Does the Force of gravity act at an object every second?

[sulemanma2]

If something has a mass of 1 kg and is at some height at rest, and I drop it, it will experience of force due to gravity of about 10 N. So does that mean that every second that passes by the objects feels a force 10 N pulling it downwards? So let's say that object falls for 20 seconds, would that mean the object experienced a total force pulling it downwards of 200 N, or would that object have only experienced a force of 10 N downwards no matter how much time passes by?

I also have another question that might build upon the right answer to this one.

 

[Pengwuino]

It will only feel a force of 10N for the entire trip. If it did have 200N, then by F=ma, either the acceleration would have had to change (but we know it's always 9.8 m/s^2 near the surface of Earth) or the mass would have to change, both of which don't change.

 

[russ_watters]

No, force is not typically multiplied by time. 20N is 20N.

If you have $10 in your pocket for 20 seconds, does that mean you have $200?

 

[sulemanma2]

Thanks for your answers. My other question is if some object that weighs 10 N is resting at a surface, the object put a downward force of 10 N on the surface, and the surface puts an upward force of 10 N to balance it out. So where does the upward force from the surface come from? Does it get deflected upwards by the downward force from object? Or does it come from inside the surface?

 

[Saladsamurai]

Thanks for your answers. My other question is if some object that weighs 10 N is resting at a surface, the object put a downward force of 10 N on the surface, and the surface puts an upward force of 10 N to balance it out. So where does the upward force from the surface come from? Does it get deflected upwards by the downward force from object? Or does it come from inside the surface?

Hi sulemanma2 I think that a good way to think about this is to think of a piece of wood that is laid across two blocks. If you were to stand on the center of the piece of wood, it is not so hard to imagine that it would begin to bend or bow or even break depending on your weight and/or the length of the board between the blocks. Let's imagine that it does not break and only bends. Clearly the board is "pushing" back at you. If you cannot imagine this, think of what would happen if you suddenly jumped off of the board; surely it would "snap back" into it's original flat state; hence, there was some force pushing back in the direction of where you were standing.

So the answer to your question is twofold: the force that pushes back comes from internal forces (i.e. the wood "trying not to break") and it acts via the surface of the wood where your feet are interacting with the board.

 

[DaveC426913]

Hi sulemanma2 I think that a good way to think about this is to think of a piece of wood that is laid across two blocks. If you were to stand on the center of the piece of wood, it is not so hard to imagine that it would begin to bend or bow or even break depending on your weight and/or the length of the board between the blocks. Let's imagine that it does not break and only bends. Clearly the board is "pushing" back at you. If you cannot imagine this, think of what would happen if you suddenly jumped off of the board; surely it would "snap back" into it's original flat state; hence, there was some force pushing back in the direction of where you were standing.

I had never thought about it that way until just this moment.

We were always taught that Newton's Law applies to standing on the ground in that you push down on the Earth and it pushes back, but I never really grokked what it meant for the Earth to "push back". (How can a passive, solid object psuh back?)

But your beam of wood shows it perfectly. The beam of wood really does push back; it yields and resists, because of its own internal structure (this becomes more obvious of you turn the wood vertically and simply push on it - it pushes back). And, of course, so does the Earth.

Moreover, it then raises the question: well, what if the ground or wood did not push back with equal and opposite force?

To which the answer is: then it is not ground or wood; it is something unresisting, like, say water. And then you push down on the water, it does not push back, so you sink!

Thank you Samurai, a childhood mystery I can finally put to bed.

 

[russ_watters]

Moreover, it then raises the question: well, what if the ground or wood did not push back with equal and opposite force?

To which the answer is: then it is not ground or wood; it is something unresisting, like, say water. And then you push down on the water, it does not push back, so you sink!

Yes, water does push back with an equal and opposite force. What you're missing is that you won't be pushing on the water with a force equal to your weight.

Forces always come in equal and opposite pairs.

 

[Saladsamurai]

Yes, water does push back with an equal and opposite force. What you're missing is that you won't be pushing on the water with a force equal to your weight.

Forces always come in equal and opposite pairs.

Hmmm... Much confusion has arisen due to this statement Russ You are right; I mean, Newton's third is Newton's third and makes no reservations as to what kind of object is experiencing a given force. But let's talk about this some more. If I were suspended such that my feet are just about to touch the water and then I was released from rest, what exactly happens? (Of course I sink, but in terms of the physics.)

At the instant I touch the water, there are two forces acting on me: My weight and something else pushing back. What exactly is that something else? It is my natural instinct to say that it must be a buoyant force, but then I immediately reject that thought since at the instant my feet touch, there is no water displaced. So I feel like it must a normal force like that of the wood in my previous example. But then the water "breaks" as a result of something. That something is probably surface tension and is maybe analogous to the wood "bending moment."

But I would love to get a real description of what is going on here.

~Casey

 

[DaveC426913]

Yes, water does push back with an equal and opposite force. What you're missing is that you won't be pushing on the water with a force equal to your weight.

Forces always come in equal and opposite pairs.

Correct me if I'm wrong but forces being balanced results in an object that is not accelerating. The fact that the object is accelerating (you are sinking into the water) is the indicator that the forces are not balanced.

Forces will only come into balance again once you reach buoyant equilibrium, or you touch bottom.

 

[Saladsamurai]

Correct me if I'm wrong but forces being balanced results in an object that is not accelerating. The fact that the object is accelerating (you are sinking into the water) is the indicator that the forces are not balanced.

Forces will only come into balance again once you reach buoyant equilibrium, or you touch bottom.

This is what confused me with Russ' post. Clearly there is an acceleration and thus an imbalance of forces. I thought that maybe by going through my thought experiment I could isolate the root of my confusion.

 

[D H]

And then you push down on the water, it does not push back, so you sink!.

Yes, water does push back with an equal and opposite force. What you're missing is that you won't be pushing on the water with a force equal to your weight.

Forces always come in equal and opposite pairs.

Correct me if I'm wrong but forces being balanced results in an object that is not accelerating. The fact that the object is accelerating (you are sinking into the water) is the indicator that the forces are not balanced.

Forces will only come into balance again once you reach buoyant equilibrium, or you touch bottom.

Dave, you and Russ are talking at cross purposes here. You are talking about the forces acting on one object. If the net force on some object is not zero it is necessarily undergoing an acceleration. That's Newton's second law. Russ is talking about Newton's third law. Newton's third law addresses the nature of the forces between two objects. If object A applies a force to object B, then object B is applying an equal but opposite force to object A.

 

[Saladsamurai]

Dave, you and Russ are talking at cross purposes here. You are talking about the forces acting on one object. If the net force on some object is not zero it is necessarily undergoing an acceleration. That's Newton's second law. Russ is talking about Newton's third law. Newton's third law addresses the nature of the forces between two objects. If object A applies a force to object B, then object B is applying an equal but opposite force to object A.

Cross purposes, maybe. But they are certainly related. What I am asserting, is that like the wood, the water exerts a reaction force that is equal and opposite. However, unlike the wood, the water allows an acceleration to occur. What I (and presumably Dave) was hoping to ascertain was exactly wherein lies the difference between the two.

Presumably it is in the tendency of the "stuff" in question (wood or water) to stay together. In water I believe this is the property surface tension and in the wood, tensile strength.

 

[Gregg]

If a force, F, is exerted on a body. It is equally correct to say that the force -F was exerted on the forcer. It is not correct to say that there is a net force of 0 though.

 

[Saladsamurai]

If a force, F, is exerted on a body. It is equally correct to say that the force -F was exerted on the forcer. It is not correct to say that there is a net force of 0 though.

If the reaction - force pair is the only pair, then what say you?

 

[D H]

If the reaction - force pair is the only pair, then what say you?

Simple: Both objects are accelerating.

Newton's third law is widely misunderstood. If object A is exerting a force on object B, object B is exerting an equal but opposite force on object A. Those equal-but-opposite forces necessarily act on the two different bodies. That the Earth is pulling you downwards via gravity and pushing you upwards due to the normal force is not an example of Newton's third law. Those are not equal but opposite forces, for one thing. More importantly, both forces (gravitation and the normal force) are acting on one body (you). Finally, the nature of the forces are very different. The third law reaction to the Earth pulling you downwards gravitationally is that you are pulling the Earth upwards gravitationally. The third law reaction to the Earth pushing you upwards via the normal force is you pushing the Earth downwards via the normal force.

 

[Saladsamurai]

Simple: Both objects are accelerating.

Newton's third law is widely misunderstood. If object A is exerting a force on object B, object B is exerting an equal but opposite force on object A. Those equal-but-opposite forces necessarily act on the two different bodies. That the Earth is pulling you downwards via gravity and pushing you upwards due to the normal force is not an example of Newton's third law. Those are not equal but opposite forces, for one thing. More importantly, both forces (gravitation and the normal force) are acting on one body (you). Finally, the nature of the forces are very different. The third law reaction to the Earth pulling you downwards gravitationally is that you are pulling the Earth upwards gravitationally. The third law reaction to the Earth pushing you upwards via the normal force is you pushing the Earth downwards via the normal force.

Splendid! I hate to say this, but I knew that! I feel like such a chump right now

 

[russ_watters]

Dave, you and Russ are talking at cross purposes here. You are talking about the forces acting on one object. If the net force on some object is not zero it is necessarily undergoing an acceleration. That's Newton's second law. Russ is talking about Newton's third law. Newton's third law addresses the nature of the forces between two objects. If object A applies a force to object B, then object B is applying an equal but opposite force to object A.

Perhaps, but the way Dave's post is worded implies that he thinks that you can apply (for example) 100N to the water while it applies 10N back at you.

 

[russ_watters]

Hmmm... Much confusion has arisen due to this statement Russ You are right; I mean, Newton's third is Newton's third and makes no reservations as to what kind of object is experiencing a given force. But let's talk about this some more. If I were suspended such that my feet are just about to touch the water and then I was released from rest, what exactly happens? (Of course I sink, but in terms of the physics.)

At the instant I touch the water, there are two forces acting on me: My weight and something else pushing back. What exactly is that something else?

If you are suspended a few mm over the water and released, your weight is pulling you down (which is a force pair between you and the Earth), but there is no other external force acting on you: your inertia and acceleration provide the reaction force via f=ma.

When you hit the water, the water provides very little resistance, so if you weigh 600N, perhaps 550 goes toward your acceleration and you apply 50N to the water.

That 50N is applied directly to the water under your feet, accelerating it down, but also sideways around your feet due to friction and viscosity. It's the equivalent of aerodynamic drag, but in the water.

 

[sulemanma2]

Simple: Both objects are accelerating.

Newton's third law is widely misunderstood. If object A is exerting a force on object B, object B is exerting an equal but opposite force on object A. Those equal-but-opposite forces necessarily act on the two different bodies. That the Earth is pulling you downwards via gravity and pushing you upwards due to the normal force is not an example of Newton's third law. Those are not equal but opposite forces, for one thing. More importantly, both forces (gravitation and the normal force) are acting on one body (you). Finally, the nature of the forces are very different. The third law reaction to the Earth pulling you downwards gravitationally is that you are pulling the Earth upwards gravitationally. The third law reaction to the Earth pushing you upwards via the normal force is you pushing the Earth downwards via the normal force.

Does that mean if an object is falling through the air with a weight of 10N, then the object is pulling the Earth up with a force of 10N?

 

[Doc Al]

Does that mean if an object is falling through the air with a weight of 10N, then the object is pulling the Earth up with a force of 10N?

Yes.

 

[my_wan]

Hmmm... Much confusion has arisen due to this statement Russ You are right; I mean, Newton's third is Newton's third and makes no reservations as to what kind of object is experiencing a given force. But let's talk about this some more. If I were suspended such that my feet are just about to touch the water and then I was released from rest, what exactly happens? (Of course I sink, but in terms of the physics.)

At the instant I touch the water, there are two forces acting on me: My weight and something else pushing back. What exactly is that something else? It is my natural instinct to say that it must be a buoyant force, but then I immediately reject that thought since at the instant my feet touch, there is no water displaced. So I feel like it must a normal force like that of the wood in my previous example. But then the water "breaks" as a result of something. That something is probably surface tension and is maybe analogous to the wood "bending moment."

But I would love to get a real description of what is going on here.

~Casey

I see this as coming down to band gaps. Without band gaps nothing we know of this Universe could exist, and the most fundamental band gap appears to be defined by quanta. What defines the board that gives it the properties we call a board. Once band gaps are allowed you can have a balance of forces that are not in equilibrium with the environment. Entropy increase is a loss of energy trapped (balanced) within these band gaps.

In the case of the wood holding you up, you have effectively reached an equilibrium state in which the extra trapped energy is not enough to exceed the energy of the band gaps defined by the wood itself. In the water case you easily exceed the energy required to break the band gaps holding the water molecules together. A needle, which is denser than water, can nonetheless be floated due to the bands gaps on the surface being stronger, because there are fewer molecules at the surface for the band gap energy to be distributed among.

Without band gaps potential energy could not be maintained in a stable or quasi-stable state. My real question involves asking what the most fundamental band gap, if most fundamental exist, consist of. QM, as noted, appears to be directly involved, as even atomic stability dissolves as h -> 0. A related question is, if band gaps exist at a fundamental level, which they apparently must, couldn't the events leading up to the Big Bang be a slow evolution of minuscule bits of energy being randomly trapped, by relatively unlikely but common events?

A lot of very fundamental questions remains open. This I find quiet intriguing:
http://arxiv.org/abs/1005.1683"

 

[Galap]

No, force is not typically multiplied by time. 20N is 20N.

If you have $10 in your pocket for 20 seconds, does that mean you have $200?

I think the OP is confusing force and momentum. The force is constant, but momentum accumulates over time. Just as the acceleration is constant, but the speed accumulates over time (same equations even, without the mass term)

 

[JDługosz]

Thanks for your answers. My other question is if some object that weighs 10 N is resting at a surface, the object put a downward force of 10 N on the surface, and the surface puts an upward force of 10 N to balance it out. So where does the upward force from the surface come from? Does it get deflected upwards by the downward force from object? Or does it come from inside the surface?

The wood acts exactly like a spring. Rather, a "spring" acts like any other solid but by removing most of the material you get movement orders of magnitude higher, making them visible. With a micrometer gauge you can measure that.