Terminal velocity - Teacher Slides.pptx

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Terminal velocity

Forces

Lesson 13 of 20

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up

Forces

Lesson 13 of 20

Answer each one, then check.

1

What is weight?

2

What is air resistance?

3

What does balanced mean?

4

What is acceleration?

5

What is the gradient of a v-t graph?

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up - Answers

Forces

Lesson 13 of 20

1

What is weight?

The force of gravity on an object

2

What is air resistance?

A force opposing motion through the air

3

What does balanced mean?

Forces equal and opposite

4

What is acceleration?

Rate of change of velocity

5

What is the gradient of a v-t graph?

Acceleration

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Learning Objectives

Forces

Lesson 13 of 20

1

Describe the forces on an object falling through a fluid.

2

Explain why the object reaches a terminal velocity.

3

Draw and interpret velocity–time graphs for terminal velocity.

4

Explain the effect of opening a parachute.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Forces

Lesson 13 of 20

TERMINAL VELOCITY

An object falling through a fluid initially accelerates due to gravity. As its speed increases, the resistive force increases until the resultant force is zero and the object moves at its terminal velocity.

At terminal velocity the weight and the resistive force (air resistance or drag) are balanced, so there is no acceleration.

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Falling to Terminal Velocity

Forces

Lesson 13 of 20

As speed rises, air resistance rises until the forces balance.

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What Happens to a Skydiver

Forces

Lesson 13 of 20

Stage by stage.

1

Start

Only weight acts (almost no air resistance): large downward resultant, acceleration about 9.8 m/s².

2

Speeding up

Air resistance increases with speed, so the resultant force and acceleration decrease.

3

Terminal velocity

Air resistance equals weight: resultant zero, constant velocity.

4

Parachute opens

Large increase in air resistance: upward resultant, skydiver decelerates.

5

New terminal velocity

A lower speed where air resistance again equals weight.

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Resultant Force

Forces

Lesson 13 of 20

A skydiver weighs 700 N. At one instant the air resistance is 400 N. Find the resultant force. What is the air resistance at terminal velocity?

1

Resultant

700 − 400 = 300 N downwards

2

At terminal velocity

The resultant is zero, so air resistance = weight = 700 N

ANSWER

300 N downwards; 700 N at terminal velocity.

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Why a Parachute Helps

Forces

Lesson 13 of 20

Explain why opening a parachute reduces the terminal velocity.

1

Area

A large surface area is opened

2

Air resistance

It is much larger at the same speed

3

Forces balance

At a much lower speed, so the new terminal velocity is lower

ANSWER

The greater air resistance balances the weight at a lower speed.

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Reading the Graph

Forces

Lesson 13 of 20

Common exam features.

Gradient

Decreases as the object falls faster.

Flat section

Terminal velocity: zero acceleration.

Steep fall after parachute

Deceleration.

Shape

Explain each part with the forces.

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Key Terms

Forces

Lesson 13 of 20

Terminal velocity

The constant velocity reached when the resistive force equals the driving force.

Air resistance

A force opposing motion through air.

Drag

A resistive force in a fluid.

Resultant force

The single force equal to all the forces combined.

Surface area

The size of the surface.

Fluid

A liquid or gas.

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Forces

Lesson 13 of 20

YOUR TASK

Parachute Test

10 minutes

A parachutist reaches 50 m/s before opening a parachute. After opening, their speed falls to 5 m/s. Explain both terminal velocities.

1

Compare air resistance and weight.

2

Say when resultant is zero.

WHAT A GOOD ANSWER SHOWS

Before: air resistance on the body equals the weight at 50 m/s. After: the parachute gives much larger air resistance, so the speed falls until it equals the weight again at 5 m/s.

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Can I...?

Forces

Lesson 13 of 20

State the forces on a falling object.

Explain acceleration at the start.

Explain why acceleration falls.

Define terminal velocity.

Interpret a graph.

Explain a parachute.

Find the resultant.

Use force balance.

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Summary & Exam Focus

At the start weight is greater than air resistance.

Air resistance increases with speed.

Terminal velocity: forces balanced, resultant zero.

Parachute: greater air resistance, lower terminal velocity.

EXAM FOCUS

Explain why a skydiver reaches a terminal velocity. (3 marks)

Air resistance rises until it equals the weight.

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Exam Practice: Terminal velocity

Forces

Lesson 13 of 20

Answer all questions. Use the mark allocation as a guide to how much to write. · 16 minutes

Question 1 · 3 marks · Explain

A skydiver jumps from a plane. Explain why the skydiver reaches a terminal velocity.

Question 2 · 5 marks · Use the graph

The graph shows the velocity of a parachutist. (a) State what happens to the velocity between A and B. (b) Explain why the velocity is…

Question 3 · 2 marks · Calculate

A skydiver has a weight of 700 N. The air resistance is 400 N. Calculate the resultant force on the skydiver and state its direction.

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Exam Practice: Terminal velocity (cont.)

Forces

Lesson 13 of 20

Question 4 · 3 marks · Explain

A parachute increases the air resistance on a skydiver. Explain why the skydiver's terminal velocity is lower with the parachute open.

Question 5 · 2 marks · State

State the size of the resultant force on a skydiver falling at terminal velocity and what this means for the acceleration.

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Question 1 · 3 marks · Explain

Forces

Lesson 13 of 20

“

A skydiver jumps from a plane. Explain why the skydiver reaches a terminal velocity.

HOW TO ANSWER IT

Command word: Explain. Worth 3 marks, so plan before writing.

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Question 1 · mark scheme

Forces

Lesson 13 of 20

3 marks available. Award a mark for each point made.

Weight greater than air resistance at first

1 mark

Air resistance increases with speed

1 mark

Forces balance: constant speed

1 mark

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Question 2 · 5 marks · Use the graph

Forces

Lesson 13 of 20

The graph shows the velocity of a parachutist. (a) State what happens to the velocity between A and B. (b) Explain why the velocity is constant between B and C. (c) At C the parachute opens. Explain what happens between C and D. (5 marks)

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Question 2 · mark scheme

Forces

Lesson 13 of 20

5 marks available. Award a mark for each point made.

A to B: accelerating, getting less

1 mark

B to C: air resistance equals weight

1 mark

Resultant force zero

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 2 · mark scheme (cont.)

Forces

Lesson 13 of 20

C to D: air resistance greater than weight

1 mark

Decelerates to a lower terminal velocity

1 mark

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Question 3 · 2 marks · Calculate

Forces

Lesson 13 of 20

“

A skydiver has a weight of 700 N. The air resistance is 400 N. Calculate the resultant force on the skydiver and state its direction.

HOW TO ANSWER IT

Command word: Calculate. Worth 2 marks, so plan before writing.

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Question 3 · mark scheme

Forces

Lesson 13 of 20

2 marks available. Award a mark for each point made.

300 N

1 mark

Downwards

1 mark

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Question 4 · 3 marks · Explain

Forces

Lesson 13 of 20

“

A parachute increases the air resistance on a skydiver. Explain why the skydiver's terminal velocity is lower with the parachute open.

HOW TO ANSWER IT

Command word: Explain. Worth 3 marks, so plan before writing.

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Question 4 · mark scheme

Forces

Lesson 13 of 20

3 marks available. Award a mark for each point made.

Larger area means greater air resistance

1 mark

Forces balance at a lower speed

1 mark

So lower terminal velocity

1 mark

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Question 5 · 2 marks · State

Forces

Lesson 13 of 20

“

State the size of the resultant force on a skydiver falling at terminal velocity and what this means for the acceleration.

HOW TO ANSWER IT

Command word: State. Worth 2 marks, so plan before writing.

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Question 5 · mark scheme

Forces

Lesson 13 of 20

2 marks available. Award a mark for each point made.

Zero resultant force

1 mark

Zero acceleration

1 mark

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