4.5.5.1.2 Pressure in a Fluid 2 (HT only)

Question 1

Equation for pressure due to a column of liquid:

Answer 1

• pressure = height of fluid column x density of liquid x gravitational field strength
• P = hpg
• p (Pa, N/m2)
• h (m)
• p (kg/m3)

Question 2

How does the depth of a liquid affect its pressure?

Answer 2

• the pressure of a liquid increases with depth (height of column) because as the depth increases there is a greater weight (of the column above) acting downwards
• liquids cant support stress within them unlike solids which is why height affects the pressure of liquids

Question 3

How does the density of a liquid affect its pressure?

Answer 3

• the pressure of a liquid increases with density
• as liquids with a greater density have a greater weight (of the column above) acting downwards

Question 4

How does pressure act in a liquid?

Answer 4

• pressure acts in all directions
• pressure increases with depth
• pressure increases with the liquids density due to the particles in a liquid being tightly packed

Question 5

Upthrust:

Answer 5

• a partially (or totally) submerged object experiences a greater pressure on the bottom surface than on the top surface
• this creates a resultant force upwards - this forces is called upthrust

Question 6

Archimedes Principle:

Answer 6

upthrust acting on an object is equal to the weight of water displaced by the object

Question 7

Why do objects float?

Answer 7

for an object to float, the upthrust must equal the object’s weight (downward force due to gravity)

Question 8

Why do objects sink?

Answer 8

• if the upthrust of an object is less than the object’s weight then the object sinks

Question 9

What happens when you lower an object into water and why?

Answer 9

• when you lower an object into water the water rises because the object displaces the water
• the size of the upthrust acting on the object is the same as the weight of the water displaced by the object
• so 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 10

What happens when an object less dense than water is put in water?

Answer 10

• object less dense than water only has to displace a small volume of water before the weight of the water displaced equals the weight of the object
• so then the upthrust equals the weight of the object and the object floats
• the object may float higher in the water

Question 11

What happens when an object with the same density of water is put in water?

Answer 11

• object that has the same density of water
• in this case the object has to displace its own volume of water in order for the weight of the water displaced to equal the weight of the object
• then the upthrust equals the weight of the object and the object floats
• the surface of the object may be at the surface of the water

Question 12

What happens when an object with a higher density than water is put in water?

Answer 12

• an object more dense than water put in water
• object cannot displace a volume of water equal to its own weight
• therefore, the weight of an object is greater than upthrust so the object sinks

Question 13

Example of upthrust:

Answer 13

• the bottom of the object is at a greater depth than the top of the object
• this means that the bottom of the object experiences a larger pressure than the top
• because of this there is a larger force acting on the bottom of the object than the top of the object
• this means that there is a resultant force acting upwards

Question 14

Example of cork, aluminium and lead in water:

Answer 14

• cork, aluminium and lead all have the same volume
• and they’re all in water which has the same volume
• this means that they all have the same upthrust as they all displace the same volume of water (so same weight of water) as they all have the same volume
• however, they all have different masses caused by different densities meaning they have different weights explaining why some float, hover or sink

Question 15

How do you calculate the weight of the aluminium in the water:

Answer 15

w = mg

p = m/V

m = pV

w = pVg

• w = p(al) x V(all) x g

Question 16

How do you calculate the upthrust on the aluminium in the water:

Answer 16

w = p(water) x V(al) x g

Question 17

Why do the volume of the lungs of a free diver decrease when they dive?

Answer 17

• volume of lungs as free diver dives decreases
• as due to them being at a higher depth when they dive the water has a higher pressure (due to there being a higher weight of the column above)
• so the lungs have a smaller volume as pressure is caused by particles colliding with the walls of the container and there are more collisions with the inside of the lungs in a smaller volume which allows the lungs to have a higher pressure allowing the diver’s lungs to have an equal pressure to the water that it is in

Question 18

What does it mean if a fully immersed object can float?

Answer 18

if an object is fully immersed the volume of fluid it displaces has a weight is equal to its own weight

Question 19

When will a fully immersed object float?

Answer 19

• a fully immersed object will float if the upthrust is greater than its weight which is only true if the weight of the fluid displaced is greater than the weight of the object itself
• the only way this can be true is if the density of the fluid is greater than the density of the object