P11 - Pressure in fluids Flashcards
What is a fluid?
anything that continuously flows and takes the shape of its container (liquid/gas)
Give the equation for the pressure caused by a column of liquid:
pressure = density x gravitational field strength x depth
Why does pressure increase with depth?
p = ρ g h
-if h increases, pressure also increases proportionally
-this is because of the increased mass of water above you, increasing the weight force acting on you, resulting in a pressure
Describe the forces acting on an object submerged in a fluid:
the fluid pressure causes a normal force acting on all surfaces of the object
A normal force is a force perpendicular to a surface
What is upthrust?
-a resultant force from the difference in pressure between the top and the bottom of an object submerged in a fluid
-this is why objects appear to weigh less in a liquid
upthrust is always constant because the difference in pressure between the top and bottom is always equal when fully immersed, as pressure increases linearly with depth
What is the size of the upthrust acting on an object equal to, and why does this allow objects to float?
-the weight of the water it displaces
-objects can float because they rise in the fluid until they eventually have some of its volume appearing outside the water, that then displaces a weight of water equal to their weight and so the resultant force is 0
Why do objects more dense than water sink?
-the object’s weight is larger than the upthrust acting on it (ie the weight of water it displaces), meaning there is a resultant force acting downwards
-the object starts to sink
How and why does atmospheric pressure change with altitude?
2 reasons
-atmospheric pressure is caused by the force of air particles colliding with a surface
-density of air particles decrease with altitude, less collisions, so less force applied per unit area
-weight of air above also decreases
-therefore atmospheric pressure decreases with height
How does a straw work?
-air sucked out of straw (into lungs)
-lower pressure in the straw
-pressure difference between atmospheric pressure and inside pressure
-atmospheric pressure causes a force on the liquid surface, equal forces everywhere in liquid, so force makes liquid go up the straw
Why is it hard to keep an inflated plastic ball underwater?
-large upthrust because of its low density
-displaces a large weight of water compared to its own weight
Why do aircraft windows have this shape?
-force from air pressure inside cabin is bigger than the force outside cabin
-holds the window in place
Describe the motion of an object falling through a fluid:
-initially accelerates due to gravity
-the opposing drag force of fluid increases with speed, and eventually equals the weight force
-resultant force equals 0
-object continues to move at a constant (terminal) velocity, by Newton’s first law of motion
