1. IGCSE Combined Science
NOT THIS ONE!
Hooke’s Law
IGCSE Combined Science

2. What did Hooke discover ?
the more force that was put on materials the more they extended
With some materials they also extended in a regular way
-eg. if the force was doubled so did the extension
this was true as long as their elastic limit was not exceeded

3. What is the elastic limit?
Is when the material no longer shows elastic behavior
-ie does not return to original size when stretching force is removed
The material is permanently deformed
-i.e. is larger or longer than originally

4. Hookes Law states: Force is proportional to extension F α x
Or if k is the proportionality constant F = kx

5. What does k mean in F=kx?
k is called the spring constant and is a measure of the stiffness of the spring or material
It has units of Nm-1 (newtons per metre)
The higher the k the stiffer the spring
Materials with a high k need a large force for a given extension

6. The spring constant k is measured in Nm-1 because it is the force per unit extension.
The value of k does not change unless you change the shape of the spring or the material that the spring is made of.

7. 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