Human on a treadmill

Factual Questions
41

One part where you are wrong:

Think of the motion when you are walking. you have one foot down, what is the other foot doing? its making its way to the front of your body, where it will hang out before soon making contact with the ground when the process will repeat. now take a snapshot where your off-ground-leg is hovering infront of you but not quite touching the ground.
When the treadmill pulls your other foot back, the law of inertia (Newton’s first law–thats annoying isnt it when I tell you what you what’s already obvious?) dictates that your center of gravity, somewhere in your crotchal region, will have a tendency to stay in place. This will cause your center of gravity to tip past the point where just one foot can support it and your body will fall the short distance until your other foot touches the ground. When the treadmill moves back, the natural tendency of your body is to tilt forward and eventually fall, NOT to move back with the treadmill as you say. THAT requires muscle tension and purposeful balancing on your part.

ETC: your assumption holds true when you (wrongly) assume that the whole human body is one rigid mass. We have joints (primarily the ankle in this case but also the hips/knees/everything else).

42

brad_d has it right. Think of it this way then, when moving forward on a road the legs have to move themselves and the mass of your upper body forward. On a treadmill they only have to move the mass of themselves forward. What we are talking about is effort expended.

He’s two Einstein examples:

  1. I’m floating in space and a rocket accelerates past me.
  2. A rocket is floating in space and I fall past it.

A camera on the rocket (or myself) will record the same event. What a camera won’t show (given my hypothetical non-flaming rocket) is the energy difference expended by the rocket.

43

How much effort is required to move your body forward once it is already moving forward?

44

Not much. Even less when something else is doing part of the work for you. (ps read my reply up there i put alot of love and care into it :stuck_out_tongue: )

45

But what about people who aren’t running on rougher terrain? Back when I used to run in “real life” I’d refuse to run on anything that wasn’t an indoor track at a gym or on an outdoor track at my old school, which were pretty much the same as the treadmill in terms of impact.

46

These words held more prescience than I had supposed at the time.

47

OMG. The lack of understanding of fundamental physics here is enough to make me think that our schools really DO fail us!!!

Folks. Let’s take this very simply.

You stand on one leg on the treadmill, what happens? The treadmill is moving backward. You go with it. You expend no energy at all to maintain your position, once the treadmill is moving; you simply stand there balanced. You are in equilibrium with the environment.

You stand on one leg on the road, what happens? The road is standing still, you stand still with it. You expend no energy at all to maintain your position, once you are stopped relative to the road; you simply stand there balanced. You are in equilibrium with the environment.

You are on a treadmill which is stopped. It starts, accellerating you backward. You expend some energy to keep from falling forward on your face, so that you can maintain your position relative to the treadmill.

You are on a road, walking forward. Suddenly, you stop. You are accellerating backward. You had best expend some energy to keep from falling forward on your face, so that you can not look like an idiot. :wink:
In each case it doesn’t matter which scenario, the result is identical. Do you comprehend? Identical.

Now let’s tackle this stupid notion of the hopping. If you are on a long treadmill, and you are being carried backward, and you start hopping, do you really think that you will suddenly stop moving “forward” relative to the treadmill??? If you do, it’s because you aren’t hopping up and down, you are hopping forward as well. Or do you think that if you hopped up and down in the airplane at 30,000’ youd end up smashed into the tail section??? :smack:

(If this notion still eludes you, try hopping up and down on one of those long moving walkways in an airport, and see if you aren’t carried forward as you go).

48

Jesus Christ, no it won’t. You’re talking about acceleration, which is NOT relative.

That’s it, goodbye cruel thread.

49

Except that you have it all wrong, again.

IF the rocket is “accellerating”, that isn’t the same as you “falling,” unless you are “falling” because of an accellerating force (say gravity). In which case there is just as much energy being expended (mass for mass) in accellerating you (assuming you and the rocket are accellerating at the same rate).

If the rocket is simply “moving” past you (no accelleration, simply velocity), and in comparison you are simply “moving” backward (no accelleration, simply velocity), then the result is identical: neither the rocket nor you are expending energy.

Now, to complete the understanding, take the accelleration hypothetical, and instead of you accellerating because of gravity, you are accellerating under your own power. Let’s say you are accellerating yourself by “swimming” through the ether. Again, kg for kg, if you are accellerating at the same rate as the rocket, you are expending the same energy as the rocket. F=ma. Don’t you get this??? :stuck_out_tongue:

50

I understand the the physics angle you are after but I don’t think it’s applicable in this example. You seem to think that once you are up to a certain speed the effort required is the same. But continued running at 10 mph is a lot more tiring than continued running at 5 mph. Again, this is about effort on the body.

51

Well then I’d be perfectly happy to learn why jogging on a treadmill is easier than the same jogging on the road. Is wind resistance enough to explain it? I always thought it was because I could read a book on the treadmill and distract myself from the effort. But others seem to agree about the less effort on the machine.

52

:smack:
embarrassing headsmack moment! I was thinking of the stopped-and-then-start case for every step, and I completely did not take into account what happens once your body’s inertia changes. Just goes to show how absolutely sure you can be about something and still be wrong (I shake my fist at you mr. 8th grade science teacher).

53

Yes. And here’s why you’re wrong. If the rocket and I accelerate past each other at the same speed, I am exerting energy consumption X. If the rocket is stationary and I am to move past it at the same (apparent) speed as the first example, I have exert energy consumption 2X. Now, let the rocket be the treadmill.

54

So Newton’s first law doesn’t apply while running on a treadmill? Recall all the physics textbooks.

I assure you, it takes exactly the same amount of energy to keep something moving at a constant speed of 10 mph as it does to keep something moving at a constant speed of 5 mph. The increased effort on the body is probably due to the need for more vertical force required for longer strides used while running 10 mph compared to 5 mph.

55

You keep using that word. I don’t think it means what you think it means.

56

In a vacuum.

And, isn’t it “the same amount of* force*”?

Or, that there are many other forces that decelerate the body.

57

I’m not a physics major and maybe I have mis-used some terms. Sorry. But we are not talking about a ball bearing floating in space. So scientific explainations aside, I’ll just state that as a long time runner my real time experience has demonstrated that it is a lot harder to sustain 10 mph than 5. And requires more effort, even if it comes from vertical forces.

58

Yea I don’t see how you could argue otherwise for a human being. Just moving your legs back and forth faster has to equal more energy expended, no question about that.

59

Of course 10mph is harder to maintain than 5mph. The much longer strides involved in running 10mph require you to exert much more effort to counter gravity than the shorter strides of 5mph. Some people in this thread were saying that running on a treadmill is easier because you are not having to propel the top half of your body on a treadmill like you do on a road. That is the reason I was breaking the forces down into vertical and horizontal. It takes exactly the same amount of horizontal force to keep your body moving at a constant 10mph, 5mph, and 0mph (neglecting air resistance). Sorry if I was unclear.

60

In the examples where people are talking about standing on one leg, having the treadmill pull back that leg, while the other leg “does nothing” until it reaches the ground… am I right in thinking that that isn’t really true, because of the moment couple it creates? As you lift one leg up, stretch it forward, and set it down again, that motion will create a turning moment on your body, which is the same as a moment and force couple on your foot, which is not the same amount of force as you were applying with that foot before you moved your other leg in the air at all. So you are actually increasing the force exerted on the treadmill by the foot in contact with it, so you aren’t just letting the treadmill take you away until the other foot hits the “ground”.

Does this even matter, and/or am I totally overthinking this? Did I even understand the analysis some people are trying to make? I did a whole semester of talking about moments, but in very simple beam and truss examples!