Upward force on ball

[Gareth56]

Is it because of Newton's 3rd Law that you find it difficult to push a beachball under water?

 

Thanks

[Rakdos]

Buoyancy

[swansont]

There's the buoyancy, which gives you the force being exerted back at you, but there's also the concept of unstable equilibrium — any force or motion not directly down is going to cause the ball to want to "squirt out" from whatever is exerting the force.

[Gareth56]

Thanks. The problem I'm having is visualizing how water can "inflict" an upward force on an object, in other words what (within the water) is providing the upward force. Is the buoyancy force a result of the downward force when the object placed in the water i.e. the buoyancy force is an example of a reaction force as described by Newton's 3rd Law?

[Sisyphus]

No, it's not that. If that were the case, once you got it under water and were no longer moving, it would stay in place. Instead, it still wants to shoot upwards.

 

The way I think about buoyancy is that the force comes from the weight of the water. By pushing it downwards, you are, by necessity, displacing that same volume of water and pushing it upwards. In other words, you're lifiting it. This is easier to see in a smaller container, where you can actually see the water level rise, than in the ocean, but it works the same way. Holding a ball full of air under water is equivalent to holding that same volume of water up in the air. (This isn't the only way to describe buoyancy, but I find it's the most intuitive.)

[Gareth56]

Does the force one experiences when holding a ball underwater come then from the fact that the water is trying to return to its original position, because it (the water) obviously prefers to be back where is was and I suppose gravity is trying to return it to its original position, but is being prevented by you holding the ball?

[Sisyphus]

Yes, basically. Although instead of saying that it wants to return to its original position, I'd just say it is pulled downwards by gravity. The ball is also pulled downwards by gravity, but, less so, because it is less massive. A cubic meter of water has a lot more downwards force than a cubic meter of air, so there is a strong tendency to push the air up and out of the way, which makes a simulated upwards force on the air. Or anything else that is less dense the medium its in. Helium is less dense than air, so a helium balloon has positive buoyancy in air, etc.

[J.C.MacSwell]

Thanks. The problem I'm having is visualizing how water can "inflict" an upward force on an object, in other words what (within the water) is providing the upward force. Is the buoyancy force a result of the downward force when the object placed in the water i.e. the buoyancy force is an example of a reaction force as described by Newton's 3rd Law?

 

It is. Every force has an equal but opposite reactionary force. One could say each is the result of the other (both the force and the reactionary force appear at the same time), but I think it would be more accurate to say they result from buoyancy, pressure differences with depth in the water, or caused by displacement of an equal amount of water as described by Sisyphus.

[swansont]

No, it's not that. If that were the case, once you got it under water and were no longer moving, it would stay in place. Instead, it still wants to shoot upwards.

 

It is a reaction force, but not a reaction force to the person pushing it down. Reaction forces always act on different objects, so the fact that they add to zero does not mean the object would like to remain at rest. (The ball pushing back on the person is the reaction force to the person pushing on the ball.) The ball is displacing water — pushing water out of the way. The water is pushing back, with an equal and opposite force.

 

The action of the ball will be due to the forces acting on it: buoyancy, gravity, and (perhaps) someone pushing on it. If the ball is submerged and then nobody is pushing on it and the buoyancy force is greater than the weight, it will push the ball up. If it is less, then the ball sinks. If it is equal, then you have neutral buoyancy.