Maximal force

[Function]

Hi everyone

 

Didn't really know where to put this; physics or biology...

 

Let's imagine an organism (human) with a mass of 80 kg.

 

My primary question: can an organism with mass m deliver a pressure greater than its mass?

I'm sorry, but I really can't explain my question better and since pressure is dependent on force and area, I don't really know if this question is even correct.

 

So let's define a 'pressure' expressed in kg. Can the man deliver a pressure greater than 80 kg?

 

At first sight: yes. Imagine him standing underneath a door opening and pushing with his arms again the door post, thus the force on the ground (his supporting surface) is (much) greater than his mass.

 

But now imagine him in space, near a big wall; Let's assume the wall is immobile. Can he push against the wall with a 'pressure' greater than 80 kg?

 

 

I'm sorry that I can't really explain the problem here. I hate that pressure is defined as the force per area, for since the area gets infinitesimale small, the pressure gets infinitely large, making this problem somewhat vague.

I could also try to explain this with force, so the force on the ground would be much larger than m*g when pressing against the door post, but since there is no g in space (let's just assume this), he can't deliver a force? Very confusing.

 

Thanks.

 

F.

Edited July 19, 2014 by Function

[John Cuthber]

At first glance I think all of these records involve people exerting forces greater (sometimes a lot greater) than their own weight.

http://en.wikipedia.org/wiki/List_of_Olympic_records_in_weightlifting

[Strange]

Pressure is different from force. Stiletto heels can do serious damage to a wooden floor because the weight (force) is concentrated in a small area.

[Function]

At first glance I think all of these records involve people exerting forces greater (sometimes a lot greater) than their own weight.

http://en.wikipedia.org/wiki/List_of_Olympic_records_in_weightlifting

 

Which includes a supporting surface and a gravitational force etc., which is why I put the example in outer space.

 

 

Pressure is different from force. Stiletto heels can do serious damage to a wooden floor because the weight (force) is concentrated in a small area.

 

Let's then from now on focus on one of those two, since they are related. First one to say "force" or "pressure" decides.

Edited July 19, 2014 by Function

[studiot]

Force is mass times acceleration.

Weight is the force a given mass applies due to the acceleration due to gravity.

So you are asking if a body can exert a greater force than its own weight.

 

(Since pressure is force divided by area there is no theoretical limit to the possible pressure that can be exerted.)

 

Since gravity is not the only acceleration a body may be subject to, given a greater acceleration than gravity, it can apply a greater force than its own weight by virtue of this acceleration. If the extra acceleration is combined with gravity the total is called apparent weight.

 

There was a recent question here as to why the apparent weight increased in a lift, accelerating upwards.

http://www.scienceforums.net/topic/84324-can-someone-help-me-understand-the-apparent-weight-of-a-body-on-a-moving-lift/

 

Note also that a body may use the principle of the lever to multiply the applied force and that impulse (impact) forces are generally greater than steady ones.

Edited July 19, 2014 by studiot

[Function]

I think I fail to explain you that we are - in our example - in space, pressing against a wall, while floating around in the vacuum. I am convinced that a body (organism) with mass m can apply a maximal force against this wall, so can someone either approve this or convince me of my physical failure?

[Strange]

I think I fail to explain you that we are - in our example - in space, pressing against a wall, while floating around in the vacuum. I am convinced that a body (organism) with mass m can apply a maximal force against this wall, so can someone either approve this or convince me of my physical failure?

 

The force is going to be proportional to how quickly you make yourself accelerate away from the wall.

[Function]

 

The force is going to be proportional to how quickly you make yourself accelerate away from the wall.

 

Isn't accelerating away from something somewhat.. "nonintuitive" in space? What is there to move in?

[swansont]

You can't compare mass directly with force or pressure; the units differ.

 

Can you exert a force greater than your weight? Yes. You do it every time you jump. Being in space doesn't change that force in any direct manner, but weight is a meaningless measure in the absence of gravity.

[Strange]

 

Isn't accelerating away from something somewhat.. "nonintuitive" in space? What is there to move in?

 

Well, initially you are stationary, relative to the wall, then you push yourself away with increasing speed. That sounds like acceleration to me.

 

And f=m.a

[pzkpfw]

Here's an experiment you can do at home:

 

Get on some bathroom scales and crouch down. Thanks to your mass and gravity, the scales will be reading your weight.

 

Stand up very slowly. Your weight may not vary much at all.

 

Try again, but standing up very fast (but not enough to jump, that'll get awkward). While you are standing, your weight will read much higher. (Maybe get someone else to watch the scales, or set your phone to video the reading).

 

To stand up more quickly, you are applying more force. That is, you accelerate more, and your weight will briefly measure more.

 

The force of gravity here is basically constant, the difference is the force applied by your legs. If you did the same thing floating in space away from measurable gravity: pushing yourself away from the wall of your spaceship slowly or quickly involves different amounts of acceleration, requiring different amounts of force.

Edited July 19, 2014 by pzkpfw

[John Cuthber]

I think I fail to explain you that we are - in our example - in space, pressing against a wall, while floating around in the vacuum. I am convinced that a body (organism) with mass m can apply a maximal force against this wall, so can someone either approve this or convince me of my physical failure?

If I put my feet against the wall with my knees bent then straighten them quickly (as if I was jumping on Earth) then I can exert a force on the wall that more than my weight.

I would accelerate away from the wall much faster than I would move if I jumped on Earth because I would only be putting effort into accelerating whereas on earth I also have to do work against gravity.

After all, I can do it on Earth, why not in space?

 

Also, here's a stupid example, but it illustrates the point.

Imagine there are two of us in space and one of us kicks the other.

That kick can generate a lot more force than someone's weight.