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

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Hooke’s law. Calculate the force from a spring when given its spring constant and deflection. Calculate a spring constant given the required force and.

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Presentation transcript:

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

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

Today’s lesson Adding forces Hooke’s law

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

Forces Force is measured in Newtons 5

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

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

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

Robert Hooke

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!)

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

Graph Force (N) Extension (cm)

Hooke’s law Let’s go!

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 http://www.youtube.com/watch?v=pVdGUTRI49E 14 14

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

Finding the gradient of a straight line

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)

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 http://www.roberthooke.org.uk/

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

Rubber Force Extension 20 20

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

Let’s try some questions!

“Ticket to exit”?!