Consider two people – call them Pat and Chris – both having the same body weight (call it 145lbs). As part of their morning exercise, Pat runs 5km while Chris cycles alongside. Chris’ bicycle weighs about 50lbs. Pat and Chris both cover the same distance in the same time, Pat running, Chris cycling. Which of the two gets a better workout from the morning’s exercise and why?
The weight is largely irrelevant, the different methods of locomotion are what’s important. It isn’t obvious that biking is easier and more efficient than jogging? Clearly the jogger works harder and gets a better workout.
It depends on what kind of exercise you want. A lot of it has to do with your age, your heart rate during the exercise, and your level of fitness.
But the bottom line is that the runner is investing a lot more energy to travel the same distance. The cyclist has the benefit of a mechanical device to help him cover the distance faster. The bicycle’s weight doesn’t really matter, since the cyclist isn’t actually bearing any of that weight.
Basically, if they’re going at the same speed, the runner is getting a better cardio workout, while the cyclist is keeping a lower heart rate that’s more efficient for burning fat.
First off, Chris needs to get a new bike, 50lbs is over twice the weight it should be. But really the weight is irrelevent for this example, although if you got it up around 100lbs, it might be more relevent. Second, Chris, going at the same speed as Pat, isn’t exercising, he’s just riding a bike. Lower heartrate? Sure, about 5 bpm over resting, you almost have to work to ride as slow as someone runs.
On the weight issue, keep in mind that the bike guy is not only not bearing the weight of the bike, he’s not even bearing his own weight. As others have said, bikes are simple machines that allow you to get more distance with less work, which is why they were invented in the first place (amusingly, nowadays our machines are so much better that I ride my bike to work specifically because it’s LESS efficient than the other machine I could take). I’m not convinced that the bike’s weight is meaningless, though–clearly, riding a heavy bike with 150 pound weights strapped to it is a better workout than riding a superlight street racer.
The key to the difference between the bike and running is the coefficient of friction, something not yet touched on in this thread. The coefficient of static friction that applies to the runner is far greater than the coefficient of rolling friction that applies to the biker. The runner must therefore do more work to move less mass than the biker once the bike is moving. The biker will have a harder time getting started, since he’ll have to overcome the inertia of both himself and the bike, but the low rolling coefficient will ensure that less momentum is lost to friction once he is moving and he will expend far less total energy as a result.
Actually, the more relevant fact is that the bicycle is actually a surprisingly energy efficient machine. Per calorie, it’s more efficient than any automobile (IIRC). The rotatinal leverage of the chainwheel imparts tremendous mechanical advantage.
Actually, the more relevant fact is that the bicycle is actually a surprisingly energy efficient machine. Per calorie, it’s more efficient than any automobile (IIRC). The rotatinal leverage of the chainwheel imparts tremendous mechanical advantage.[/quote]
True, but that doesn’t decrease the total amount of work required to move a mass (common misconception). Mechanical advantage means that less force can be applied over a longer period to achieve the same result. A lever doesn’t reduce the amount of total energy expended to move an object. Lifting a 1000 pound rock one foot against 1 G requires 1000 foot-pounds whether you use a lever or not. However, with a long enough moment arm, you can get the job done by applying just 100 pounds of force, force that is applied over a longer period to give the same 1000 foot pounds of total work figure.
What makes any wheeled conveyance energy efficient is the difference between static and rolling friction. Streamlining, lower weight, and efficient engines help too, but friction is the key. Mechanical advantage enables you to use less force, not do less work. Work is constant.
Edit: first sentence of second paragraph said “being” instead of “between” and was borderline nonsense as a result. :shock:
Uh… Tom… even I’m starting to think you’re getting a little pathological.
Sorry. I just liked the way that fit in there.
I ride a bike and run, and they’re very different kinds of exercise. Tell Pat and Chris that they’re both very good for you, but what it comes down to is that running is more hardcore. Riding a bike a great entry into regular exercise and a perfect “gateway” workout if you want to work your way up to running.
However, it sounds like Brewer’s question was one of physics, so congrats to Dave for getting the right answer.
I agree that rolling efficiency is important. But the bicycle allows more of the energy from your pedaling to be converted to work, even if you ignore friction.
In terms of exercise, it’s more than just work. It’s how much work you get out versus how much energy you put in. Another factor here is that, when running, the impulse comes when you push off the ground – the rest of the leg motion is “wasted”.
When you’re pedaling, more of the leg’s effort is converted into real work. And if you’re clipped to the pedals, even more of that energy is converted when you pull up.
So I take back part of what I said. It’s not the leverage per se, but the fact that the chainwheel arrangement allows a greater proportion of your effort to be converted to “real” work. But the fact that rolling friction is less than what a runner encounters when pushing off is certainly a factor, too.
Mechanical advantage may not reduce the amount of work that needs to be done to accomplish a task, but it does reduce the amount of work you end up doing. That is because muscle (as well as many other things) is much more efficient at lower levels of force.
e.g. 10 reps of 40 pounds and 4 reps of 100 pounds is the same amount of real work done, but the latter makes you much more tired.
My point is that the mechanical advantage really does make a bycicle more efficient, by the nature of the person riding the thing.
Some background to the question: I run and cycle too and know from first hand experience that the workout I get from running a given distance is better than the workout I get from cycling that same distance. However, a friend and I were having a discussion about the amount of energy / work involved and her argument was that the cyclist actually expends more energy over the same period of time since they are moving a greater mass.
I suspect Dave’s discussion of friction and Case’s discussion of ‘wasted’ energy on the part of the runner suffice to explain why the runner gets a better workout than the cyclist.
I would also advise people not to run as regular excercise. There are better things you can be doing that aren’t quite so bad, in the long term, on your knees. That isn’t to say running is bad for you, but like anything, there is a price to pay.