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

JimFiore wrote:

If you held your torso at 45 degrees, it seems to me that you be expending extra energy to do so versus being more or less vertical (that is, there's a greater torque). I suspect that some deviation from perfect vertical may be in order depending on individual biomechanics and other factors (velocity, incline, wind, etc.)

Yes, I suspect so too (so ignore my previous rambling).

roger penrose wrote:

However, there is no work done in keeping something in a certain position. So your muscles do not do any work to "keep you in a specific position." For example, when pushing on a large object, you have done no work until that object actually changes position.

Not to put words in Jim's mouth, but I think he was referring to maintaining a specific posture (e.g., forward lean) rather than to maintaining a specific position along a flat plane.

roger and Elmer,

While it's true in the classic sense that no work is done to keep a body in a specific position, this is not the case when it comes to energy expenditure in the real world of humans. If I grab a box and proceed to carry it one level mile, placing it on a shelf at the same height as I grabbed it, I have no done no work on the box (PE is constant). I have, however, expended energy to do so (I have burned calories that I wouldn't have had I just stood still next to the box). Energy is, of course, the ability to do work. If I lean forward at a certain angle my muscles will burn calories to keep me in that position. I am, afterall, not a rigid body. The more I lean, the greater the torque, and the more difficult in general it will be for me to maintain that position.

Don't forget, from the classical viewpoint of PE, even if you run to the top of a mountain, when you return to your starting point you will have done no work as your vertical position (elevation) has not changed.

But ultimately, as many have stated, this is all window dressing. What matters here is the improvement of running mechanics. How do we propose to do that? I prefer to have someone simply run and apply basic drills (hills and so forth) instead of saying "try to run so that you look like this". A nice form does not necessarily mean good economy, but lots of running is an almost sure way of making one more efficient. The body WILL adapt.

Bum wrote:Given the innaccuracies posted on this thread, I find it completely implausible that humans actually invented the atomic bomb and landed a man on the moon. Both must have been pure luck.

If I put a ball on an incline and let go, gravity acts in a downward fashion, however, the ball goes forward as well.

If I stand montionless on skis on an incline with no snow, gravity is acting on me, however I do not move in a forward direction because my skis are held by friction.

If I stand montionless on skis on an incline with snow, and the horizontal component (caused entirely by gravity) exceeds friction then I will slide downnwards AND forward.

What do you call the horizontal vector that was caused entirely by the force of gravity acting in a vertical direction?

Question for Jim Fiore wrote:

What do you call the horizontal vector that was caused entirely by the force of gravity acting in a vertical direction?

The horizontal component of the normal force (acting perpindicular to your inclined plane)?

Try a force diagram. The horizontal force was not caused at all by the vertical force of gravity. Is science education in the US really this lacking?

Gravity forces the ball downwards at all times. The normal force pushes the ball outwardly perpendicular to the surface of the incline at all times. When you add those two vector forces, you get a force in a direction parallel to the inclined plane.

One foot pushes off the ground briefly launching the body into the air. Gravity accelerates the launched body back towards the earth as the other foot gets in position to catch the body. Once caught, the foot and leg carry the body forward in preparation for another launch. Once in position, the launch occurs while the other foot positions itself for another catch.

Running is nothing more than keeping your forward momentum going through a series of launches, catches, and carries as your upper body does what it must to maintain balance. This balancing act may or may not involve a forward lean, depending on your specific body shape and mechanics.

I call that horizontal vector, 'F minus.'

I gather you're familiar with that 'grade.'

Asterix wrote:

The horizontal component of the normal force (acting perpindicular to your inclined plane)?

Try a force diagram. The horizontal force was not caused at all by the vertical force of gravity.

Really? Without the vertical force of gravity the object (ball or skier) would not move at all.

-9.8 m/s squared. bling bling.

Asterix wrote:

asdef wrote:

How is this any different from running downhill? You're changing gravitational potential into kinetic energy.

But unless you have an infinite downwards slope, you will need to regain that lost altitude by the application of a vertical force greater in magnitude than gravity.

There is no such thing as a free lunch.

Because you had to overcome gravity (store energy) to get to the top of the hill/half-pipe/whatever in the first place. There is no free lunch in physics.

Question for Jim Fiore wrote:

Really? Without the vertical force of gravity the object (ball or skier) would not move at all.

Yes, really. Draw a force diagram. Study the response given more clearly by 'roger penrose'.

Since you are dealing with an inclined plane, there is a component of the vertical force of gravity that is parallel to your surface. But this is only the result of you being on an inclined plane.

Add your force of gravity and the normal force (which is perpindicular to your plane), you get a resulting vector

When your plane is truly horizontal, there is NO component of the force of gravity that acts parallel to your surface. See trigonometry. No matter what your surface, the force of gravity NEVER pulls in any direction but straight down.

tank wrote:

Lets think clearly about this and not over complicate and obfuscate the critical issues.

Can gravity enhance forward momentum?

Although gravity is a downward force it can under some circumstances be redirected into forward motion. The examples given by others including the ball in the half pipe, upsidedown tipping pyramid, rollercoaster are all examples of this. Its specious to try and refute this by stating that the forward momentum is all due to energy expended by having previously lifting the object. All gravity created velocity is due to previous energy used to lift the object (stored enegy).

In the examples given the objects move faster forward due to the force of gravity.

No one has refuted that the upside down pyramid falls forward faster due to gravity.

Please don't obfuscate that basic observed fact with miss directed application of common formula's.

The upside-down pyrimid does not fall forward at all, unless it is pushed forward. The push gives it any forward (horizontal) momentum that it has. Without horizontal momentum from an external force it will do nothing at all. Gravity only pulls it straight down.

Very good. Well put.

There are very many intelligent people becoming obtuse on many obvious points. Lets walk through this in simple logical steps.

1) Gravity's pull is always downward. (everyone agrees)

2) In any sport like crosscountry skiing or running gravity slows down your performance. You must expend energy to overcome the effects of gravity. (agreed)

3) At certain points of an activity the effects of gravity are lessened (downhill running) or in the case of crosscountry skiing even helpful. In skiing as you are skiing down the hill the effect of this downward force to to increase your forward speed. (Does that get us around the semantics)

4) There are certain ways to ski and run that to some degree minimizes or maximizes the negative affect of gravity on performance. Runners who take high bounding steps are fighting gravity more than runners who don't bound as high.

Case closed.

I'm a bit rusty on my physics 101 I must admit. Could someone provide me the answers (step by step) to the following questions.

1) I have an object on a frictionless incline (30 degree slope) that rests 3 meters high. If I let the object go, what will be the horizontal speed of the object when it gets to the end of the incline?

2) Same object on a frictionless incline, same height (3 m), 45 degree angle.

Asterix wrote:

Yes, really. Draw a force diagram. Study the response given more clearly by 'roger penrose'.

Since you are dealing with an inclined plane, there is a component of the vertical force of gravity that is parallel to your surface. But this is only the result of you being on an inclined plane.

Question for Jim Fiore:

don't listen to this asterix guy. he is clueless and contradicts himself. not only that but he is another of the supercilious kind who only replies to your question to condescend you and not to help you. at first he says gravity only pulls down and then goes on to say there is a component of gravity that is parallel to the surface. what a goofball.

let me answer your question. yes, gravity can cause objects on an inclined surface to move forward. to prove that all you need to do is imagine a skier on a 30 degree slope in the absence of gravity. they will not move. with gravity they slide down and forward. HOWEVER, it is NOT gravity that is pulling the objects forward. you have to remember that for every action there is a reaction. in this case the action is the force you exert against the surface, the force caused by gravity. the reaction is the force exerted by the surface onto you. this vector force by the surface is perpendicular to the surface. on a 0 degree plane this vector force is going to be exactly opposite of the force of gravity and the same amount. as the slope increases this vector rotates with the slope and also decreases so that on a 90 degree slope there will be no force exerted on you by the surface, but on an 89 degree slope there will be a miniscule force directed onto you at 89 degrees. part of this vector force counteracts gravity so that your vertical acceleration down the slope will be less than that of a freefall, and part of this vector force is horizontal such that you will go forward. in summary, what pushes you forward down the slope is the slope itself, and this is a reaction to the force of gravity. so you are correct in saying gravity causes you to move forward down the slope, but it is not actually gravity that pulls you forward. this is similar to saying that what causes a lightbulb to light up is the flipping of the switch, even though what actually makes the lightbulb light up is the electromagnetic radiation of the filament as electricity passes through it.

if gravity cant pull you forward im assuming it cant pull you back, so why cant we lean back when we run?