This document explains Hooke's law, which states that the extension of a spring is proportional to the applied force. It defines elastic and plastic behavior, the spring constant, and the elastic limit on a graph. An example calculation demonstrates determining the spring constant k from measurements of force and extension using the equation F=kx. Key definitions are provided for Hooke's law, the spring constant, and a diagram to represent Hooke's law.

1. Matter and Forces:
Hooke’s Law (2.3)

2. Lesson Objective / We Are Learning
Today.....
To explain the concept of Hooke’s Law.
Lesson Outcome / What I’m Looking
For.....
• 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.

3. Hooke’s Law
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 and
wrote the first scientific best-
seller, Micrographia.
He even coined the word ‘Cell’.

4. 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.

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

11. Since Force is proportional to
extension Hookes Law could be
put as
F x
Where F is the applied force in Newtons
x is the extension in metres
Or if k is the proportionality constant
F=kx

12. 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).

13. 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 F
2.0N = k x 0.04m k x
So, k = 2.0 N
0.04 m = 50 N m-1

17. 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