# Springs and Things…Robert Hooke

Purpose:

1.      To investigate Hooke’s Law (The relation between force and stretch for a spring)

2.      To investigate Newton’s Laws and the operation of a spring scale.

Discussion

Everybody knows that when you apply a force to a spring or a rubber band, it stretches. Everything stretches when it’s pulled, as it happens, from concrete to human hair. A scientist would ask, “How is the force that you apply related to the amount of stretch?” This question was answered by Robert Hooke, a contemporary of Newton, and the answer has come to be called Hooke’s Law.

Hooke’s Law, believe it or not, is a very important and widely-used law in physics and engineering. Its applications go far beyond springs and rubber bands.

You can investigate Hooke’s Law by measuring how much known forces stretch a spring. A convenient way to apply a precisely-known force is to let the weight of a known mass be the force used to stretch the spring. The force can be calculated from weight (N) W = mass m(kg) x gravitational field strength, g. (10 N/kg)The extension (new length – original length ) of the spring can be measured with a ruler

If you do this in the lab, measure the original length of the spring carefully by looking perpendicularly to the spring with a clear plastic ruler. Have a pencil and notebook handy to write down the results, having first made a results table like this…

 Added mass /g Applied force /N Original length /cm Final length /cm Extension /cm 0 0 2.0 2.0 0 100 1 2.0 5.0 3.0

Don’t exceed a load of 600g, otherwise the spring will extend beyond its elastic limit, and deform permanently

Plot a graph of added mass /g (y-axis) against extension  in cm (x-axis) the slope or steepness of the line is a measure of how stiff the spring is, or, how resistant it is to deforming forces. It should look like this.

Try again, this time attaching 2 springs end-to-end. What happens? Repeat, this time interleaving the coils in parallel. Your teacher will show you how. Plot all your graphs on the same axes.Can you explain what you see in terms of stiffness?

If you get all this finished, you MIGHT be allowed to test one spring to destruction. Ask your teacher. Finally,  Hooke’s Law works just fine if a spring is compressed, too. The compression is proportional to the applied force if the spring is squashed, like a bedspring.