12.2 Upthrust + Flotation Flashcards
What is upthrust ?
Upthrust is the upward force exerted on an object that is completely or almost submerged. It happens when fluid pressure is greater at the bottom of an immersed object compared to pressure above it.
Why does this mean the object is pushed upwards?
As the resultant force will be in effect upwards. Due to difference in pressures
How does this apply to the p = hpg equation?
The change in h means the pressure is greater at greater h under the fluid. The difference in pressure causes an upward force (upthurst).
What is Archimedes’ Principle?
When a body is completely or partially immersed in a fluid, it experiences upthrust equal to the weight of the fluid it has displaced.
How do submarines use upthrust to dive underwater and come back out again?
To sink, large tanks are filled with water to increase the weight of submarine, so that it exceeds upthurst.
To rise to the surface, the tanks are filled with compressed air to reduce weight, so that it is less than upthrust.
Prove that upthrust is equal to weight displaced.
F1 = p1A = h1pgA where F1 is the force at the top F2 = p2A = h2pgA where F2 is the force at the bottom U = F2-F2 = (h2-h1)pgA = (h2-h1)Apg = Vpg = mg
Why does this proof work ?
As the upthust is the difference in pressure between top and bottom and so the force.
How will an object float?
If the upthrust (the weight of fluid displaced) is equal to objects weight.
Can upthrust change after an object has been fully submerged?
No, unless water density changes. Otherwise the difference in pressure will be the same as it depends of h2-h1
How will upthrust change as we submerge an object?
It will increase as h2-h1 will get bigger, increasing the difference in pressure
Why will a ship sink more in the less dense water?
As the density is lower means the mass (pV) will be lower and so more of the fluid will need to be displaced (as less of the volume makes up the mass)to balance the weight of the ship.
Why will a ship rise more to the surface of the water is the water is more dense?
The density is higher so that pV (mass) is greater, so that means less of it will need to be displaced (as more of the volume of the water makes up the mass) to balance the weight of the ship
