P11

Question 1

What is pressure ?

Answer 1

force per unit area, measured in pascals (Pa)

Question 2

Give the equation for pressure when the force is acting evenly perpendicular to the surface of area

Answer 2

pressure = force/area

Question 3

How does the pressure in a liquid change with depth ?

Answer 3

increases with increasing depth
- if along the same constant horizontal, will be at same pressure at all points along it

Question 4

How does a liquid’s density affect its internal pressure ?

Answer 4

more density = more pressure

Question 5

Give the equation for pressure at the base of a liquid column

Answer 5

pressure = density x height (m) x gravitational field strength

Question 6

What is atmospheric pressure ?

Answer 6

• when particles of air collide with surfaces or each other and thus generate pressure
  [] higher at sea level than further up in the atmosphere due to differences in density and thus frequency of collision of air particles
• Earth’s average atmospheric pressure at sea level is 100 kPa

Question 7

Describe some examples of how atmospheric pressure is useful for daily life

Answer 7

• suction cup
  [] pushing the cup on a surface squeezes out any air from between the cup and the surface
  [] no/little air = no/little pressure from inside
  [] atmospheric pressure is thus greater on the outside, and pushes the cup against the surface without any/too much opposite force, so it stays on the surface
• straws
  [] sucking air out = atm pressure pushes liquid up straw
• breathing

Question 8

Describe how up-thrust works to make objects float/sink

Answer 8

• columns of liquids have pressure acting upwards
• when an object is placed into the liquid column, it has its own downwards force due to gravity
• up-thrust is the resultant upwards force when the force of the liquid is greater than the gravitational force
• floating:
  [] object must displace a volume of the liquid equal to its own weight to be able to float
  [] if it is less dense than the liquid, it will float
  [] if it is the same density as the liquid, it will float but its surface will be at the liquid’s surface
• sinking:
  [] if the object is not able to displace a volume equal to its own weight, it will sink
  [] the object is more dense than the liquid

Question 9

What is up-thrust ?

Answer 9

upward force on an object due to the fluid, and is caused by the pressure of the fluid