Matter and Forces:Hooke’s Law (2.3)

Lesson Objective / We Are Learning Today.....

• All pupils will be able to clarify Hooke’s law and identify the point on a graph in an experiment on a spring, where Hooke’s law no longer applies.

• Some pupils will be able to successfully use the equation to work out the spring constant of a spring when a force is applied.

To explain the concept of Hooke’s Law.

Lesson Outcome / What I’m Looking For.....

Hooke’s LawIn the 1600s, a scientist called Robert Hooke discovered a law for elastic materials.

Hooke's achievements were extraordinary - he made the first powerful microscope and wrote the first scientific best-seller, Micrographia.

He even coined the word ‘Cell’.

Hooke's Law, elastic and plastic behaviour

• If a material returns to its original size and shape when you

remove the forces stretching it we say that the material - elastic behaviour.

• A plastic (or inelastic) material is one that stays deformed after you have taken the force away - plastic behaviour.

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

• 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.

The elastic limit can be seen on the graph. This is where it stops obeying Hookes law.

Since Force is proportional to extension Hookes Law could be put as

Or if k is the proportionality constant

F=kx

xF Where F is the applied force in Newtonsx is the extension in metres

K = Spring Constant?!• 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).

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

2.0N = k x 0.04m So, k = 2.0 N

0.04 m

= 50 N m-1

F

k x

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