1. Do now!
Can you read through the slides you stuck in last lesson on measuring density?

2. Do now!
Can you plot a graph of last lesson’s experiment? (Force on the y axis, extention on the x axis)

3. Today’s lesson
Adding forces
Hooke’s law

4. Forces
Remember a force is a push (or pull) 4

5. Forces
Force is measured in Newtons 5

6. Forces
There are many types of forces; electrostatic, magnetic, upthrust, friction, gravitational……… 6

7. Representing Forces
Forces can be represented by arrows. The length of the arrow indicates the magnitude, and the direction the direction! 7

8. Adding forces
Copy please!
For example;
Resultant force
2 N
6 N
4 N 8

9. Robert Hooke

10. Investigating forces and springs
You are going to investigate how much a spring stretches when a force is applied to it.
The amount a spring stretches is called its extension. This is the difference in length between the stretched spring and the length of the spring when it was unstretched.
(Remember we are looking at the force on the spring. A mass of 100g will have a weight (force of gravity pulling it down) of 1 Newton.
Add masses to your spring and carefully measure its extension. You can do this until the spring breaks! (but you must wear goggles and be careful during this investigation).
Your experimental report will include the following
A table of results
A graph of your results
A conclusion describing what you have discovered (think about this!)

11. Table of results Mass (g) Force (N) Length of spring (cm)
Extension (cm)
2.4
100 1
5.4
3.0
200 2
7.4
5.0
300 3
9.4
7.0

12. Graph
Force (N)
Extension (cm)

13. Hooke’s law
Let’s go!

14. Hooke’s law (F = kx) Force (N) Extension (x) (cm)
Limit of proportionality
Force (N)
Extension (x) (cm)
The extension of a spring is proportional to the force applied (until the limit of proportionality is reached). The gradient of the graph is equal to k, the spring constant.
Hooke's Law and springs - Physics – YouTube 14 14

15. 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 N/m (newtons per metre)
The higher the k the stiffer the spring
Materials with a high k need a large force to for a given extension

16. Finding the gradient of a straight line

17. Calculating spring constant
Can you use your graph to calculate the spring constant of your spring? (The gradient of the straight line part of your graph – the units will be N/cm)

18. More info on Hooke at http://www.roberthooke.org.uk/
COPY!
A material is said to obey Hookes Law if its extension is directly proportional to the applied force
More info on Hooke at

19. Steel, glass and wood!
Force
Even though they don’t stretch much, they obey Hooke’s law for the first part of the graph
Extention 19 19

20. Rubber
Force
Extension 20 20

21. Spring constant in series and parallel?
Plot graph (same axes)
Calculate spring constant (gradient)
Write a conclusion
Force (N) ? k
Extension (x) (cm) ?

22. Let’s try some questions!

23. “Ticket to exit”?!