[paul cotter]

I think I know the error this poster is making. In a healthy person of ~70kg the nervous system has adapted to the work in, for example standing up from a seated position, and the process feels close to effortless. Now that same person is asked to lift 20kg and a significant effort is required because that is an extra load that his(her) neuromuscular system is not accustomed to. If one was to strap a 20kg to oneself it would be quite noticeable as requiring an extra effort to stand up. Left there over a prolonged period it would become unnoticeable as the neuromuscular system adapts to the new load. Good to see you posting again, Origin, best wishes from "da Grinch"

[Bored chemist]

The final theory is available at SSRN:
The Biological Leverage in Humans and Animals' Self-Movement
https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4794848

https://www.quora.com/Is-the-journal-International-Journal-of-Applied-Science-and-Technology-a-good-journal-or-a-predatory-journal

[alancalverd]

Predatory and amateur.

[alancalverd]

I lift with my only feet muscles

There's the trick! No human has significant muscles in his feet. YAS is an octopus or an alien.

[Yahya A.Sharif (OP)]

It is SSRN, it is not a predatory journal it is not even a journal:
Biological Leverage: A Novel Principle in Human Biomechanics
https://papers.ssrn.com/sol3/papers.cfm?abstract_id=4794848

[Yahya A.Sharif (OP)]

I lift with my only feet muscles

YAS is an octopus or an alien.

Its along time since your funny comments, thanks Alan I actually adopted that acronym.

[Bored chemist]

it is not even a journal:

[evan_au]

A different take...

Can a mass be lifted with force less than its weight ?

Yes, if it is in a liquid to provide some buoyancy...
- and the weight is measured in air

That's why elderly people do "Aqua-Aerobics".

[Yahya A.Sharif (OP)]

A different take...

Can a mass be lifted with force less than its weight ?

Yes, if it is in a liquid to provide some buoyancy...
- and the weight is measured in air

That's why elderly people do "Aqua-Aerobics".

Hello Evan_au,
Yes it's true that doing that exercise will require less force, but the resultant force is the force by muscles plus the buoyant force  the same force when doing that outside water. A similar example is a mass balanced with a pulley; pulled upwards by another mass less in weight you will always lift the bigger mass with force less than its weight because it is pulled upwards by the rope by the smaller mass gravity.

[alancalverd]

No. The force in the part of the rope attached to the bigger mass is exactly the weight of the mass. A single pulley won't make life any easier but a multiple sheave block acts as a force multiplier: the small mass moves further than the big one and energy (force x distance) is conserved.

[Yahya A.Sharif (OP)]

I think you didn't get me. The pulley is free to rotate, the smaller mass is exerting force equals to its gravity on the bigger mass, so you need to exert force equals:
Mg-mg
M: mass of the bigger object
m: mass of the smaller object
Actually if the masses are equal, in theory you can exert the smallest force but to accelerate it in a short time to lift it you need to exert the force for the time you want. See the drawing.

[alancalverd]

Your wording was inexplicit and implied what I wrote.

[paul cotter]

It is still wrong.

[alancalverd]

The weight of an object is defined as the force required to lift it. So the question is absurd.

[Yahya A.Sharif (OP)]

The weight of an object is defined as the force required to lift it. So the question is absurd.

Hello Alan, yes the weight of an object is defined as the force required to lift it, now an answer to my question might be yes via levers now I want to answer it further by adding the biological leverage. My experiments use weight and mechanical advantage which violates your defintion above unless there is a sort of biological leverage. Tools, devices and systems have corresponding biological ones there is a possibility that the lever as a tool is originaly biological.

[paul cotter]

You are confusing perceived effort with physical measurement.

[alancalverd]

Hello Alan, yes the weight of an object is defined as the force required to lift it,

So you agree that the answer to your question is "no". 

You might generate that force by applying a smaller force to a lever/crowbar/pulley block, but the force applied to the object will still be the weight of the object. The ratio of output force/input force is called "mechanical advantage".

I can't think of any biological lever with a mechanical advantage greater than 1.

[Eternal Student]

Hi.

I can't think of any biological lever with a mechanical advantage greater than 1.

    I don't know.....  my sister had a nasty technique.
    Put their head between your left upper arm and your body,  then use your other arm, the right arm to grab the edge of your left arm and pull it inwards.    You should have your right arm further from the fulcrum (your armpit) than their head.

Best Wishes.

[Yahya A.Sharif (OP)]

I can't think of any biological lever with a mechanical advantage greater than 1.

That caught my interesting. The human skeleton can not contain a lever that has mechanical advantage greater than one since the characteristics of such lever requires a fulcrum outside the human body the biological leverage actually is embedded physiologically inside the body making it the greater-than-one lever for organisms reminds me of differences in mechanisms in organisms and machines like an airplane vs a bird.

[Yahya A.Sharif (OP)]

Hi.

I can't think of any biological lever with a mechanical advantage greater than 1.

    I don't know.....  my sister had a nasty technique.
    Put their head between your left upper arm and your body,  then use your other arm, the right arm to grab the edge of your left arm and pull it inwards.    You should have your right arm further from the fulcrum (your armpit) than their head.

Best Wishes.

Hello Eternal Student,
Your sister exactly used her head as an external fulcrum to build a lever but tha is not possible for a natural body formation.