What Causes the Upward Force Counteracting Gravity?

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In summary, the pressure in a fluid is the same in all directions because the weight of the fluid pushes the particles out in all directions.
  • #1
rafterman
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I know that gravity causes the downward force, but to keep us in ballance there is an upward force.Can someone tell me what causes this upward force.
 
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  • #2
normal reaction force
 
  • #3
Your weight is trying to push the atoms in the floor closer together, the electrical charges in the atoms repel each other and push back.

Strangley most phyiscs classes don't seem to mention this - the reaction force is treated as just a book keeping exercise to make all the forces balance.
 
  • #4
mgb_phys said:
Your weight is trying to push the atoms in the floor closer together, the electrical charges in the atoms repel each other and push back.

Strangley most phyiscs classes don't seem to mention this - the reaction force is treated as just a book keeping exercise to make all the forces balance.

I get this, but I don't understand why the pressure must be the same in every direction. If you take a small cube in a fluid at rest, the net force must be zero. So pressure must be equal on opposite sites. But why does this mean that the horizontal pressure must equal to vertical pressure?
 
  • #5
That's a slightly different question - but yes.
It's difficult to immediately see why the force on the walls of the bottom of a tank of water is the same as the force on the base.
Imagine something heavy that can easily flow, like oiled ball bearings.
Now picture a large box full of them and you cut a hole in the side - you would need a lot of force to hold back the balls. The force of them coming out of a hole in the side, near the bottom, is pretty much the same as the force of them coming out of the bottom - there is still the same weight of all the balls on top of them.

The same thing applies to water or air, or any other fluid - the pressure acts in all directions because the particles are being pushed out in all directions by the weight of the fluid above them
 
  • #6
I see, thank you!
 
  • #7
Speaking of pressure, is there a way to show a fluid exerts equal pressure in all directions mathematically? Like for ideal gases we can use the assumption that collisions with the walls of the container are perfectly elastic to show that pV =1/3Nm<v^2> where <v^2> is the mean square speed of the particles.

Is there some other mathematics for a fluid as well?
 

1. What is upward force?

Upward force is a type of force that acts in the opposite direction of gravity. It is a force that pushes objects upwards and is present in many natural phenomena, such as when objects float or when we jump.

2. What causes upward force?

Upward force is caused by a variety of factors, such as air resistance, buoyancy, and electromagnetic forces. For example, when an object falls through the air, it experiences a force pushing upwards due to the resistance of the air. In the case of buoyancy, objects immersed in a fluid experience an upward force due to the difference in pressure between the top and bottom of the object. Electromagnetic forces can also cause upward force, such as when two magnets repel each other.

3. How is upward force related to Newton's third law of motion?

Newton's third law of motion states that for every action, there is an equal and opposite reaction. This means that when an object exerts a downward force on a surface, the surface exerts an equal and opposite upward force on the object. This upward force is what we refer to as the upward force.

4. How does the surface area of an object affect upward force?

The surface area of an object can affect upward force in a few ways. Generally, objects with a larger surface area experience greater air resistance, which can result in a larger upward force. Additionally, when an object is submerged in a fluid, the surface area can affect the buoyant force acting on the object, which in turn affects the upward force.

5. Can upward force be greater than downward force?

Yes, upward force can be greater than downward force. This can occur in situations where there are multiple upward forces acting on an object, such as when an object is being lifted by multiple people or when an object is submerged in a fluid with a high buoyancy force. In these cases, the combined upward forces can be greater than the downward force acting on the object, resulting in a net upward force.

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