0
$\begingroup$

If I were to have a block falling in the water, the upthrust/force upwards on the block is basically due to the liquid pressure experienced by the bottom surface of the block.

My question is, why is there no upward force due to the normal force by the water on the block due to the weight of the block?

When the block is in the water, the block has weight, which is acting against the water. So why is there no normal force by the water on the block upwards due to the weight?

$\endgroup$
6
  • 1
    $\begingroup$ Related : physics.stackexchange.com/questions/148112/…. $\endgroup$
    – Cross
    Commented May 23, 2021 at 17:09
  • $\begingroup$ yea I read that already but the answer wasn't really that clear. So why does the water not exert an upward force due to the block pushing the water down? $\endgroup$ Commented May 23, 2021 at 17:14
  • $\begingroup$ The water does exert an upward force, called buoyancy. Buoyancy is the liquid equivalent of normal force in most contexts. $\endgroup$
    – Cross
    Commented May 23, 2021 at 17:16
  • $\begingroup$ I know that buoyancy exist and it is due to the force from the liquid pressure against the bottom surface. But why don't this buoyancy force also include the force that the water exerts on the block when the block pushes down the water due to its weight? $\endgroup$ Commented May 23, 2021 at 17:24
  • 1
    $\begingroup$ @zenaiderrrr The buoyancy force IS the normal force. You're totally confusing the same thing for two different things. What displaces the water in the first place is the weight of the block and equal and opposite reactions dictates the water exert a force right back on the block. At least when the block is partially submerged. When the block is fully submerged then it is the drag force that becomes the normal force preventing the block from free-falling. $\endgroup$
    – DKNguyen
    Commented May 23, 2021 at 17:57

4 Answers 4

2
$\begingroup$

When the block is in the water, the block has weight, which is acting against the water. So why is there no normal force by the water on the block upwards due to the weight?

There is a bit of a mistake here that may be the overall source of your confusion. The weight of the block does not act against the water. The weight is a gravitational force from the whole earth, acting on the block. The third law force opposite the weight is the gravitational force of the block acting on the whole earth.

There is a normal force, called buoyancy. This is the contact force from the water acting on the block. The third law pair of the buoyant force is an unnamed contact force from the block acting on the water.

The weight (gravitational) and the normal/buoyant (contact) forces are independent in general. It is only in special cases, such as a floating object, where they are equal. But even in those cases they are not part of a 3rd law pair

$\endgroup$
1
  • $\begingroup$ yea I get that. But the weight causes the block to exert a force downwards against the water. Not the weight itself, but the force due to the weight. $\endgroup$ Commented May 25, 2021 at 1:05
0
$\begingroup$

Buoyance deserves more the name Normal Force than the usual one, found in body diagram, because it relies only on Normal stresses.

Suppose an object laying on the floor of an apartment building (like the couch where I am sitting now). It doesn't sink because the thickness of the slab (and the steel rebars inside) have enough shear strenght. If for example, instead of the slab, there was only the tiles of the floor, the sofa would sink by exceding the floor shear strenght.

Liquids don't develop shear stresses, except for the surface tension, that allows tiny objects like some insects to stay on the surface, as they were on a floor.

Because Normal Force is perpendicular to the surface it is called like that. But for solid-solid contact it is really a consequence of shear stresses.

$\endgroup$
0
$\begingroup$

There is an upward force. It is called Buoyancy. That acts as the normal force in this example. It is different from the solid block on a table normal force. Over there, the normal force prevents the block from drilling through the table. In this example, Buoyancy acts as the normal force. You can think of Buoyancy as the normal force offered by a fluid. The expression is not the same for a normal force offered by a solid i.e. -mg. It has a similar expression -pVg, where p is density of fluid, and V is volume displaced. As you can see, pV acts as a substitute for mass here.

$\endgroup$
3
  • $\begingroup$ I know that buoyancy is due to the liquid pressure at the bottom surface of the liquid. I know how to calculate everything. Isn't the block pushing the water down? So shouldn't the water also push the block up as normal reaction force? Hence silent the buoyancy force include both the force due to the liquid pressure at the bottom surface and the normal force due to the block pushing the water down? $\endgroup$ Commented May 23, 2021 at 17:19
  • 1
    $\begingroup$ No, the Buoyancy itself is the normal force. It is just not strong enough to resist the sinking. But remember, any reaction force that acts on a body when it pushes against anything, is called the normal force. If instead of the water, I had been physically pushing the block upwards, my push would have been the normal force. If someone was pulling the block from outside, that would be the normal force. Any force that acts perpendicular to the surface is normal force. The block is pushing water down. The water pushes back, that is Buoyancy. That is normal force. $\endgroup$ Commented May 23, 2021 at 17:49
  • 1
    $\begingroup$ Normal force isn't a special kind of force that is created when things are pushing other things. It is just the reaction to your action. This reaction can have several forms. It could be 'the normal force' you are talking about. It could also be the Buoyancy. Buoyancy is normal force, all of it. How it is created is irrelevant. The liquid pressure at the bottom surface is created because the block is pushing the liquid from the top. This pressure pushes back. That is Buoyancy. That is normal force, the liquid pushing back because of pressure. $\endgroup$ Commented May 23, 2021 at 17:52
0
$\begingroup$

A fluid doesn't exert a normal force in exactly the same way as a solid. If you set a block on a table, for example, the molecules of the solid table aren't free to flow out from under it. It therefore exerts a normal force on the block not allowing it to sink into the table. When the block is sinking in a fluid the bottom of the block could be said to have a normal force against the viscosity of the fluid. It sinks because the fluid molecules are free to move out from under the block from the extra pressure its weight exerts on them.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.