There can be no argument that humans constantly do work (meaning they constantly transfer energy).
A human being is a complex system that constantly transforms chemical energy into other forms.
A human being is a complex system that constantly transforms chemical energy into other forms.
Let's consider a man supporting a brick in his hand.
The man must exert an upward force equal to the weight of the brick, but this force does no work on the brick.
So why will the man eventually feel tired just holding the brick if he is not doing any work on it?
The answer is that muscle filaments in his arm are doing work as they contract and relax inside his arm thus converting chemical energy to heat.
The man must exert an upward force equal to the weight of the brick, but this force does no work on the brick.
So why will the man eventually feel tired just holding the brick if he is not doing any work on it?
The answer is that muscle filaments in his arm are doing work as they contract and relax inside his arm thus converting chemical energy to heat.
If you invoke ‘fictitious force’ as you have done, and Einstein did not provide an explanation for why inertial gravity and gravity on the surface of the earth were the same, or behaved the same, it cannot be ‘elementry physics’ can it? 😊
The brick has gravitational potential energy and all sorts of other energy types as I explained. If you move it on a solid surface, you are adding an additional energy source to do so. Similarly, if you want to change the trajectory of a body in flat space time, you will have to expend energy to create a force to do that. In both cases, the object requires energy in order to change its position/trajectory. In both cases the objects are moving - it’s just that one is on the surface of a large mass that itself is moving.
Please note that I uninvoked pseudo force back in my post #4,774, but shall repeat it here:
"To be clear, I was wrong to mention "fictitious force" in my post #4,769.
As I have since said, there is no fictitious force in the accelerating clock scenario that you originally introduced.
The recent discussion on fictitious force may simply be regarded as an aside."
Yes, and the additional energy source would be you, or more exactly, the chemical energy in your food.
And that chemical energy is what enables your muscles to produce a force that can transfer energy to the brick.
Are we getting somewhere?
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We’re still on post 4791 IMV. Inertial mass and gravitational mass were shown by Einstein to be equivalent. Do we agree on that?
If yes, I proposed that in both cases the common factor was energy transformation. On Earth it’s the energy transformations of all kinds within the mass of the Earth that curves space time and gives a body weight. In flat space time, it’s the energy transformation provided by some motive energy that creates the 1g force. I’ve used weight and inertial mass interchangeably- the fact that one is in newtons and the other in kg is neither here nor there IMV since it’s just driven by convention. Both describe a force brought about by energy transformations.
You stated a body on the Earth has no energy input requirement at rest since gravity and the ‘upward force’ balance each other out. I disagree. There is no ‘upward force’ just solid ground that prevents the object falling further to the centre of the Earth. However, said object has potential gravitational energy which manifests as a force (weight) so therefore you need additional energy input aka energy transformations to move it, unless you throw it off a building in which case the potential gravitational energy transforms into kinetic energy as it falls.
Is this a fair summary of what we’ve covered thus far?
If yes, I proposed that in both cases the common factor was energy transformation. On Earth it’s the energy transformations of all kinds within the mass of the Earth that curves space time and gives a body weight. In flat space time, it’s the energy transformation provided by some motive energy that creates the 1g force. I’ve used weight and inertial mass interchangeably- the fact that one is in newtons and the other in kg is neither here nor there IMV since it’s just driven by convention. Both describe a force brought about by energy transformations.
You stated a body on the Earth has no energy input requirement at rest since gravity and the ‘upward force’ balance each other out. I disagree. There is no ‘upward force’ just solid ground that prevents the object falling further to the centre of the Earth. However, said object has potential gravitational energy which manifests as a force (weight) so therefore you need additional energy input aka energy transformations to move it, unless you throw it off a building in which case the potential gravitational energy transforms into kinetic energy as it falls.
Is this a fair summary of what we’ve covered thus far?
1. We need to take into account all the forces which act on a body that sits on a floor.
Firstly, there is a pair of non-contact forces - planet Earth pulls the body down while the body pulls planet Earth up.
Secondly, there is a pair of contact forces - the body pushes down on the floor while the floor pushes up on the body.
Note that the only forces acting on the body are the downward pull of the Earth and the upward push of the floor. Crucially, if the body were a person, it is the upward push of the floor that gives him the sensation of weight.
2. Now put a person in a lift.
To accelerate the occupant upwards, the upward push of the floor of the lift must exceed the the downward pull of the Earth. During the brief period of upward acceleration the occupant would therefore feel that their weight had increased.
Now consider what happens when lift and occupant begin to accelerate downwards. The upward push of the floor on the occupant must become smaller than the downward pull of the Earth. During the brief period of downward acceleration the occupant would therefore feel that their weight had decreased.
Now extend this to the lift cable breaking, causing both the occupant and the lift to accelerate downwards at 1 g. The occupant would no longer feel an upward push from the floor of the lift and consequently would feel 'weightless'.
Kinetic energy=force*distance. No movement, no energy dissipated. Objects at rest on earth surface consume no energy. potential energy is not energy consumed unless potential is turned into movement.
To lighten the mood, here's seven minutes of fun!
In particular, at 4 mins in, watch the falling leaky bottle of water.
In particular, at 4 mins in, watch the falling leaky bottle of water.
You haven’t answered post 4806 Galu. You’re on about Newtonian gravity.
Potential energy. As I noted, try picking the brick up or pushing it off the side of a building. There you have to do work to create potential energy and it changes to kinetic energy when it gets dropped. But the brick has other forms of energy as well as I explained. It’s all these other forms of energy that curve space time.
Even the earth has kinetic energy because it is in freefall around the sun, but it contains thermal, atomic etc energy.
Potential energy. As I noted, try picking the brick up or pushing it off the side of a building. There you have to do work to create potential energy and it changes to kinetic energy when it gets dropped. But the brick has other forms of energy as well as I explained. It’s all these other forms of energy that curve space time.
Even the earth has kinetic energy because it is in freefall around the sun, but it contains thermal, atomic etc energy.
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I pointed out a glaring error in your post 4806. The upward force exerted on the brick by the floor is very real, yet you said "there is no 'upward force'"!
When the brick pushes down on the floor (action), the floor pushes up on the brick (reaction).
Physics text books refer to the "normal reaction" as being the force that gives a person the sensation of weight.
P.S. Gravity acts on every cell/atom in the human body and not on some particular area of the body, and I believe that is why we don't perceive it directly. The normal reaction does act on a particular area, e.g. on the soles of the feet, and it is this force that we perceive as the force of gravity.
Potential energy is just stored energy. Energy can be stored in the gravitational field where it is available to do work.
In physics 'doing work' means applying a force over some distance in the direction of the force. When work is done, energy is transferred.
Let's take the energy stored in the water piled up behind a dam in a hydroelectric scheme as an example.
Open up the dam gates and potential energy will be converted into kinetic energy which provides the force to do work in turning the generators.
I presume this is what you mean when you say that gravitational potential energy "manifests" itself as a force.
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I'm learning nothing new about Einsteinian gravity, but I have picked up some misconceptions regarding Newton's laws of motion.
I pointed out a glaring error in your post 4806. The upward force exerted on the brick by the floor is very real, yet you said "there is no 'upward force'"!
Wot, like a 'fictitious force'?
There is a point contact force. You may want to draw it with a vector but the fact of the matter is it's the weight of the object on the surface that is pushing down (or getting pulled down), not something 'pushing up' - its a solid surface that can not move. It hardly makes sense to talk about a 6 x10^24 kg body 'exerting an upward force' on a 3 kg brick does it? This reminds me of descriptions of the equivalence principle where the surface of the Earth is described as 'accelerating upward' (if you google around you'll find it).
Potential energy is just stored energy. Energy can be stored in the gravitational field where it is available to do work.
In physics 'doing work' means applying a force over some distance in the direction of the force. When work is done, energy is transferred.
Let's take the energy stored in the water piled up behind a dam in a hydroelectric scheme as an example.
Open up the dam gates and potential energy will be converted into kinetic energy which provides the force to do work in turning the generators.
I presume this is what you mean when you say that gravitational potential energy "manifests" itself as a force.
Doing work is the same as energy Galu. You can talk about it taking place over distance or time. So I do not see any contradiction with your statement.
Back to the brick: If I want to add potential gravitational energy to my brick, I pick it up - the act of moving it up adds potential energy and I can calculate that from the weight of the brick and the distance I moved it up by. I drop the brick, it has kinetic energy and it impacts the Earth, exerting a force at the point of impact. That energy dissipates (transforms) into heat and movement of the soil etc.
Wot, like a 'fictitious force'?
There is a point contact force. You may want to draw it with a vector but the fact of the matter is it's the weight of the object on the surface that is pushing down (or getting pulled down), not something 'pushing up' - its a solid surface that can not move. It hardly makes sense to talk about a 6 x10^24 kg body 'exerting an upward force' on a 3 kg brick does it? This reminds me of descriptions of the equivalence principle where the surface of the Earth is described as 'accelerating upward' (if you google around you'll find it).
Potential energy is just stored energy. Energy can be stored in the gravitational field where it is available to do work.
In physics 'doing work' means applying a force over some distance in the direction of the force. When work is done, energy is transferred.
Let's take the energy stored in the water piled up behind a dam in a hydroelectric scheme as an example.
Open up the dam gates and potential energy will be converted into kinetic energy which provides the force to do work in turning the generators.
I presume this is what you mean when you say that gravitational potential energy "manifests" itself as a force.
Doing work is the same as energy Galu. You can talk about it taking place over distance or time. So I do not see any contradiction with your statement.
Back to the brick: If I want to add potential gravitational energy to my brick, I pick it up - the act of moving it up adds potential energy and I can calculate that from the weight of the brick and the distance I moved it up by. I drop the brick, it has kinetic energy and it impacts the Earth, exerting a force at the point of impact. That energy dissipates (transforms) into heat and movement of the soil etc.
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You obviously paid no attention to my explanation in post #4,807. I suggest you brush up on Newton 3, balanced forces and free-body diagrams.
It is a common misconception that there is a downward force on the surface, but no upward force, and that the surface just gets in the way.
You might find the video in this link helpful: https://conceptualacademy.com/course/conceptual-physical-science-explorations/28-support-force—why-we-don’t-fall-through-floor
We don't fall through the fall because there's a solid surface that is difficult to pass through Galu. How do you reconcile that with F=ma? To make that work, you now have to say 'The Earth is accelerating upwards'. Really? If someone is going to invoke 'we don't fall through the floor because there's an upward force pushing on a downward force' then what does that make of atoms and molecules bound together that form the solid structure the smaller object is resting on?
Hmm... I figure the surface of the earth, being solid, resists the downward force that gravity exerts on our bodies.
That energy is dissipated as heat.
That energy is dissipated as heat.
Newton's 1st law of motion dictates that, since the object on the floor is stationary, it must be acted upon by balanced forces.
In this case that's two forces that are equal in size and opposite in direction, i.e., the downward pull of the Earth and the upward push of the surface.
If the surface were not present, the pull of the Earth would be an unbalanced force and the object would accelerate downwards according to Newton's 2nd law as represented by F = ma.
What happens to the a in F=ma in that?
(Newton provides a decent framework for how gravity behaves along with equations that are almost but not quite 100% correct, but as Einstein showed, they do not explain it perfectly).
(Newton provides a decent framework for how gravity behaves along with equations that are almost but not quite 100% correct, but as Einstein showed, they do not explain it perfectly).
When the object is in free-fall (falling without air resistance):
F = W (the pull of gravity on the object)
m = m (the mass of the object)
a = g (the acceleration an object undergoes while in free fall)
The equation W = mg is a special case of Newton's second law of motion, F = ma.
Objects sitting on solid ground on earth do not accelerate.
Flesh is not solid, so we feel the force of gravity as a deformation of flesh in the lower parts at rest. As standing we are an unstable pendulum with pivot in the ankles, we rock back and forth on the feet. An unconcious movement after the age of one.