Density, Pressure and Flotation

Question 1

what is a fluid

Answer 1

a material that flows

Question 2

what is the equation for density

Answer 2

density = mass / volume

Question 3

how do the volumes of fluids change as they are heated and why

Answer 3

• their volumes increase
• this is because their particles which can slide past each other gain more kinetic energy when heated
• meaning that the amplitude of their vibrations increase so the space between them increases
• leading to an increased volume

Question 4

if the volume of a fluid with a fixed mass increases, what happens to its density

Answer 4

• its density decreases
• using the formula density = mass / volume
• a constant mass with a larger volume (denominator) leads to a lower density

Question 5

what is the main difference between how the particles in liquids and gases can be handled or manipulated

Answer 5

• liquids are more or less incompressible
• due to their particles always touching but being able to move around past each other
• however gases are compressible or can be readily squeezed
• due to their particles not always being in contact with each other

Question 6

what is the symbol for density

Answer 6

rho (greek p)

Question 7

what conditions should densities normally be measured in

Answer 7

• RTP

- 298K and 101kPa

Question 8

if you dive to the bottom of a swimming pool, why do you feel the pressure pushing into your ears

Answer 8

• the pressure is created by the weight of the water above you
• so the deeper you go, the more water there is above you so more of its weight or pressure would be exerted on you

Question 9

what is the pressure on the earths surface

Answer 9

101kPa or 1atm

Question 10

what is the normal equation for pressure

Answer 10

• pressure = force / area

- p = F / A

Question 11

if you had a column of liquid with the density p, that had a height and a circular base, what would the initial equation for the pressure exerted on the base by that column of water be

Answer 11

pressure at the base = weight of column / area of base

Question 12

how would you manipulate that equation to include volume

Answer 12

• weight of column / area
• weight = mass x gravity (mg)
• mg / A
• as density = mass / volume, m = Vp
• replace m in the equation to get Vpg / A

Question 13

what would the volume of the column equal

Answer 13

volume = base x height or Ah

Question 14

what would the final formula for the pressure at the base then be if that equation for volume is implemented

Answer 14

• as V = Ah
• Vpg / A = Ahpg / A
• simplifies to phg
• so pressure due to a column of liquid (p) = p(density)hg

Question 15

is the pressure on earth constant through out and why

Answer 15

• it is not
• the earths atmosphere has a lower density and therefore pressure at higher altitudes
• this is because more of the air particles have low energy levels so they rest near the surface

Question 16

why does upthrust occur

Answer 16

• because the pressure of a fluid being exerted on the bottom of an object is larger than the pressure being exerted at the top
• leading to a resultant force pushing the object upwards and out of the fluid

Question 17

a cylinder is submerged in a body of water, with the distance between the top of it and the water surface being h1 and the distance between the bottom of it and the surface being h2. The force
pushing it downwards from the top is F1 and from the bottom is F2. What would be the equations for the forces on the top and bottom of the cylinder that include the equation for the pressure due to a column of liquid

Answer 17

• p = F / A
• p = pgh
• F1 = p1 A = h1 g p A
• F2 = p2 A = h2 g p A

Question 18

how would you use these two equations to prove that the upthrust is equal to the weight of the fluid displaced

Answer 18

• upthrust (U) = F2 – F1
• F2 – F1 = h2 g p A – h1 g p A
• U = (h2 – h1) g p A
• V = Ah so (h2 – h1)A = V
• U = Vpg
• mass = volume x density (m = Vp)
• U = mg
• weight = mass x gravity

Question 19

what is this proof called and what is the definition of that proof

Answer 19

• it is Archimedes principle
• which states that when a body is totally or partially immersed in a fluid
• it experiences an upthrust equal to the weight of the displaced fluid

Question 20

how do the flotation of ships differ depending on the temperature of the water

Answer 20

ships sink into the water more the hotter the water gets

Question 21

why do ships sink into the water as the water gets warmer

Answer 21

• because the water gets less dense
• this means that more of the ship would need to be immersed into the water to displace the same volume of water it normally would in order for it to equal its weight

Question 22

what property of the water could also affect how much the ship sinks

Answer 22

• whether it is fresh or salt water
• salt water is denser than fresh water so less of the ship would need to be immersed in order for it to displace a volume of water equal to its weight

Question 23

what do ships do or have to decrease their risk of succumbing to a lack of upthrust

Answer 23

• all cargo ships have maximum load levels on their hulls so their weight isnt too high
• the ships have a number of lines painted on the vessels used for international trade
• these represent the safety levels in different waters