Pressure

Question 1

Pressure

Answer 1

-force per unit area
-determines the effect of a force on a surface

Question 2

Shoe example:
Flat shoes spread the force over a large area so the pressure…

Answer 2

….is reduced.
-snow shoes have larger area than feet and spread force over more area, reducing pressure on snow which stops ppl sinking

Question 3

Why will it hurt more if a person steps on a foot in high heels than in flats?

Answer 3

-high heeled shoes transfer the force through a much smaller area
-causing greater pressure

Question 4

Fluid

Answer 4

-substance that can flow, such as liquid or gas

Question 5

What angle do the forces due to pressure in fluids act to a surface?

Answer 5

-force normal (90 degrees)

Question 6

What is a force normal?

Answer 6

-force that acts 90 degrees to a surface

Question 7

Pressure equation

Answer 7

p (Pa) = F(N) / A (m squared)

Question 8

Pressure in fluids example question:
A fluid exerts a force of 150 N over an area of 1.2 m2 Calculate the pressure on the surface

Answer 8

150N/1.2m2
=125Pa

Question 9

What happens to pressure in a liquid at a deeper depth?

Answer 9

-pressure increases as the depth increases
-this is because the deeper you go, the greater the height of water there is above the area
-the weight (force) increases, which increases pressure

Question 10

Why does pressure act in all directions?

Answer 10

-particles in fluids move randomly and collide with surfaces from every angle
-collisions generate a force that spreads equally in all directions, rather than in just one

Question 11

Explain why dam walls are usually wedge-shaped

Answer 11

-pressure is greater at the bottom, so the walls need to be stronger there
> deeper in a liquid something is, more column of liquid there is above it
> so there is more weight about that depth, so more force, increasing pressure

Question 12

Pressure in a fluid calculation:

Answer 12

pressure (Pa) = height of column (m) × density of the fluid (kg/m3) × gravitational field strength (N/kg)

Question 13

Pressure in a liquid example question:
A stone is dropped into a lake. Calculate the increase in pressure on the stone caused by the water when it sinks from 1 m deep to 6 m deep. (The density of water is 1,000 kg/m3 and gravitational field strength is 9.8 N/kg).

Answer 13

1000kg/m3 x (6-1)m x 9.8 N/kg =
49000Pa

Question 14

What is the atmosphere?

Answer 14

-thin layers of gases that surround the Earth

Question 15

Key features of the atmosphere

Answer 15

-thin compared to the size of the Earth
-becomes less dense as the altitude increases

Question 16

Explain why atmospheric pressure decreases with an increase in height

Answer 16

-air pressure is created by collisions of air molecules, exerting a force on a surface

-as height increases, quantity of air molecules (air density) decreases and so weight of the air decreases

-will be less collisions between less molecules

> atmospheric pressure decreases with a height increase

Question 17

Why must aircraft that fly at high altitudes be pressurised?

Answer 17

-if air pressure is too low, humans cannot take in oxygen quick enough to meet their bodies’ needs

Question 18

Upthrust

Answer 18

-upwards force exerted by a liquid or gas on an object partly/completely submerged in it

Question 19

If the upthrust is less than the weight of the object…

Answer 19

….the object will sink.

Question 20

How does pressure in fluids increase with greater depth?

Answer 20

-the greater the depth, the greater the height of the column of liquid above that depth
> greater weight of liquid, and so greater force acting on the object, pressure will increase

Question 21

Why is the bottom surface of an object submerged in a fluid experience greater pressure than the top surface?

Answer 21

-upper surface isnt as deep as the lower surface

> smaller height of column of liquid above it, less weight of water pushing down on it

-bottom surface experiences greater pressure as there is a greater weight of water

> greater force pushing down on it

Question 22

Displaced fluid

Answer 22

-the fluid that rises when an object is submerged in a fluid

Question 23

Upthrust is equal to the weight of…

Answer 23

…displaced fluid.

Question 24

If an object doesn’t need to sink down very far into the fluid until it displaces enough of it to equal its own weight….

Answer 24

…it is a light object

Question 25

If an object does need to sink down very far into the fluid until it displaces enough of it to equal its own weight...

Answer 25

....it is heavier and denser.

Question 26

If an object has the same density as the fluid, to make the upthrust equal to the object's weight, it will need to...

Answer 26

...displace a volume of fluid equal to its own volume -results in the objects surface being at the same height as the fluids surface.

Question 27

If an object has a higher density than the fluid, it is unable to displace enough fluid to make the upthrust equal to the objects weight...

Answer 27

...so it sinks.

Question 28

What is upthrust directly proportionate to?

Answer 28

-upthrust depends on the density of the displaced fluid

Question 29

What causes upthrust?

Answer 29

-differences in pressure on the top and bottom surface surface of the submerged object -which causes an upward resultant force -you float or sink, depending on whether the upthrust is more/less than your weight

Question 30

What factors influence whether an object will sink or float?

Answer 30

-upthrust -weight -density of fluid

Question 31

Why will an object with a density greater than water not float?

Answer 31

-upthrust is equal to the weight of the displaced fluid -if the density of the object is high, there would not be enough displaced to produce an upthrust larger than the object's weight

Question 32

Why does a packet of crisps swell up on a plane ride?

Answer 32

-at ground level, pressure inside bag is the same as pressure outside -but when planes fly, air pressure in the cabin is slightly lower than air pressure on the ground -so air pressure inside the bag is greater than the air pressure in the cabin, making it swell up

Question 33

A stone is dropped into a lake. Calculate the increase in pressure on the stone caused by the water when it sinks from 1 m deep to 6 m deep. (The density of water is 1,000 kg/m3 and gravitational field strength is 9.8 N/kg)

Answer 33

1 x 1000 x 9.8 = 9800 6 x 1000 x 9.8 = 58800 58800-9800=49000

Question 34

Why are nails pointed?

Answer 34

-this concentrates the force through a smaller area, creating a greater pressure -this allows the nail to be hammered more easily into something

Question 35

When a plunger is pushed down into a syringe, the pressure is increased. Why?

Answer 35

-as plunger descends, volume decreases and gas particles are closer together >will collide more frequently with eachother and the walls of their container, exerting more force -so there is a larger force exerted on a smaller area, increasing the pressure

Question 36

As depth increases, the pressure in a fluid...

Answer 36

...increases.

Question 37

In which direction does pressure act on something?

Answer 37

-in all directions

Question 38

If the upthrust is less than the weight of the object, the object will.... If the upthrust is larger than the weight of the object, the object will...

Answer 38

..sink. ...float

Question 39

How does the structure of a tractor create lower pressure?

Answer 39

-tyres have large surface area -force is spread out over a larger area, so pressure is decreased (as p =f/a)

Question 40

What is the size of the upthrust equal to? (Archimedes principle)

Answer 40

-to the weight of the liquid displaced by the object submerged in a fluid

Question 41

What is upthrust due to?

Answer 41

-due to the difference in pressure between the bottom

Question 42

What happens to the density of the atmosphere with increasing altitude?

Answer 42

decreases

Question 43

Why are objects in a fluid subject to upthrust?

Answer 43

-an object submerged in a fluid has a higher pressure below than above c. more weight above it at the bottom than there is at the top -differences in pressure leads to upwards resultant force of upthrust

Question 44

What factors influence whether an object will sink or float?

Answer 44

-upthrust -weight -density of fluid

Question 45

Explain why an object with a density greater than water cant float

Answer 45

-upthrust is equal to the weight of the displaced fluid -if object density is high, there wouldnt be enough liquid displaced to produce an upthrust larger than the object's weight > it will sink

Question 46

Why do high-heeled shoes cause a much greater pressure than flat shoes?

Answer 46

-they transfer force through a much smaller area, which increases the pressure -flat shoes transfer force through a wider area

Question 47

A fluid exerts a force of 50 N over an area of 2 m2. Calculate the pressure on the surface.

Answer 47

25 Pa

Question 48

A fluid exerts a force of 150 N over an area of 1.2 m2. Calculate the pressure on the surface.

Answer 48

125 Pa

Question 49

The density of water is 1,000 kg/3. Calculate the pressure exerted by the water on the bottom of a 2.0 m deep swimming pool.

Answer 49

2 x 1000 x 9.8 = 19, 600

Question 50

A stone is dropped into a lake. Calculate the increase in pressure on the stone caused by the water when it sinks from 1 m deep to 6 m deep.

Answer 50

5 x 1000x 9.8 = 49 000 Pa

Question 51

A force of 40 N is applied to a spanner to turn a nut. The perpendicular distance is 30 cm. Calculate the moment of the force.

Answer 51

40 x 0.3 = 12Nm