Pressure in a column of liquid and upthrust.pptx

AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Pressure in a column of liquid and upthrust

Forces

Lesson 8 of 20

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AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Warm-up

Forces

Lesson 8 of 20

Answer each one, then check.

1

What is density?

2

Write the equation for pressure.

3

What unit is pressure in?

4

What is g?

5

What is a fluid?

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Warm-up - Answers

Forces

Lesson 8 of 20

1

What is density?

Mass per unit volume

2

Write the equation for pressure.

p = F ÷ A

3

What unit is pressure in?

Pascal

4

What is g?

Gravitational field strength, 9.8 N/kg

5

What is a fluid?

A liquid or gas

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Learning Objectives

Forces

Lesson 8 of 20

1

Explain why pressure at a point in a liquid increases with depth and with density.

2

Apply p = hρg and calculate pressure differences at different depths.

3

Explain why a submerged object experiences an upward force called upthrust.

4

Describe the factors that influence floating and sinking.

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AQA GCSE PHYSICS · PAPER 2 · HIGHER TIER ONLY

Forces

Lesson 8 of 20

PRESSURE IN A LIQUID

pressure = height of column × density × gravitational field strength p = hρg

The equation is given on the Physics equation sheet. In any calculation g will be given. Pressure is in pascals (Pa), h in metres, ρ in kg/m³ and g in N/kg.

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Pressure Increases with Depth

Forces

Lesson 8 of 20

More liquid above means more pressure.

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Upthrust

Forces

Lesson 8 of 20

Pressure is greater at the bottom than the top.

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Pressure at a Depth

Forces

Lesson 8 of 20

Calculate the pressure due to the sea water at a depth of 20 m. Density of sea water = 1030 kg/m³; g = 9.8 N/kg.

1

Write the equation

p = hρg

2

Substitute

p = 20 × 1030 × 9.8

3

Answer

p = 201 880 Pa, about 2.0 × 10⁵ Pa

ANSWER

About 2.0 × 10⁵ Pa

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Pressure Difference

Forces

Lesson 8 of 20

Calculate the difference in pressure between depths of 2.0 m and 5.0 m in fresh water (density 1000 kg/m³). g = 9.8 N/kg.

1

Difference in depth

5.0 − 2.0 = 3.0 m

2

Substitute

p = 3.0 × 1000 × 9.8

3

Answer

29 400 Pa

ANSWER

29 400 Pa

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Upthrust on a Cube

Forces

Lesson 8 of 20

A cube of side 0.10 m is submerged with its top face at a depth of 0.20 m. Calculate the upthrust. Density of water = 1000 kg/m³; g = 9.8 N/kg.

1

Pressure difference between bottom and top

0.10 × 1000 × 9.8 = 980 Pa

2

Area of a face

0.10 × 0.10 = 0.010 m²

3

Force difference

F = pA = 980 × 0.010 = 9.8 N

ANSWER

Upthrust = 9.8 N, upwards.

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Floating and Sinking

Forces

Lesson 8 of 20

Compare upthrust with weight.

Float

Upthrust equals weight when an object floats; its average density is less than the liquid's.

Sink

If the weight is greater than the maximum upthrust, the object sinks.

Ships

A steel ship floats because its shape displaces a lot of water, giving it a low average density.

Submarines

Change their average density by taking in or pushing out water.

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Key Terms

Forces

Lesson 8 of 20

Upthrust

The resultant upward force on a submerged object due to pressure.

Column of liquid

The liquid above a point.

Density

Mass per unit volume.

Submerged

Completely under the surface.

Displaced

Pushed out of the way.

Average density

Total mass divided by total volume.

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Forces

Lesson 8 of 20

YOUR TASK

Dive Down

12 minutes

A diver is 15 m deep in water (density 1000 kg/m³, g = 9.8 N/kg). Calculate the pressure due to the water. Explain why the pressure is greater than at 5 m.

1

Use p = hρg.

2

Explain with depth.

WHAT A GOOD ANSWER SHOWS

p = 15 × 1000 × 9.8 = 147 000 Pa. There is more water above the diver, so its weight produces a greater pressure.

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Can I...?

Forces

Lesson 8 of 20

State p = hρg.

Calculate pressure at a depth.

Find a pressure difference.

Explain why pressure increases with depth.

Explain why pressure increases with density.

Explain upthrust.

Explain floating.

Explain sinking.

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Summary & Exam Focus

p = hρg.

Pressure increases with depth and density.

Upthrust = difference in force on bottom and top.

Floats if upthrust equals weight.

EXAM FOCUS

Calculate the pressure due to the sea at a depth of 20 m. Density = 1030 kg/m³; g = 9.8 N/kg. (2 marks)

Substitute into p = hρg.

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