T5 pressure and pressure differences in fluids

Question 1

what is meant by the term “fluid”

Answer 1

a liquid or a gas

Question 2

in any fluid, at what angle do the forces due to pressure act on a given surface

Answer 2

at right angles (normal to) the surface

Question 3

equation used to calculate pressure

Answer 3

pressure = force / area

pressure (pascals), force (newtons), area (metre^2)

Question 4

what happens to the density of the atmosphere with increasing altitude

Answer 4

becomes less dense as altitude increases

Question 5

explain why atmospheric pressure decreases with an increase in height

Answer 5

• pressure is created by collisions of air molecules
• quantity of molecules (and so weight) decreases as the height increases
• this means atmospheric pressure decreases with an increase of height

Question 6

what is the Earth’s atmosphere

Answer 6

a thin (relative to the magnitude of the Earth) layer of gas surrounding the Earth

Question 7

how does pressure in fluids increase with depth

Answer 7

as the depth increases, the mass of liquid above that depth also increases

this means that the force due to the mass increases

since the force has increased whilst the area has remained constant, the pressure will increase

Question 8

why does pressure in fluids increase with density

Answer 8

as the density of a fluid increases, the number of particles in a given volume increases

consequently the weight of the fluid is greater

this means that the force from the fluid above a certain point is larger

since the force has increased, the pressure also increases

Question 9

what equation shows the magnitude of pressure in liquids at different depths

Answer 9

pressure due to a column of liquid (Pa) = column height (m) x density (kg/m^3) x gravitational field strength (N/kg)

P = hρg

Question 10

why are objects in a fluid subject to an upthrust

Answer 10

when an object is submerged in a fluid, it has a higher pressure below it than it does above, as there is more weight above it at the bottom than there is at the top

this leads to an upwards force called upthrust

you float or sink depending on whether the upthrust is more or less than your weight

Question 11

what is upthrust always equal to

Answer 11

weight of the fluid that the object displaces

Question 12

what factors influence whether an object will sink or float

Answer 12

• upthrust
• weight
• density of fluid

Question 13

explain why an object with a density greater than that of water can never float

Answer 13

upthrust is equal to the weight of the displaced fluid

if the density of the object is high, there wouldn’t be enough volume displaced to produce an upthrust larger than the object’s weight

this means that it will sink