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Hooke’s Law Book pg 19 and 20

Syllabus 1.29 – 1.31

25/01/2016

© cgrahamphysics.com 2016

Do you know what Hooke’s

Law is telling us?

Why are seat belts elastic?

Why not have rigid seatbelts that would keep you firmly in place?

Hooke’s Law has to do

with elasticity

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Aim

Explain the concepts of Hooke’s law

Identify the point on a graph where Hooke’s law no

longer applies.

Explain the differences between reversible

deformation and irreversible deformation.

Use the equation to work out the spring constant of

a spring when a force is applied.

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Key words

Elastic 탄력, 신축성이 있는

Plastic 플라스틱, 비닐

Stretch 늘이다, 늘어 지다

Extension 확대, 연장

Linear 정기선

Proportional 비례

Helical 나선(형)의

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There are 1000s of different springs in

the world…

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What do we know about springs?

In the 1600s, a scientist called Robert

Hooke discovered a law for elastic

materials.

Hooke's achievements were

extraordinary - he made the first

powerful microscope

But he is not Captain Hook, for sure

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If a material returns to its original size and shape it shows elastic behaviour.

A plastic (or inelastic) material stays deformed

If you apply too big a force a material will lose its elasticity.

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Hooke’s Law Hooke discovered that the amount a spring

stretches is proportional to the amount of force

applied to it.

This means if you double the force its extension will

double, if you triple the force the extension will triple

and so on.

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The elastic limit can be seen on the

graph.

This is where it stops obeying Hookes

law.

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You can write Hooke's law as an equation:

F = k ∆ x

Where:

F is the applied force (in newtons, N),

x is the extension (in metres, m) and

k is the spring constant (in N/m).

The extension ∆x (delta-x) is sometimes

written e or ∆l. You find the extension from:

∆x = stretched length – original length.

k x ∆𝑥

F

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Force is directly proportional

to extension

𝐹∞∆𝑥

What does ‘directly

proportional’ mean?

Well, it’s related to ratios…

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Directly Proportional

F (N) x (cm)

10 3

20 6

50 15

Two variables are directly proportional

if they are always in the same ratio.

Force

F : x

10:3

20:6 = 10:3

50:15 = 10:3

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Direct Proportional Graphs

If you plot two variables which are

directly proportional, you will always

get a straight line which goes through

the origin (0,0).

Force

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Spring Constant k The spring constant measures how stiff the spring is.

The larger the spring constant the stiffer the spring.

You may be able to see this by looking at the graphs below:

k is measured in units of newtons per metre (Nm -1).

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Example A spring is 0.38m long. When it is pulled by a force of

2.0 N, it stretches to 0.42 m. What is the spring

constant? Assume the spring behaves elastically.

Extension, ∆x = Stretched length – Original length =

. 0.42m – 0.38m = 0.04 m

So, k = 2.0 N

0.04 m

= 50 N m-1

F = k ∆x

𝑘 =𝐹

∆𝑥

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Elastic behaviour – Car Safety Elastic behaviour is very important in car

safety, as car seatbelts are made from

elastic materials. However, after a crash

they must be replaced as they will go past

their elastic limit.

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Key Definitions

Hooke’s Law = The amount a

spring stretches is proportional to

the amount of force applied to

it.

The spring constant measures

how stiff the spring is. The larger

the spring constant the stiffer the

spring.

A Diagram to show Hooke’s Law

F = k ∆ x

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Plenary In as many sentences as possible link

the two pictures using the keywords.

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hooke’s law - wordpress.com explain the concepts of hooke’s law identify the point on a graph...

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