GRAVITY DOES NOT PULL YOU FORWARD!!!!!!!!!!

True, the best illustration of this is if you stand vertically on a track and lean forward you'll fall forward and your feet will remain in the same position but you center of mass will have moved forward. Do the same thing on an ice-rink wearing skates and your feet will slide backwards as you fall and your center of mass will fall vertically (frictional force on skate blades being approximately zero)

The center of mass will fall vertically, but the point here that everyone is missing is the fact that a human is not treated as a particle. Instead this is an example of rigid body motion. With this in mind, a lean forward or propulsive backextension of the leg creates a net torque.

Also note that friction is not working against you all the time.

On the propulsive (concentric) contraction friction is moving parallel and positive to the direction of motion. Therefore, friction is doing positive work on the push off.

On the landing (eccentric) contraction friction is moving parallel but negative to the direction of motion. Therefore, friction is doing negative work on the landing.

This is a difficlult concept to grasp because it is a continuous cycle. However, there is energy loss and this is what you must put into the system to keep it moving at a constant velocity or accelerate.

Anyway, back to the original question at hand, does gravity propel you forward or do work?

Gravity does produce a net torque forward on propulsion and to compensate this torque the leg must swing forward ahead of the center of mass. Upon footstrike the reaction force creates a torque in the opposite direction to stop you from spinning on your face. So this too is a continuous cycle.

Saying that torque(also called moment) cannot produce work is ridiculous. This is like saying that the torque in your car engine is not possible of doing work.

The work of a couple of moment M acting on a rigid body is:

dU = Md(theta)

What this simply means is that througout the cycle, gravity is doing work.

Now at contant velocity the net moment of the cycle is zero, because when you land you are experiencing a reaction to the ground (the normal) which is working against gravity's spin, and it is working against the forward motion.

I have spent some time studying the Pose method and even met him on a couple of occasions. I think that his explanation is flawed because he does not address the whole cycle of the running motion. I think from the "frozen section" of the running pose it is easier to see why people would argue with him. From the froze "Pose" the center of gravity is directly beneath the legs so there is no net torque do to gravity. But if you watch the video he leans forward.

He's dead wrong to say the gravity is the sole force propelling you forward. It is only part. The elastic energy stored (eccentric) in your muscles stored with propulsive (concentric) contraction also contributes. The torque produce by gravity works as a restoration component to bring the leg forward (but propulsion does too).

I'm not particularly opposed to the "Pose method", it looks to be economical. I just think it would be better to say that the method optimizes gravity's contribution to the running cycle (and some elastic properties). Instead of saying that gravity is the only thing propelling you.

esmoke

Friction never does positive work. It is a dissipative force. It allows the foot to propel the runner (otherwise it would be like running on ice), but there is energy lost to the motion.

By leaning to induce a gravity torque, one requires the muscles to do work to counter that torque (otherwise, the runner falls down). This work is internal, and does not result in translational motion. The fact that one moves one's limbs faster to keep from falling doesn't mean that the movement is more efficient than running properly balanced.

The question is not, is gravity doing work - is it doing useful work? Unless the person is running downhill, the answer is no.

Being well endowed will pull you forward.

Here's a free physics lecture, don't try to argue, because if you are trying to argue with me I've had more physics than you.

Orbits - momentum is what keeps you moving forward, not gravity. Gravity pulls you down, not forward. Ever heard of the centrifugal force? It's a fake force, it doesn't exist. It's not actually a force keeping you moving forward, it's momentum. Gravity constantly accelerates you inward, there is no change in velocity, only direction. Change in direction is acceleration (change in velocity, velocity is speed and direction).

Downhill - Gravity still ONLY pulls you down, you have a horizontal component to motion but that is due to the normal force of the road. Gravity still points straight down, the normal force points perpendicular to the plane of the road (on a hill, up and forward).

FORCES CAN ONLY ACCELERATE YOU IN THE DIRECTION OF THE FORCE. GRAVITY DOWN, not forward. Key concept here.

Gravity on space and time. Yes gravity warps space, but that has absolutely no place here because it's too advanced for our conversation or the simple minds of the people who can't figure this out.

On another note, planes on treadmills will not take off. No air is moving underneath the wing. If a ball is on a treadmill will it curve (okay not a great example)? No, there is no upward force (due to a difference in pressure because air is moving at different speeds above and below the wings.

I'm really excited about what I've learned here ... according to the POSE proponents, I should be able to build a car that "leans forward" -- I'll put most of the weight in the front and lift the rear end a bit. Then I'll drop in a tiny motor and still be able to achieve high speeds and conserve fuel.

Then I'll just "fall forward" on the road to Moronville. Brilliant!

A real physics major wrote:

On another note, planes on treadmills will not take off. No air is moving underneath the wing. If a ball is on a treadmill will it curve (okay not a great example)? No, there is no upward force (due to a difference in pressure because air is moving at different speeds above and below the wings.

Oh darn, you were doing soooo well until you got here, but you need to read that problem again. I am not going to explain it here because this is the wrong thread, but there are many explanations on the other thread that you should be able to grasp if you are "a real physics major."

Good luck!

You Wrote: "This work is internal, and does not result in translational motion...Is it doing useful work?"

As a restoration force in the rotational cycle of the rigid body, it is. This is useful because it is conserving energy.

As a direct force applying to a PARTICLE with no moment. Then yes, downhill the component of gravity parallel and positive to the direction of motion is Fg sin(theta).

You Wrote: "The fact that one moves one's limbs faster to keep from falling doesn't mean that the movement is more efficient than running properly balanced."

Yes, it does. When you let gravity do work as a restoration force, and take more frequent strides then you are conserving energy...instead of straining to over extend and drive during what should be gravity's contribution. I think Pose would argue that his method IS PROPERLY BALANCED. But I think that is subject to debate.

The whole argument of the Pose method is that it is conserving energy and thus the component done by gravity is useful work.

Valid or not if you watch the whole documentary of the “BBC Tomorrow World” comparing the oxygen consumption and heart rate of the same runner, running the same speed before and after the pose method...the runner used less energy running the Pose Method.

You Wrote: "Friction never does positive work."

YES, IT DOES!!!

What do you think makes your car go forward?

When you are accelerating the friction of your tires to the street propels you forward. Friction will always oppose the force applied or direction of motion. In the case of the car accelerating, the wheels spin backwards so friction propels your forwards. This is why formula 1 cars have air foils. It increases the normal force so that more friction car be generated to propel the car forward without spinning the tires.

This is very, very basic physics.

It becomes more complex because there are other loses lumped into internal energy losses/friction such as heat losses, and wind resistance etc.

But upon propulsion, you can rest assured that friction with the ground points forward and does positive work.

Think about it.

esmoke

My god, what idiocy.

"When you are accelerating the friction of your tires to the street propels you forward."

Obviously friction can have a positive effect (sometimes quite pleasant).

That doesn't mean it's adding energy to a system. It can't.

Also ... Let's see someone design a car that falls forward and gains power from gravity. Can a car roll down hill? Yes.

Can it roll on a flat without a push? No.

Get it? You can't "lean forward" and gain energy. It's that simple.

Biomechanically, things are far more complicated than arguing simple physics problems. But if leaning forward was really more biomechanically efficient, wouldn't humans naturally have adapted to a POSE style of running? We didn't because it's not a more effecient way to run.

Finally, elite runners are the best model for effecient biomechanics. Good running posture varies slightly be the individual, but it's plain to see that oddball theories break down when real runners seek out and find the most efficient way for a human to run.

roger penrose, I was looking for that thread, but I couldn't find it. And again, I told you not to argue with me.

Any of you ding dongs figure out that gravity is not gravity. Gravity is the centrifugal force produced from the spinning action of the planet on it's axis?

Anyway, I think Pose sounds like anothe rreligion.

As George Michael said:

"You gotta have faith, faith, faith".

Even if you are a bi-metro-exposing-wannabe-ding-dong-diddler.

You Wrote: "Biomechanically, things are far more complicated than arguing simple physics problems."

You are the one oversimplifying this situation.

The cyclic effect of a runner propelling off and then braking upon landing is a very complex system...far more than a symmetric wheel spining.

Friction does positive work to accelerate a car or a person forward.

W = F*d cosine (theta)

In the case of friction and the ground, on propulsion the angle between friction and the displacement is 0.

cosine 0 degrees = +1

Hence the work is positive.

In the case of friction and the ground, on landing the angle between friction and the displacement is 180.

cosine 180 degrees = -1

Hence the work is negative.

Therefore when the force of friction points in the direction of displacement the work is positive.

Therefore when the force of friction points in the opposite direction of displacement the work is negative.

It may be hard for you to accept but this is basic physics my friend.

I'm not sure why you are even disputing this!

The argument for the work of gravity is dealing with work created by the torque as defined by:

dU = Md(theta)

The work done by gravity is the moment created by gravity and as it passed through the angle (theta) to help restore the leg initial position.

The argument is that this restoration force by gravity helps to conserve energy, rather having a person drive the leg with exaggerated motion. Pose argues that instead it is more efficient to let the leg fall underneath one's self than to "drive" it through. For this reason, I don't think the Pose method is valid for sprinters who need to "drive" for power. But for distance runners concerned with economy while “cruising” it offers an interesting argument.

The fact that Dr. Romanov is able to run on ice, demonstrates the he is at least “minimalizing” the propulsion due to friction. There is “some” friction on the ice, but the fact the he is able to run across it, is rather impressive and demonstrates the fact that he is using gravity as a restoration force to move the leg forward.

It is interesting to say the least.

Here are a couple of links:

The first one is the BBC show demonstrating the “improved” economy.

http://www.posetech.com/video/index.php/weblog/bbc_tomorrows_world/

The second one is him running on ice.

http://www.posetech.com/video/index.php/weblog/running_on_ice/

http://www.posetech.com/video/index.php/weblog/running_on_ice_part_ii/

Hopefully this topic can be explored more without all the name calling, nastiness and hate.

Remember its still the HOLIDAYS!!! :)

esmoke

I will follow the plan when i see the example of the guy running on top of the water......

Explore away, slowpoke.

I'll reconsider when I see Geb adopt the Pose method.

Happy New Year.

No gravity is the force created by the attraction of two masses.

Fg= G*M1*m2/r^2

G= Universal Gravitational Constant

M1 = Mass of Earth

r = Is the radius of the Earth

m2 = Your mass

G*M1/r^2 actually creates the little g which = -9.8 m/s^2.

Its actually not a constant, but at the surface of the earth we treat it as such.

It is maximum at the surface of the earth.

As you approach the center of the planet “g” decreases as a linear function (and is zero at the center of the earth).

As you leave the planet upwards “g” decreases as a hyperbolic function of 1/r^2 (theoretically never reaching zero).

The spinning action of the earth actually decreases our apparent weight (also called the Normal) slightly through centripetal acceleration.

esmoke

Thank you for that nice explanation esmoke, but I think Ballpoint was joking, so he still wins.

Actually little "g" is a positive constant. If it were negative, g would increase as you moved away from the surface of the earth, but you said it decreases, so it must be postive.

And the only reason that guy is able to run on the ice is because there is still a small amount of friction between his feet and the ice, which is evidenced when he slides on the ice and SLOWS DOWN.

I really can't believe this discussion has continued this far. It must be the work of some very talented trolls.

you guys are all a bunch of idiots.

ok, here's the situation: i'm standing on the ground and i allow myself to fall down. the truth is that i will fall forward. try it yourself. the fact is, i can do that all day and my legs won't get tired, so it has to be some other force that is pushing me forward. that other force is obviously gravity. try the same experiment repeatedly yourself, maybe you will knock some sense into your over inflated heads and you will realize that i am right, and i am always right.

so yes, gravity can push you forward. the pose method works, and until you realize that, you will never be faster than me.

Gravity pulls objects directly down to the ground. However, if something gets in the way, gravity will pull the object forward, even though its still just pulling straight down. Consider a slide. Gravity pulls a child down, but the slid makes them move forward. When a runner leans forward, gravity pulls them down, but the runner's legs keep them from falling all the way to the ground. Like the slide, they move the runner forward. The gravity helps maintain the runner's momentum. Consider objects in orbit. The gravity is not pulling the object forward, but it is keeping the object in motion so that it maintains its momentum. Leaning forward while running is a similar situation.

you wish you actually knew what you are talking about.