Pressure + Hydraulics Flashcards by A A

Equation for pressure

Pressure = Force normal to the surface / Area of that surface

p =F/A

p in N/m^2 or Pascals (Pa)

How well did you know this?

Not at all

Perfectly

1 Pa in N/m^2

1Pa = 1N/m^2

How well did you know this?

Not at all

Perfectly

What is a fluid

A fluid can be either a liquid or a gas.

How well did you know this?

Not at all

Perfectly

What do pressure in fluids cause

The pressure in fluids causes a force normal (acting at right angles) to any surface.

How well did you know this?

Not at all

Perfectly

How is atmospheric pressure created

Air molecules colliding with a surface create atmospheric pressure.

How well did you know this?

Not at all

Perfectly

What is the atmosphere

The atmosphere is a thin layer (relative to the size of the Earth) of
air round the Earth.

How well did you know this?

Not at all

Perfectly

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.

How well did you know this?

Not at all

Perfectly

at what point is the atmosphere most dense

The atmosphere is most dense at the surface of the Earth

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

1m^2 into cm^2

1m^2 = 10,000 cm^2

How well did you know this?

Not at all

Perfectly

Atmospheric pressure at sea lecel

Atmospheric pressure approx. 100,000 Pa

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

Describe how pressure increases with depth

In a liquid, pressure increases with depth. This is due to the increased weight of liquid acting downwards.

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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

How well did you know this?

Not at all

Perfectly

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.