P11-Force and Pressure Flashcards
When is pressure caused?
Pressure is caused when objects exert forces on each other or when a fluid (can be a liquid or a gas) exerts a force on an object in contact with the fluid.
What is pressure measured in?
Pressure is the force per unit area. The unit of the pressure is the pascal (Pa).
Equation for pressure
pressure (Pa)=force (N)/area (m squared)
Pressure in a liquid at rest
The pressure of a liquid increases with depth. The further the hole is below the level of water in the bottle, the greater the force with which the jet leaves the bottle. Water pressure at each hole is greater the further the hole is below the water level. A liquid flows until the pressure along the same horizontal line is constant. The greater the density of a liquid, the greater the pressure in the liquid.
When water is poured into one side of a U-shaped tube, and then oil is poured into the other side, why is the oil level higher than the water level?
When the liquid settles, the oil level is higher than the water level on the other side because oil is less dense than water so a greater depth of oil is needed to create the same pressure.
Pressure of a liquid column
You can calculate the pressure at a certain depth due to the column of liquid above by doing:
pressure=height of the column of liquid in m (depth) X gravitational field strength (N/kg) X density of the liquid (kg/m cubed)
Atmospheric Pressure
Atmospheric Pressure is created on a surface by air molecules colliding with the surface
Why does the density of the atmosphere decrease with increasing altitude?
The density of the atmosphere decreases with increasing altitude because as the altitude increases, the atmosphere gets less dense so there are fewer air molecules that are able to collide with the surface. (Fewer air molecules above a surface as the height increases-wight of air above it decreases with altitude)
Proof 1: Crushing can experiment
When a can filled with hot water is closed and is cooled down rapidly by pouring cold water on it, it will crush instantly.
This experiment proves that there is a huge atmospheric pressure exerts on everything on the surface of the earth.
Proof 2: Water cover with cardboard does not flow out
The cardboard does not fall and the water remains in the glass even though it’s not supported by anything.
This is because the force caused by the atmospheric pressure acts on the surface of the cardboard is greater than the weight of the water in the glass. This experiment proves that atmospheric pressure is present on the surface of the earth.
Proof 3:Magdeburg Hemisphere
When the air inside the hemisphere is pumped out so that it becomes a vacuum, the hemisphere cannot be separated even by a very great force.
This is because when the air is pumped out, the pressure inside the hemisphere becomes very low.
The atmospheric pressure exerts a strong force on the outer surface of the hemisphere, holding the hemisphere tightly together.
Why does a rubber suction cup pressed onto a wall tile stay on the tile and does not fall off?
Atmospheric Pressure acts on the outside of the cap but not on the inside between the cap and the wall. The action of pressing the cap on the wall squeezes any air trapped between the cap and the wall out. The force due to atmospheric pressure acts on the outside surface of the cap only and keeps the cap on the wall.
Why does a drinking straw only work when the air in the straw is sucked out?
Without any air in the straw, atmospheric pressure acting on the liquid surface outside the straw pushes liquid up the straw.
Upthrust and flotation
Why does the water level in a water container rise when an object is lowered into the water?
The object displaces some of the water. The more the object is lowered into the water, the bigger the volume of water displaced and the bigger the upthrust. When an object is fully immersed, the volume of water displaced is equal to the volume of the object.
Figure 2: Cylinder fully immersed in water
Pressure increases with depth so the pressure of the water at the bottom of the cylinder is greater than the pressure on top of the cylinder so the upward force of water on the bottom of the cylinder is greater than the downward force of the water on top of the cylinder. The upthrust is the resultant of these two forces.
