[Yahya A.Sharif (OP)]

Here is my discovery
I can lift my whole body " 60kg" with my feet , but I can't lift a 60kg mass with my whole body.
Explanation
When a human uses his own body muscles to lift his own body mass he can do the moves with smaller force than lifting or moving any other object with the same mass, I can jump pushing my own body up high but I can't even move a rock of the same mass from the ground.

[Origin]

I can lift my whole body " 60kg" with my feet , but I can't lift a 60kg mass with my whole body.

Explanation:  you can't lift it because that would be a total 120 kg you would be trying to lift.

[Yahya A.Sharif (OP)]

I can lift my whole body " 60kg" with my feet , but I can't lift a 60kg mass with my whole body.

  you can't lift it because that would be a total 120 kg you would be trying to lift.

Let it be sitting down on a chair " no effect of body weight on my feet  " I put 20 kg on my thighs above my knees and I lift it up by moving my feet up. What would be easy ? to lift the 20 kg or to lift my body 60 kg?

[evan_au]

Let it be sitting down on a chair

When you lift weights to a standing position, you engage your gluteus maximus (butt muscles), which are some of the largest muscles in the human body.

These can't help much when you are seated, so you can lift a reduced weight with your feet.

[Origin]

Go to the gym and use the leg press equipment and you will find that you can easily lift more than your body weight.  There is no magic going on here.

[Yahya A.Sharif (OP)]

Let it be sitting down on a chair

When you lift weights to a standing position, you engage your gluteus maximus (butt muscles), which are some of the largest muscles in the human body.

These can't help much when you are seated, so you can lift a reduced weight with your feet.

I might be misunderstood in my previous post :
 Raising my body 60 kg is while standing and that by moving my feet up. Raising the 20 kg is while sitting putting the 20 kg on my thighs above my knees and move my feet up both I lift with my only feet muscles without using my leg muscles. I claim it would be far more easy to left the 60kg"my body" than to lift the 20kg because a body can lift itself easier than it lift a mass.

[evan_au]

I claim it would be far more easy to left the 60kg"my body" than to lift the 20kg, because a body can lift itself easier than it lift a mass.

Your body does have mass, so you have to include that in what the body is lifting.

We are so used to holding up the body's mass while standing or walking, that it seems "effortless".
- Now try walking up 10 floors of a building via the stairs - you are lifting your body mass against gravity, and that takes energy.

On Earth's surface, we can often ignore the difference between weight and mass, because a stationary mass of 1kg exerts a force of 1kg-Force =9.8 Newtons.
- One place where these are very different is in the "vomit comet": planes used for astronaut training by NASA, ESA and the Russian space agency (and able to be hired for joy rides, too).
- During the microgravity part of the flight, you could (slowly) lift a ton of mass - because it is weightless.
- But you need to get out of the way quickly, because after 30 seconds, the plane bottoms out, and the weight increases to 2 tons, which would crush you (and damage the plane).
- In this phase of the flight, your weight increases to twice normal, which most people can withstand by standing while holding onto a handrail. But they require that you don't try to walk in this situation, as you are likely to fall and break bones. (The safety staff on the flight are trained to walk under these G-Forces - but they do so slowly and carefully!)
- Under 2g acceleration, I expect that you would struggle to lift yourself alone, with no weights.
See: https://en.wikipedia.org/wiki/Reduced-gravity_aircraft
https://omegataupodcast.net/330-parabolic-flights-at-airzerog/

[Yahya A.Sharif (OP)]

- Now try walking up 10 floors of a building via the stairs - you are lifting your body mass against gravity, and that takes energy.

You didn't compared it with walking up carrying a load.

[alancalverd]

Your legs account for about 1/3 of your mass, so the muscles in your legs are only lifting 40 kg, and they are, as pointed out, huge.

The argument above is considerably confused but it seems to revolve around either "deadlifting" another 60 kg to the mass your legs are lifting, which is manageable with training but not what we evolved to do, or "curling" 20 kg from a seated position, using the puny muscles in your skinny arms - again manageable with training, but easier if you are some other kind of ape. Adult humans differ from other primates in  being obligate bipeds, with long rear limbs and very little capacity for brachiation (though babies can grasp and hang long before they can walk), which is why we are reluctant to swing through the trees and sleep in a nest.   

The trick of a good backpack is that the frame transfers the load to your hips. I often wear a "lead apron" for radiation protection at work: the best modern design uses a skirt and jacket to distribute the weight better than a simple tabard.

[Yahya A.Sharif (OP)]

Your legs account for about 1/3 of your mass

It is absolutely easy to raise up my legs of 20 kg while sitting and it is absolutely hard to raise legs up in a case 20 kg is tied to them calculating the weight of my legs 20 kg adding to them the mass 20 kg will not give the normal results the difference will be huge.

[alancalverd]

Definition of weight: force required to lift a mass. If you get the physics right, it will be easier to discuss the physiology.

[Kryptid]

 lifting the body "60kg"is while standing, lifting the 20 kg is while sitting  , both I lift with my only feet muscles without using my leg muscles. I claim it would be far more easy to left the 60kg"my body" than to lift the 20kg, because a body can lift itself easier than it lift a mass.

The distribution of mass is different and the muscle groups engaged are also different. Have you heard of the concept of leverage? https://en.wikipedia.org/wiki/Lever#Force_and_levers If you consider your legs to be levers and the load is placed at your feet, then that is the worst possible place to put the weight in terms of lifting difficulty. If you put the load on your knees or thighs, it would be easier to lift despite having exactly the same mass.

[Yahya A.Sharif (OP)]

Have you heard of the concept of leverage? https://en.wikipedia.org/wiki/Lever#Force_and_levers

Yes I have.

If you consider your legs to be levers and the load is placed at your feet, then that is the worst possible place to put
the weight in terms of lifting difficulty.

That doesn't  have significant effect

If you put the load on your knees or thighs, it would be easier to lift despite having exactly the same mass.

I don't think putting a 60 kg load on my thighs is a good idea.

[evan_au]

My great new discovery : A mass can be lifted with force less than its weight

This is not a new discovery.

This was achieved by an invention known since antiquity: The "Block and Tackle":
https://en.wikipedia.org/wiki/Pulley#Block_and_tackle

(Kryptid also mentions levers, above, which achieve the same force-multiplying effect by a different mechanism...)

[Kryptid]

That doesn't  have significant effect

It has a very significant effect:

Double the length of the lever, and you double the force produced. So placing a 20 kilogram weight on your knees would require you to exert only about half as much force as you would need to lift that same weight if it was at your feet.

I don't think putting a 60 kg load on my thighs is a good idea.

Like I said before, the distribution of mass and the muscle groups employed are different in that scenario than they are for when you stand up from a sitting position.

[Bored chemist]

both I lift with my only feet muscles

Feet have very little muscle.
So, it's clear that, as usual, you don't know what you are talking about.

Why do you do this?

[Yahya A.Sharif (OP)]

The distribution of mass is different and the muscle groups engaged are also different. Have you heard of the concept of leverage? https://en.wikipedia.org/wiki/Lever#Force_and_levers If you consider your legs to be levers and the load is placed at your feet, then that is the worst possible place to put the weight in terms of lifting difficulty. If you put the load on your knees or thighs, it would be easier to lift despite having exactly the same mass.

I can lift my 60 kg body with only my weak feet muscles when trying to pick a fruit on a tree.

Let's for the example above  the foot is 30 cm or 0.3 meters, Now let's calculate for a 0.3 lever:
First the lever will be class 3 :
The weight for 60 kg will be 60*9.8=588 Newtons.
The force of my feet  is distributed along the feet  , from both  toes to both  heel .Let’s  the muscles force  is  focused on the heel which is the maximum force of muscles could be.
Class 3  is the fulcrum at the toes  , and in this case both the weight of my body and the force of my feet will be exactly at the heel:
F: force of my weight
f: force of  muscles strength
L: the distance  of the weight from the heel to the toes.
l: distance of the muscles force from the heel to the toes.
f * l=FL
F=588   and L=l =0.3

f*0.3=588*0.3
f=588  Newton
The muscles force to lift 60 kg according to current physics and the lever idea must be  588 Newtons
The muscles force  actually is far less than 588 Newton but it  lift the body .

[Bored chemist]

I can lift my 60 kg body with only my weak feet muscles when trying to pick a fruit on a tree.

Actually, it is mainly your calf muscles doing that, and they are (obviously) rather strong.

[Bored chemist]

What you seem to have noticed- it isn't news to science- is that a thing can  produce a force that is greater than its own weight.
So what?
A hammer does that.

[jeffreyH]