Newtons Second Law.pptx

AQA GCSE PHYSICS · PAPER 2 · FOUNDATION & HIGHER

Newton's Second Law

Forces

Lesson 15 of 20

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Warm-up

Forces

Lesson 15 of 20

Answer each one, then check.

1

What is a resultant force?

2

What is acceleration?

3

What is the unit of force?

4

What is proportional?

5

What does inversely proportional mean?

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Warm-up - Answers

Forces

Lesson 15 of 20

1

What is a resultant force?

The single force with the same effect as all the forces

2

What is acceleration?

Rate of change of velocity

3

What is the unit of force?

Newton

4

What is proportional?

Doubling one doubles the other

5

What does inversely proportional mean?

Doubling one halves the other

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Learning Objectives

Forces

Lesson 15 of 20

1

State Newton's Second Law.

2

Recall and apply F = ma.

3

Describe how to investigate the effect of force and mass on acceleration (Required Practical 7).

4

Explain inertial mass (Higher tier).

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Forces

Lesson 15 of 20

NEWTON'S SECOND LAW

resultant force = mass × acceleration F = ma

The acceleration of an object is proportional to the resultant force and inversely proportional to its mass: a ∝ F and a ∝ 1/m.

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Required Practical 7: Force, Mass and Acceleration

Forces

Lesson 15 of 20

The weight of the hanging masses provides the force.

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Required Practical 7: Method

Forces

Lesson 15 of 20

Two investigations.

1

Force varied

Keep the total mass constant by moving masses from the trolley to the hanger; the force is the weight of the hanging masses.

2

Measure acceleration

From the light gates: a = (v − u) ÷ t.

3

Plot

Acceleration against force: a straight line through the origin.

4

Mass varied

Keep the force constant and add masses to the trolley.

5

Plot

Acceleration against mass (a curve) or against 1/mass (a straight line).

6

Repeat

Take repeat readings and compensate for friction by slightly tilting the runway.

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Finding Acceleration

Forces

Lesson 15 of 20

A car of mass 1200 kg has a resultant force of 3000 N. Calculate its acceleration.

1

Rearrange

a = F ÷ m

2

Substitute

a = 3000 ÷ 1200

3

Answer

a = 2.5 m/s²

ANSWER

2.5 m/s²

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Finding the Force

Forces

Lesson 15 of 20

A 0.50 kg trolley accelerates at 4.0 m/s². Calculate the resultant force.

1

Write the equation

F = ma

2

Substitute

F = 0.50 × 4.0

3

Answer

F = 2.0 N

ANSWER

2.0 N

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Using a Graph

Forces

Lesson 15 of 20

A graph of acceleration against force for a trolley is a straight line through (2.0 N, 1.0 m/s²). Find the mass.

1

Gradient

a ÷ F = 1.0 ÷ 2.0 = 0.50 kg⁻¹

2

Since a = F ÷ m

Gradient = 1 ÷ m

3

Mass

m = 1 ÷ 0.50 = 2.0 kg

ANSWER

2.0 kg

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Inertial Mass (Higher)

Forces

Lesson 15 of 20

Explain what is meant by inertial mass.

1

Definition

The ratio of force to acceleration

2

Meaning

A measure of how difficult it is to change the velocity of an object

ANSWER

Inertial mass = force ÷ acceleration: how hard it is to change an object's velocity.

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Estimating

Forces

Lesson 15 of 20

Use sensible values.

Everyday values

A car accelerating from 0 to 27 m/s (60 mph) in 10 s has a ≈ 2.7 m/s².

Force

For a 1000 kg car: F = 1000 × 2.7 ≈ 2700 N.

Symbol ~

Means approximately.

Units

Mass in kg, a in m/s², F in N.

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Key Terms

Forces

Lesson 15 of 20

Resultant force

The net force acting on an object.

Acceleration

Rate of change of velocity.

Inertial mass

Force divided by acceleration.

Proportional

Increasing by the same factor.

Inversely proportional

One quantity halves as the other doubles.

Light gate

A sensor that times how long a card takes to pass.

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Forces

Lesson 15 of 20

YOUR TASK

Predict the Acceleration

10 minutes

A trolley has a mass of 1.0 kg and accelerates at 2.0 m/s² under a resultant force. What acceleration would the same force give a 2.0 kg trolley? What force is needed for 2.0 kg to accelerate at 2.0 m/s²?

1

Use F = ma.

2

Compare the masses.

WHAT A GOOD ANSWER SHOWS

Half the acceleration: 1.0 m/s². Force = 2.0 × 2.0 = 4.0 N.

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Can I...?

Forces

Lesson 15 of 20

State Newton's second law.

Recall F = ma.

Rearrange for a or m.

Describe Required Practical 7.

Interpret a force–acceleration graph.

Explain inertial mass.

Estimate forces on a car.

Use units.

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Summary & Exam Focus

F = ma.

Acceleration ∝ force; ∝ 1/mass.

Inertial mass = force ÷ acceleration.

Use light gates to measure acceleration.

EXAM FOCUS

A 1200 kg car has a resultant force of 3000 N. Calculate its acceleration. (2 marks)

a = F ÷ m.

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