Resultant forces - Teacher Slides.pptx

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Resultant forces

Forces

Lesson 3 of 20

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up

Forces

Lesson 3 of 20

Answer each one, then check.

1

What is a force measured in?

2

What does balanced mean?

3

Is force a vector?

4

What does a resultant force of zero mean for motion?

5

Add 40 N and 15 N.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Warm-up - Answers

Forces

Lesson 3 of 20

1

What is a force measured in?

Newtons

2

What does balanced mean?

Equal and opposite

3

Is force a vector?

Yes

4

What does a resultant force of zero mean for motion?

No change in velocity

5

Add 40 N and 15 N.

55 N

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Learning Objectives

Forces

Lesson 3 of 20

1

Define a resultant force.

2

Calculate the resultant of two forces acting in a straight line.

3

Describe the forces on an isolated object using a free body diagram (Higher tier).

4

Use scale drawings to find the resultant of two forces, and resolve a force into components (Higher tier).

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Forces

Lesson 3 of 20

RESULTANT FORCE

A number of forces acting on an object may be replaced by a single force that has the same effect: the resultant force.

Forces in the same direction are added. Forces in opposite directions are subtracted. If the resultant is zero the forces are balanced.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Finding the Resultant

Forces

Lesson 3 of 20

Add same-direction forces; subtract opposite ones.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Forces in a Straight Line

Forces

Lesson 3 of 20

A car's engine produces a driving force of 1800 N. Friction and air resistance give 1200 N in the opposite direction. Find the resultant force.

1

Opposite directions

Subtract

2

Calculate

1800 − 1200 = 600 N

ANSWER

600 N in the direction of the driving force.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Free Body Diagram (Higher)

Forces

Lesson 3 of 20

Draw a free body diagram for a book at rest on a table.

1

Isolate the book

Draw the book as a box on its own

2

Weight

An arrow down from the centre of the box

3

Normal contact force

An arrow up of equal length

4

Resultant

Zero, so the forces are balanced

ANSWER

Equal arrows up and down: the resultant force is zero, so the book stays at rest.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Scale Drawing (Higher)

Forces

Lesson 3 of 20

Two forces of 3.0 N (east) and 4.0 N (north) act on an object. Find the resultant using a scale drawing.

1

Scale

1 cm = 1 N

2

Draw

3 cm east, then 4 cm north from the end of the first arrow

3

Join start to finish

The arrow is 5.0 cm

4

Direction

Measure the angle: about 53° north of east

ANSWER

Resultant 5.0 N at about 53° north of east.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Resolving a Force (Higher)

Forces

Lesson 3 of 20

A force of 10 N acts at 30° above the horizontal. Find the horizontal and vertical components using a scale drawing.

1

Scale

1 cm = 1 N

2

Draw

A 10 cm line at 30° to a horizontal line

3

Drop a perpendicular

Make a right-angled triangle

4

Measure

Horizontal about 8.7 cm and vertical 5.0 cm

ANSWER

Components: about 8.7 N horizontally and 5.0 N vertically.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Key Points

Forces

Lesson 3 of 20

Remember in an exam.

Direction

Give the direction of the resultant.

Balanced

Zero resultant means constant velocity (Newton's first law).

Free body diagram

Show only the forces on the object, as arrows from its centre.

Units

Forces in newtons.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Key Terms

Forces

Lesson 3 of 20

Resultant force

A single force with the same effect as all the forces on an object.

Balanced forces

Forces that are equal and opposite.

Free body diagram

A diagram showing all forces acting on an isolated object.

Component

One of two perpendicular forces that together have the same effect as one force.

Resolve

Split a force into two components at right angles.

Equilibrium

When the forces on an object are balanced.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Forces

Lesson 3 of 20

YOUR TASK

Find the Resultant

10 minutes

Find the resultant force: (a) 50 N right and 20 N left (b) 30 N up and 30 N down (c) 12 N right and 8 N right.

1

Same direction: add.

2

Opposite: subtract.

WHAT A GOOD ANSWER SHOWS

(a) 30 N right (b) 0 N (c) 20 N right.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Can I...?

Forces

Lesson 3 of 20

Define a resultant.

Add forces in the same direction.

Subtract opposite forces.

Recognise balanced forces.

Draw a free body diagram.

Use a scale drawing.

Resolve a force.

Give direction.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Summary & Exam Focus

Resultant: same direction add, opposite subtract.

Zero resultant: balanced.

Higher: free body diagrams and scale drawings.

Resolve into perpendicular components.

EXAM FOCUS

A driving force of 1800 N and a resistive force of 1200 N act on a car. Calculate the resultant force. (2 marks)

Subtract the smaller from the larger and give the direction.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Exam Practice: Resultant forces

Forces

Lesson 3 of 20

Answer all questions. Use the mark allocation as a guide to how much to write. · 14 minutes

Question 1 · 2 marks · Calculate

A car engine produces a driving force of 1800 N. The total resistive force is 1200 N. Calculate the resultant force on the car and state…

Question 2 · 2 marks · Use the diagram

The diagram shows the forces acting on a box. Calculate the resultant force.

Question 3 · 2 marks · Explain

A skydiver has a weight of 800 N and air resistance of 500 N. Calculate the resultant force and state its direction.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Exam Practice: Resultant forces (cont.)

Forces

Lesson 3 of 20

Question 4 · 3 marks · Draw

Draw a free body diagram for a stationary book on a table. Label the forces.

Question 5 · 4 marks · Determine

Two forces act on an object at right angles to each other: 3.0 N east and 4.0 N north. Use a scale drawing to find the magnitude and…

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 1 · 2 marks · Calculate

Forces

Lesson 3 of 20

“

A car engine produces a driving force of 1800 N. The total resistive force is 1200 N. Calculate the resultant force on the car and state its direction.

HOW TO ANSWER IT

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

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 1 · mark scheme

Forces

Lesson 3 of 20

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

600 N

1 mark

In the direction of the driving force

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 2 · 2 marks · Use the diagram

Forces

Lesson 3 of 20

The diagram shows the forces acting on a box. Calculate the resultant force. (2 marks)

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 2 · mark scheme

Forces

Lesson 3 of 20

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

Subtracts 15 from 40

1 mark

25 N to the right

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 3 · 2 marks · Explain

Forces

Lesson 3 of 20

“

A skydiver has a weight of 800 N and air resistance of 500 N. Calculate the resultant force and state its direction.

HOW TO ANSWER IT

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

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 3 · mark scheme

Forces

Lesson 3 of 20

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

300 N

1 mark

Downwards

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 4 · 3 marks · Draw

Forces

Lesson 3 of 20

“

Draw a free body diagram for a stationary book on a table. Label the forces.

HOW TO ANSWER IT

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

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 4 · mark scheme

Forces

Lesson 3 of 20

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

Downward arrow labelled weight

1 mark

Upward arrow labelled normal contact force

1 mark

Arrows equal in length

1 mark

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 5 · 4 marks · Determine

Forces

Lesson 3 of 20

“

Two forces act on an object at right angles to each other: 3.0 N east and 4.0 N north. Use a scale drawing to find the magnitude and direction of the resultant force.

HOW TO ANSWER IT

Command word: Determine. Worth 4 marks, so plan before writing.

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AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Question 5 · mark scheme

Forces

Lesson 3 of 20

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

Correct scale chosen

1 mark

Right-angled triangle drawn accurately

1 mark

5.0 N

1 mark

Direction about 53° north of east

1 mark

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