Hydrostatics

Question 1

What is a fluid?

Answer 1

Substance with no fixed shape that takes on the shape of its container and yields easily to external force. It is unable to resist shearing stress or deformation.

Question 2

What is a compressible substance?

Answer 2

One that has a reduction in volume in response to pressure.

Question 3

What is the relationship between fluids and gases and density, temperature and pressure?

Answer 3

Temp and pressure alter density.
Liquids show negligible changes to changes in pressure e.g. don’t change density and are therefore incompressible.
Gases do change density and so are compressible.

Question 4

What kind of fluids does pascals principle relate to?

Answer 4

Ones that are compressed to their maximum and can be compressed no further.

Question 5

What is pascals principle?

Answer 5

If a pressure is exerted at any point in a liquid, it is transmitted without loss to every portion of the fluid e.g. applying 10 N of pressure to one side of a liquid, will mean it exerts 10 newtons of pressure on all parts of its container.

Question 6

According to pascals principle, what angle does fluid exert pressure to its container?

Answer 6

Perpendicular to the walls.

Question 7

What angle will fluids escape through a hold in a container if pressure is exerted from the right side?

Answer 7

It doesn’t matter what angle the pressure is exerted on. The jet will always escape perpendicular to its opening.

Question 8

What factors is the pressure that a fluid escapes a hole in its container dependent on?

Answer 8

The force applies and the area of the hole in the container.

Question 9

What would happen to pressure of a liquid leaving a hole in a container if we increased the amount of holes in the container?

Answer 9

The total area would increase but the force would be the same.

Using pressure = force/area, means the pressure in each jet would be proportionally less.

Question 10

What happens to a jet once it escapes a container?

Answer 10

It is at atmospheric pressure so relies on kinetic energy for continued movement. The behaviour outside of the container is not relevant to the calculation that shows the pressure of the fluid leaving the container.

Question 11

Describe the pressure exerted by liquid in a container?

Answer 11

All of the layers of molecules will be exerting equal force due to pascals law.

The pressure exerted on the container is due to collisions, but all of the layers are exerting equal force.

The liquid doesn’t escape as the container exerts equal and opposite pressure.

Question 12

What is Stevins law?

Answer 12

The pressure exerted by liquid at the base of a column is not due to the diameter, but instead due to the height of the column.

F = pghA

P = liquid density
G = acceleration from gravity
H = height
A = cross sectional area

Question 13

How can we rearrange Stevins equation to get pressure?

Answer 13

F = pghA

P= F/A

So

F/A = pghA/A = pressure

Therefore pressure = pgh
(Pressure = density x acceleration x height)

Question 14

What is the relationship between pressure and height of a straight column?

Answer 14

Pressure increases linearly with height.

Question 15

If we have two columns of the same height, but one is 5 times the diameter, what will the pressures at the base be in relation to each other?

Answer 15

The same as pressure at the bottom is only related to height.

Question 16

If we have a upside down t shaped container and combine Stavin’s and pascals law. What is the pressure difference along the bottom of the T shaped area e.g. under the column and to the sides of the column?

Answer 16

The pressure will be the same at all points as pressure is being exerted on one part of the bottom liquid and so this is transmitted equally throughout the liquid.

Question 17

If we take a straight column and angle it, will the pressure at the bottom remain the same?

Answer 17

No as the overall height is reduced due to the angle.

(The height is counted as the overall vertical height not the angled height)

(Diagram page 50 physics module 1 - two triangle diagram)

Question 18

What equation do we get if we introduce atmospheric pressure to Stevins equation?

Answer 18

Stevins formula only accounts for gravity

Pa = Patm + pgh

So for any pressure being applied, it will be:

Pa = Pb + pgh

(Pb being any pressure e.g. a piston)

Question 19

In terms of how much liquids in containers are effected by atmospheric pressure, compare a shallow tub and a submarine at depth?

Answer 19

The shallow tub is very affected as the height of the column is so small.

The pressure on a submarine would be negligible as the water pressure far outweighs the atmospheric pressure.

Question 20

Is the relationship between atmospheric pressure and height, linear or exponential?

Answer 20

Exponential, NOT linear

Question 21

What is the equation for work?

Answer 21

Work = force x difference.

Question 22

How does a hydraulic press work?

Answer 22

We have a long slim tube with a smaller weight on the other end of a u bend of a shorter tube that is larger in diameter and so can hold a large weight at the top e.g. a car. The u shaped tube is filled with liquid.

Due to pascals law, the heavier mass can be moved a shorter distance, by using a far smaller mass being moved over a longer distance.

(Diagram page 51 of physics module 1).

Question 23

In a hydraulic lift, if there is no frictional forces, what will be the relationship between work inout and work output?

Answer 23

Input = output

Question 24

We have a hydraulic press with the small piston being of 1.25cm in diameter and the second being 25cm in diameter. We then put a car with weight of 6000N on the large piston. How much force do we need to put on the small piston to move the can 10cm?

Answer 24

Areas = pi r2
Gives ratio of 1:400

So to move a 6000N force = 6000/400 = 15N of pressure needs to be applied.

However to move this car 10cm, you would need to move the smaller column x 400 = 4000cm or 40m.

This can be achieved by pumping oil into the smaller tube via a compressor.

(Elfh, YouTube and page 51)

Question 25

Describe hydraulic brakes?

Answer 25

Page 51 diagram.

Force is applied to a centre chamber from above. This transmits equal pressure to both brake sides of the chamber via pascals law.

The sides are attached to brake pads, which are them pressed against the metal drum. Friction then causes the car to stop.

Question 26

What is the principle behind using an air bag instead of a seatbelt alone?

How does it deploy?

Answer 26

Air is compressed to its maximum in its casing (pascals law)

Released and expands equally in all directions.

The large surface area spreads the pressure equally over the persons whole body as opposed to the seat belt, which applies over a small area.

Question 27

Are hydrostatics time sensitive?

Answer 27

No

Question 28

How do air/pressure mattresses work?

Answer 28

Air is pumped in and once it is compressed to its max, pascals law spreads the pressure evenly over the whole body. Also has small particles that vibrate to massage the body.

This is how air mattresses prevent the pressure of rocks and twigs being transmitted.

Question 29

How does pascals principle relate to eye injury?

Answer 29

The blow to the front, will be transmitted equally to all areas e.g. the delicate retina at the back.