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NEWTON’S 2ND LAW7 May 2023

ObjectivesTo understand why different objects accelerate at different rates and be able to use the equation for

Newton’s 2nd Law.HSW: AF1 – Thinking scientificallyUsed before in: lesson 1 – resultant forces Will use again in:

PLTS: Independent enquirers - support conclusions, using reasoned arguments and evidence.Used before in: Science A Will use again in:

KeywordsForce, Mass, Acceleration, N, Kg, m/s/s,

proportional, inverse.

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All students should be able to: • Describe that resultant forces make objects

accelerate and the more force you give something the more it will accelerate.

Most students should be able to:• Calculate the force required to produce a given

acceleration of an object of known mass.• State that objects of larger mass require greater

forces to cause large acceleration.• Determine the direction of the acceleration on an

object.Some students should be able to:• Rearrange and use the equation:

force = mass x acceleration.

OUTCOMES

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Introducing unbalanced forces

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If the resultant force acting on an object is not zero, all the forces are said to be unbalanced.

This forms the basis of Newton’s second law of motion, which states:

What is Newton’s second law?

If the forces on an object are unbalanced, two things about the object can change: the speed of the object may change – it may either

increase or decrease the direction of motion may change.

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Demos

Qualitative description

Classic demo

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How is movement calculated from force?

The resultant force acting on an object is related to the object’s mass and acceleration. These three factors are linked by the following equation:

Resultant force is measured in newtons (N).

Mass is measured in kilograms (kg).

Acceleration is measured in metres per second per second (m/s2).

force = mass x acceleration

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How do we use Newton’s second law?

A car has a mass of 1,000 kg. What force must the car’s engine supply to cause an acceleration of 2 m/s2?

force = mass x acceleration

= 1,000 x 2

= 2,000 N

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Using a formula triangle

A formula triangle helps you to rearrange a formula. The formula triangle for force (f), mass (m) and acceleration (a) is shown below.

x

Cover the quantity that you are trying to work out, which gives the rearranged formula needed for the calculation.

So to find force (f), cover up f…

…which gives the formula…

f = m x a

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How do we use Newton’s second law?

A lorry has a mass of 12,000 kg. What acceleration is caused by a force of 10,000 N?

force = mass x acceleration

= 0.83 m/s2

= 10,000 12,000

forcemass

acceleration =

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Questions. – use sheet if necessary

1 Complete the table below showing the resultant force, mass and acceleration of objects in

different situations.

Resultant force (in newtons)

Mass (in kilograms)

Acceleration (in m/s2)

a) Athlete accelerating at start of 100 m race 70 8.0

b) Car accelerating 3000 1200

c) Lorry braking 16 000 0.8

d) Plane taking off 8000 5.0

2,A vehicle of mass 1500 kg braked to a standstill from a velocity of 24 m/s in 12 s.

a) Show that the deceleration of the vehicle was 2.0 m/s2.

b) Calculate the resultant force on the vehicle.

3.a) A cyclist accelerated along a flat road from a standstill to a velocity of 12 m/s in 60 seconds. The mass of the cyclist and the bicycle was

80 kg.

i) Show that the acceleration of the cyclist was 0.2 m/s2.

ii) Calculate the resultant force on the cyclist and the bicycle.

b) On reaching a velocity of 12 m/s, the cyclist in a) stopped pedalling and slowed down to a velocity of 8 m/s in 10 s, when she started

pedalling again.

Calculate:

iii) The deceleration of the cyclist when she slowed down.

iv) The size and direction of the resultant force on the cyclist when she slowed down.

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Answers1 a) 560 N,

b) 2.5 m/s2,

c) 20 000 kg,

d) 40 000 N.

2 b) 3000 N.

3 a) ii) 16 N,

b) i) 0.4 m/s2,

ii) 32 N in the opposite direction to her direction of motion.

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Plenary Game

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