5.5 Forces (pressure and pressure differences in fluids)

Question 1

What is meant by the term ‘fluid’?

Answer 1

A liquid or gas

Question 2

In any liquid, 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

State the equation used to calculate pressure?

Answer 3

Pressure = Force / Area

Question 4

What are the units for pressure?

Answer 4

Pascals (Pa) or N/m2

Question 5

What are the units for area?

Answer 5

m2

Question 6

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

Answer 6

The atmosphere becomes less dense as altitude increases

Question 7

Explain why atmospheric pressure decreases with an increase in height?

Answer 7

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

Question 8

What is the Earth’s atmosphere?

Answer 8

A thin layer of gas surrounding the earth

Question 9

How does pressure in fluids increase with depth?

Answer 9

• 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 10

Why does pressure in fluids increase with density?

Answer 10

• As the density of fluid increases, the number of particles in a given volume increases
• So the weight of the fluid is greater
• This means the force from the liquid above a certain point is larger
• Since the force has increased, pressure also increases

Question 11

What equation shows the magnitude of pressure in liquids at different depths?

Answer 11

pressure due to a column of liquid = column height x density x gravitational field strength

Question 12

What is upthrust?

Answer 12

the upward force that a liquid or gas exerts on a body floating in it

Question 13

Why are objects in fluid subject to upthrust?

Answer 13

• When an object is submerged in a fluid, it has a higher pressure below it than it has 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

Question 14

How does floating and sinking work?

Answer 14

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

Question 15

What is upthrust always equal to?

Answer 15

The weight of the fluid that the object displaces

Question 16

What factors influence whether an object will sink or float?

Answer 16

• upthrust
• weight
• density of fluid

Question 17

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

Answer 17

• Upthrust is equal to the weight of the displaced fluid
• If the density of the object is high, there would not be enough volume displaced to produce an upthrust larger than the object’s weight
• this means that it will sink