running?

[…]*

We can now appreciate why bicycle riders are willing to propel the extra weight of a bicycle, even when going uphill. The cost of transport on a bicycle is low because active muscles are not stretched while pedaling, and mean muscle efficiency is about $.25$, nearly its maximum value. The wheels stabilize the rider's center of mass. Even if the rider accelerates the center of mass vertically by pedaling while standing up, active muscles need not be stretched. When the center of mass falls, the cranks, sprockets, chain, and rear wheel constitute a system of levers that transposes the vertical motion to a horizontal one by supplying a perpendicular force. Thus, humans can use external machinery to move along a level surface with the same muscular efficiencies that swimming and flying animals achieve naturally.

The Energetic Cost of Moving About: Walking and running are extremely inefficient forms of locomotion. Much greater efficiency is achieved by birds, fish—and bicyclists. V. A. Tucker, American Scientist, Vol. 63, No. 4 (July-August 1975), pp. 413-419


*Of course, most of the article is where I put "[...]". It's quite a good, fun read. There's even some kind of Galilean experiment with dropping pigeons and rats from heights.

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    $\begingroup$ "active muscles are not stretched while pedaling". That is the exact crux of the issue. I am sorry to see this did not become the top voted answer. See also an answer to a duplicate question - I think we agree... $\endgroup$ – Floris Oct 1 '14 at 19:16
  • $\begingroup$ There is at least one error in Tucker's analysis, which is in his discussion of efficiency compared to the maximum efficiency of muscle contractions. He says that efficiency when pedaling a bike is close to the maximum efficiency of muscle contractions. He seems to imagine that this maximum is never achieved without a bike, but that's not the case. In steep uphill running and walking, the same efficiency is achieved. See Minetti, J Physiol 471, 725, 1993; Minetti, J Exp Biol 195, 211, 1994. $\endgroup$ – Ben Crowell Jun 12 at 18:34
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  • Bicycles make better use of inertia/momentum. As Nathaniel said, one push and you can coast for quite a while. That's just not possible while running.

  • Running wastes energy moving up and down. In addition to moving forward, running requires a substantial upward push to get your body airborne, giving you time to bring your other foot forward. You then cushion and spring forward and upward again. While bicycling does have an up-and-down component to the pedaling, because the bike doesn't leave the ground, the energy you use in pedaling is converted much more efficiently to forward motion.

  • Bicycling can translate weight to propulsion. While most serious bicyclists will tell you that pedaling is about spinning, not stepping, any 10-year-old can tell you that going butt-in-the-air and transferring your weight from left to right gets you going pretty quickly.

  • Pedaling with toe clips makes use of the entire leg motion. When your feet are locked to the pedals, it isn't just the pushing-down portion of the pedal stroke that is used; lifting your foot, pulling it forward, pushing down and pushing backward all keep tension on that chain and so add power to the stroke. When running, fully half of your foot's cycle is wasted energy from a forward-motion perspective.

  • Bicycling gives you mechanical advantage. Even with a single-gear bike, the motion of your foot is magnified when translated to the wheel. On a multi-speed bike, the ratio of the top gear is pretty high indeed. This allows two things; first, your effort is magnified, and second, your tempo slows, which reduces the amount of energy wasted moving the weight of your legs around.

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    $\begingroup$ The last three points of this answer make no sense when speaking about energy-efficiency... $\endgroup$ – Steven Roose Apr 15 '13 at 18:57
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    $\begingroup$ Are you kidding? All three of them have a profound effect on energy-efficiency of bicycling versus running. You can bicycle as fast as jogging (at least) just by straightening your leg and shifting your weight to one foot, using that weight to push the pedal down, then shifting your weight to the other leg. The "entire leg motion" point specifically states how it's more efficient than a running stride, and the mechanical-advantage point says when the motion does more work, the percent of energy spent moving the weight of your own legs versus pushing you forward is lower (aka more efficient). $\endgroup$ – KeithS Apr 15 '13 at 19:10
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    $\begingroup$ Agree with Steven Roose. High gears only waste $\endgroup$ – Kaz Apr 15 '13 at 23:00
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    $\begingroup$ Steven Roose is right: the last three points are no good in this context, where we're concerned with the overall energy requirement. Cycling sitting down, without toe clips, in a low gear takes less energy than walking: therefore, standing up, toe clips, and high gears can't be the explanation. $\endgroup$ – user20432 Aug 19 '13 at 15:50
  • $\begingroup$ forget friction at joints? (foot-to-ground, ankle, knee, hip) Remember that running on easily deformed surfaces (like sand or soft ground) makes people tired a lot quicker even when the deformation after each step is small. $\endgroup$ – user121963 Jun 29 '16 at 6:38
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  • Running requires intense muscle contractions with a low duty cycle, whereas cycling uses long, smooth contractions. If you work at running, you can easily get to the point where it is no longer an aerobic challenge, yet still tough: you hardly have to breathe, yet the legs struggle with lactic acid buildup.

  • Running wastes a lot more energy on compensating motions: the runner has to move her torso and arms to compensate the kicking motion of the legs. The leg motion is not symmetric in running: the forward kick of the recovering leg is faster than the backward movement of the drive leg, and so the arm on the same side as the recovering leg has to swing backward to compensate its motion. Compensation on a bike is mostly limited to a side-to-side rocking during a hard effort.

  • The runner basically flies through the air, but periodically comes down and touches the ground with one foot, and then exerts a force in order to get back up into the air. However, this is not done by an efficient, elastic bounce. The landing energy is dissipated, rather than stored and reused. In fact, the runner must exert energy in order to absorb the landing, and then exert more energy to get back into the air. The runner thus wastes considerable energy to stay in the air.

  • Depending on the nature of the footstrike, the runner may counter-productively be exerting energy which retards (brake) his forward motion, and then has to exert more energy to recover momentum.

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  • $\begingroup$ Running requires intense muscle contractions with a low duty cycle, whereas cycling uses long, smooth contractions No, this is wrong. Elite runners typically do 90 stride cycles per minute. Elite cyclists typically do 70-90 revolutions per minute. $\endgroup$ – Ben Crowell Jun 12 at 19:48
  • $\begingroup$ If you work at running, you can easily get to the point where it is no longer an aerobic challenge, yet still tough: you hardly have to breathe, yet the legs struggle with lactic acid buildup. This is wrong. All running events at distances of about a half mile or greater (2 or more minutes) are aerobically limited. $\endgroup$ – Ben Crowell Jun 12 at 19:49
  • $\begingroup$ @BenCrowell Middle distance running (half mile to three miles or so) is aerobically limited only in those trained/talented athletes whose bodies can actually develop the output that requires the oxygen supply: they are able to run these distances at their VO2Max. Secondly, it drops off with increasing distance; past 3 miles or so, racing isn't done at VO2Max. Whether it remains aerobically challenging depends again on the individual. The ordinary recreational runner is severely limited by their lactate threshold, so when they run distances, it's nowhere near their aerobic capacity. $\endgroup$ – Kaz Jun 12 at 20:17
  • $\begingroup$ @BenCrowell Elite athletes are a tiny, cherry picked minority, not a statistically unbiased population sample. $\endgroup$ – Kaz Jun 12 at 20:19
  • $\begingroup$ @BenCrowell The muscle contractions in running are much shorter and more intense than those in cycling, at equal stride counts. $\endgroup$ – Kaz Jun 12 at 20:20
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A lot of the answers here go into movement of your center of gravity etc. I think it's a lot simpler than that.

When you are cycling your vertical movement on the pedals is translated into horizontal movement of the wheels, coupled with inertia a small amount of energy can go a long way.

Whereas whilst running you are putting energy into moving horizontally, to get places, as well as vertically to reduce friction with the floor. However all the vertical movement is fighting gravity, and wasted as your vertical movement gets you no closer to your destination

This answer assumes that the distance is over a flat surface. As soon as you throw a three dimensional aspect on it then the system turns on its head.

For horizontal movement the bike is best as its wheels are designed to reduce friction, unlike feet.

However as soon as you come to an incline the lack of friction will cause the bike to roll backwards unless constant energy is put in to the system, whereas the increased friction one has from running allows anyone to stop and stay put.

As soon as you get to an incline from a standing start, running is much more efficient than cycling.

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    $\begingroup$ whilst running you are putting energy into moving horizontally, to get places While you're running, the time-averaged change in your KE is zero. However all the vertical movement is fighting gravity Gravity does zero net work over the course of one stride on level ground. $\endgroup$ – Ben Crowell Jun 12 at 19:23
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To expand on Nicks answer, when you run, you sort of jump a little bit so that you raise your center of mass, which costs you energy equal to

$$\Delta E = m g \Delta h$$

Now when you lower your center of mass, the energy is dissipated as the vertical acceleration gained by going down is not increasing your horizontal speed.

This is definitely one of the causes.

Also, one can think about dissipated power, which might give us more insight. For example we have the following identity:

$$ P = \cfrac{\operatorname{d} W}{\operatorname{d} t} = \vec{F} \cdot \cfrac{\operatorname{d} \vec{x}}{\operatorname{d} t} = \vec{F} \cdot \vec{v}$$

Also, let's assume, that power losses in both cases are similar and that the input power is the same in both cases (we use similar muscles in both cases after all).

Now we have the velocity very different in both cases, and we could probably also agree, that there is much more force produced if we run as we can speed up to our maximum speed very quickly. So far everything agrees with the formulae.

Now we can easily convince ourselves, that even if the efficiency is similar in both cases, the loss of energy in running would be greater because we do it for longer for the same distance travelled.

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  • $\begingroup$ when you run, you sort of jump a little bit so that you raise your center of mass, which costs you energy...Now when you lower your center of mass, the energy is dissipated as the vertical acceleration gained by going down is not increasing your horizontal speed. You seem to be assuming that the vertical and horizontal motions decouple, but I see no reason to believe that's true. $\endgroup$ – Ben Crowell Jun 12 at 19:18
  • $\begingroup$ After this, you give a completely separate argument about power and speed. This model makes incorrect predictions. It predicts that cyclists and runners should have the same efficiency at the same speed, which is false on flat ground. It also predicts that the energy required to run or bike a certain distance should be a monotonically decreasing function of the time, but actually there is an optimal time. I use much more energy sprinting 100 meters than walking 100 meters. $\endgroup$ – Ben Crowell Jun 12 at 19:21
  • $\begingroup$ In your use of the identity $P=Fv$, you also haven't specified what force or what power you have in mind. If you're running or cycling in a straight line at constant speed, the time-averaged net force on your body is zero. And the power that's of interest as a physiological limit on performance is not the power delivered as mechanical work (which is ordinarily zero on the flats), it's the rate at which energy dissipated from the body's supply of fuel. If you're running downhill, the power delivered as mechanical work is negative, while the body's rate of energy expenditure is positive. $\endgroup$ – Ben Crowell Jun 12 at 19:33
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It's down to the biological inefficiency of maintaining the mechanical constraint of one foot stationary with regard to the floor, with the rest of the body moving.

Let's suppose a skater on ice and another on rollers use approximately the same amount of energy to get to say 5 km/s. Both possess a different, but efficient compared to walking, mechanism that maintains that velocity, while satisfying any mechanical constraints. For the one on rollers, the point of contact between the ground and a roller must be stationary. For walking, the foot in contact with the ground must be stationary, and the biological mechanism for this introduces far greater biological losses.

However, there are far more efficient modes of biological transport such as hopping as in the case of Kangaroos, which uses half the energy of a marathon runner. Kinetic energy is converted into potential energy in the tendons while the mechanical constraint is maintained, with most converted back into kinetic energy upon leaving the gound.

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  • $\begingroup$ It's down to the biological inefficiency of maintaining the mechanical constraint of one foot stationary with regard to the floor, with the rest of the body moving. This doesn't make sense. A bike's wheel is rolling without slipping, so the tire is at rest relative to the ground as the bike is moving, just like the runner's foot while it's on the ground. Both the bike-body system and the runner's body have parts that decelerate and accelerate. $\endgroup$ – Ben Crowell Jun 12 at 19:12
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This is easiest to understand if you start by considering the extreme cases of steep uphill and downhill slopes.

Actually it isn't true in all cases that running is less efficient than cycling. In work by Minetti et al. ("Energy cost of walking and running at extreme uphill and downhill slopes," DOI 10.1152/japplphysiol.01177.2001.), it was found that when elite mountain runners run uphill on a treadmill, at slopes greater than about 0.20, the efficiency becomes about 0.25, which is the efficiency of concentric muscle contractions. ("Concentric" means that the direction of motion is in the same direction as the contraction of the muscle, as when you do a pull-up.) This is an upper limit on the efficiency of any human-powered mode of going uphill, so since trained runners achieve it, they are not less efficient than cyclists on these slopes.

On a steep downhill, a cyclist can coast while the leg muscles expend zero energy. Running downhill does consume energy. In fact, the runner's efficiency is negative, because the gravitational potential energy of the person's body decreases, while the body's energy reserves are depleted. Minetti measured the downhill efficiency on grades steeper than $-0.20$ to be $-1.20$, and this is approximately the efficiency of muscles in eccentric contraction (like letting yourself down from a pull-up).

So if we can understand why lowering yourself down from a pull-up expends energy, then we automatically also have a physiological explanation of the imperfect efficiency of downhill running on the steepest grades, and then by interpolation we have an explanation of why running is less efficient than cycling on ordinary slopes or on the flats.

The reason that muscle tissue expends energy in eccentric contractions is that there are processes in the body that dissipate heat when a muscle is under tension. For example, the body has to burn fuel to maintain muscle tension, and there is also internal friction in the muscle as the muscle moves.

In addition to the processes described above, there are other energy-dissipating mechanisms as well, including the dissipation of energy into vibration and sound on a runner's foot strike.

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A quick guess would be that for running you are lifting your centre of mass with every step while with cycling your centre of mass has a constant height. Therefore you are only doing work against air resistance/friction in the bike mechanics. While running you are also working against gravity.

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  • $\begingroup$ You're working against gravity in a wasteful way: not like an elastic bouncy ball which stores energy and comes back to almost the same height, but more like a sack of rice. $\endgroup$ – Kaz Apr 15 '13 at 22:45
  • $\begingroup$ This explanation doesn't work. When you run on level ground, you're lowering your legs just as often and as much as you're raising them, so that the net work done in gravitational PE is zero. $\endgroup$ – Ben Crowell Jun 12 at 19:08
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The difference is in the underlying mechanism which transforms the chemical energy into kinetic energy of the vehicle or body. The answer is that the bicycle mechanism (due to having wheels etc..) is able to transform this energy better.

A classic analog is the lever or a pulley. One can use a lever or a pulley to lift a weight which would be very difficult (or even impossible) to lift with bare hands.

So to maintain same average speed with a bicycle one needs to use less chemical energy (than running) and as a result produce less heat.

"Give me a place to stand and I can move the earth"

                                            -- Archimedes on the principe of the lever (allegedly)

A related answer on the principle of the mechanical lever (and variations)

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  • $\begingroup$ Does this sound kind of strange to you: Assuming that the people don't even move their center of gravity and that the only parts that have gravity on them during pedaling are the light weight legs. In that case, cycling is still less tiring. Archimedes' words on the lever do not mean the energy used is better. You have less force but longer distance in that case. At higher value of force, the structural integrity of your soft body starts breaking down making the energy spread to doing the structural deformation. You get more tired, and you get tired quicker. $\endgroup$ – user121963 Jun 29 '16 at 7:02
  • $\begingroup$ The answer is that the bicycle mechanism (due to having wheels etc..) is able to transform this energy better. This isn't an explanation, it's just a statement of the definition of efficiency. The question is why the efficiency is different. A classic analog is the lever or a pulley. One can use a lever or a pulley to lift a weight which would be very difficult (or even impossible) to lift with bare hands. No, this is wrong. The mechanical advantage provided by a simple machine doesn't affect the amount of work done, so it has no effect on the efficiency. $\endgroup$ – Ben Crowell Jun 12 at 19:06
  • $\begingroup$ So to maintain same average speed with a bicycle one needs to use less chemical energy... This is a non sequitur. $\endgroup$ – Ben Crowell Jun 12 at 19:06
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As there are already answers which explained it very well, we can have an analogy to understand it further.

enter image description here

Consider that square wheel is us and round wheel is cycle. Now let's try to rotate the square wheel. When you do, a normal force will act from the right edge of the wheel instead of center to prevent it from toppling. (Torque will act in opposite direction and stop it from rotating). So the only possible option is to pick this wheel in air and put on next step.

But in case of round wheel, normal reaction always act at the bottom most point of the wheel and torque because of the normal force will always be zero. So there is no force acting on the round wheel to stop it's rotation. Because of this once a round wheel is given a push, will keep on rotating without any further force. (As there is no opposite force acting).

Now if we compare both wheels, picking up a wheel and putting down on each step will always be more energy consuming or will require more work.

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protected by Qmechanic Apr 15 '13 at 21:14

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