Pressure + Hydraulics Flashcards
Equation for pressure
Pressure = Force normal to the surface / Area of that surface
p =F/A
p in N/m^2 or Pascals (Pa)
1 Pa in N/m^2
1Pa = 1N/m^2
What is a fluid
A fluid can be either a liquid or a gas.
What do pressure in fluids cause
The pressure in fluids causes a force normal (acting at right angles) to any surface.
How is atmospheric pressure created
Air molecules colliding with a surface create atmospheric pressure.
What is the atmosphere
The atmosphere is a thin layer (relative to the size of the Earth) of
air round the Earth.
explain why atmospheric pressure varies with height above a surface.
The atmosphere gets less dense as we increase in altitude
The pressure of the atmosphere is caused by air molecules colliding with a surface
As a surface increases height above ground level, the number of air molecules above the surface decreases (and so the weight of air)
This means that atmospheric pressure decreases with an increase in height.
at what point is the atmosphere most dense
The atmosphere is most dense at the surface of the Earth
What does this statement mean:
The atmosphere is most dense at the surface of the Earth
At the Earth’s surface there are the greatest number of air molecules in a given volume
1m^2 into cm^2
1m^2 = 10,000 cm^2
Atmospheric pressure at sea lecel
Atmospheric pressure approx. 100,000 Pa
Explain how a suction cup works
A suction cup sticks to smooth objects because of atmospheric pressure
When it is pushed down, the air under it is force out creating very low pressure
The external pressure produces a force on the suction cup which keeps it attached to the surface
Describe how pressure increases with depth
In a liquid, pressure increases with depth. This is due to the increased weight of liquid acting downwards.
equation for pressure due to a column of liquid
pressure = height of the column × density of the liquid
× gravitational field strength
[ p = h ρ g ]
pressure, p, in pascals, Pa
height of the column, h, in metres, m
density, ρ, in kilograms per metre cubed, kg/m3
Figure 1 shows a spouting can
A student fills the can with water and allows it to drain
Explain why water escaping from the bottom of the spouting can does so at a faster rate than water from the top
Pressure increases with depth (due to increased weight of water above) therefore the water at the bottom has a greater pressure
This means that the water comes out of the bottom hole with a greater force and velocity due to p= F/A
Suggest why the diver experiences different pressures in fresh water and sea water despite swimming at the same depth
The sea water has a greater density than the fresh water
p = h ρ g , therefore the greater the density, the greater the pressure
Why do things float
At the depth, that the base of the object is at, there is pressure in the liquid
The pressure acts on the area, A (of the base of the object), producing a force,
This force is called upthrust, and it is equal to the pressure x area of the object.
weight of object = Upthrust = phg x A
What must happen for an object to float
For an object to float, the upthrust must equal the object’s weight
What happens if the upthrust is less than the objects weight
Then the object sinks
Explain why when an object is lowered into water, the level of water rose
When we lowered the object, the level of the water rose.
This is because the object displaced the water.
What is the size of the upthrust acting on the object the same as
The size of the upthrust acting on the object is the same as the weight of the water displaced by water displaced by the object
What happens if an object can displace its own weight of water
If an object can displace its own weight of water, then the upthrust will equal the object’s weight and the object will float
Describe what happens when an object that is less dense than water is placed into the water
This object only has to displace a small volume of water before the weight of water displaced equals the weight of the object.
Now the upthrust equals the weight of the object and the object floats
In this case the water floats high in the water
Describe what happens when an object that has the same density as the water is placed into the water
The object has to displace its own volume of water in order for the weight of water displaced to equal the weight of the object
The upthrust equals the weight of the object and the object floats
In this case, the surface of the object is at the surface of the water
Describe what happens when an object that is more dense than water is placed into the water
The object cannot displace a volume of water equal to its own weight
Therefore the weight of the object is greater than the force of upthrust (acting upwards) so the object sinks
Describe an experiment a student can do, to show that pressure in a liquid increases with depth
Make several holes down the side of an empty plastic bottle (at different heights), then fill the bottle with water in a sink
What would happen is that the water from the bottom hole squirts out much further (an at a faster rate, with greater force) than the water from the top hole
Why are liquids used in hydraulics
Liquids are used because they are incompressible and therefore the pressure acts at all point in the liquid
therefore able to transmit much greater forces
How can hydraulics be described as
Hydraulics are force multipliers
What is a hydraulic system
A hydraulic system uses liquids (normally oil) to transmit forces from one place to another
Explain how a hydraulic braking system is a force multiplier
When a force is applied to the master piston, it creates a pressure in the liquid
The pressure is the same at every point in the liquid
Since liquids are incompressible, the same pressure acts on the slave piston.
The force applied on the slave piston is equal to pressure x area of the slave piston
Sine the area of the slave piston is greater than the area of the master piston, then the force which acts on the slave piston will be greater than the force which is applied by the master piston
There are two brake blocks, one on each side of the rim
Explain how the system ensures that the same force is applied by each brake block
They have the same area and therefore they produce the same force
Explain how snowshoes help people to walk in deep snow?
p = F/A
The person’s force that is acting down on the snow, is the same when he is wearing snowshoes and when he is wearing regular shoes
The snow shoes have a very large area compared to the area of the soles of shoes alone.
This therefore reduces the pressure the person’s weight is exerting on the snow, therefore he will not sink as deeply into the snow and will be able to walk on it.
________________
Pressure is what makes you sink into the snow. Pressure is force divided by contact area.
The snowshoes have a big contact area with the snow compared to the area of the soles of shoes alone.
Therefore they reduce the pressure the person’s weight exerts on the snow by spreading it out over a bigger area than would be the case if he just wore shoes. He therefore will not sink as deeply into the snow.
Is it easier to cut with a sharp knife or a blunt one?.
It is easier to cut with a sharp knife
p = F/A
The force needed to cut the object is the same whichever knife you use (when using the sharp knife or the blunt knife)
The tip of the sharp knife has a smaller area than the tip of the blunt knife.
As a result, there is more pressure exerted, for the same applied force, making it easier to cut the object
____________________________
It is easier to cut with a sharp knife than with a blunt one because, for the same applied force, the pressure exerted (force/area) is more in the case of a sharp knife than in the case of a blunt one as the sharp knife has lesser area.
Pressure = Force / Area;
pressure is the deciding factror to cut the material using knife.
if the knife is sharp, contact area with the material to be cut is less. If it is blunt, contact area will be more.
hence for the given force, pressure is more with sharp knife and pressure is less with blunt knife.
Hence sharp knife cut the material easier than blunt one.
