Pressure + Hydraulics

Question 1

Equation for pressure

Answer 1

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

p =F/A

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

Question 2

1 Pa in N/m^2

Answer 2

1Pa = 1N/m^2

Question 3

What is a fluid

Answer 3

A fluid can be either a liquid or a gas.

Question 4

What do pressure in fluids cause

Answer 4

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

Question 5

How is atmospheric pressure created

Answer 5

Air molecules colliding with a surface create atmospheric pressure.

Question 6

What is the atmosphere

Answer 6

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

Question 7

explain why atmospheric pressure varies with height above a surface.

Answer 7

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.

Question 8

at what point is the atmosphere most dense

Answer 8

The atmosphere is most dense at the surface of the Earth

Question 9

What does this statement mean:

The atmosphere is most dense at the surface of the Earth

Answer 9

At the Earth’s surface there are the greatest number of air molecules in a given volume

Question 10

1m^2 into cm^2

Answer 10

1m^2 = 10,000 cm^2

Question 11

Atmospheric pressure at sea lecel

Answer 11

Atmospheric pressure approx. 100,000 Pa

Question 12

Explain how a suction cup works

Answer 12

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

Question 13

Describe how pressure increases with depth

Answer 13

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

Question 14

equation for pressure due to a column of liquid

Answer 14

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

Question 15

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

Answer 15

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

Question 16

Suggest why the diver experiences different pressures in fresh water and sea water despite swimming at the same depth

Answer 16

The sea water has a greater density than the fresh water

p = h ρ g , therefore the greater the density, the greater the pressure

Question 17

Why do things float

Answer 17

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

Question 18

What must happen for an object to float

Answer 18

For an object to float, the upthrust must equal the object’s weight

Question 19

What happens if the upthrust is less than the objects weight

Answer 19

Then the object sinks

Question 20

Explain why when an object is lowered into water, the level of water rose

Answer 20

When we lowered the object, the level of the water rose.
This is because the object displaced the water.

Question 21

What is the size of the upthrust acting on the object the same as

Answer 21

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

Question 22

What happens if an object can displace its own weight of water

Answer 22

If an object can displace its own weight of water, then the upthrust will equal the object’s weight and the object will float

Question 23

Describe what happens when an object that is less dense than water is placed into the water

Answer 23

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

Question 24

Describe what happens when an object that has the same density as the water is placed into the water

Answer 24

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

Question 25

Describe what happens when an object that is more dense than water is placed into the water

Answer 25

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

Question 26

Describe an experiment a student can do, to show that pressure in a liquid increases with depth

Answer 26

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

Question 27

Why are liquids used in hydraulics

Answer 27

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

Question 28

How can hydraulics be described as

Answer 28

Hydraulics are force multipliers

Question 29

What is a hydraulic system

Answer 29

A hydraulic system uses liquids (normally oil) to transmit forces from one place to another

Question 30

Explain how a hydraulic braking system is a force multiplier

Answer 30

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

Question 31

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

Answer 31

They have the same area and therefore they produce the same force

Question 32

Explain how snowshoes help people to walk in deep snow?

Answer 32

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.

Question 33

Is it easier to cut with a sharp knife or a blunt one?.

Answer 33

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.