There is a separate activity on Hooke’s law experiment – we will not be covering the experiment in this activity. If you are looking for Hooke’s law experiment, then please search the required practical’s for forces experiment.
Standing on the side of a bridge with a river running underneath the person to your left asks you to step off the scales and puts some numbers into a computer. They look at you and smile as the as you ‘how much do you want to go into the water?’ You look scared, adrenaline running through your body as you say, ‘About up to my waist?’. They play with some numbers on the screen and then ask you if you are ready. You slowly nod and they start a countdown ‘3, 2, 1’ with a slight hop, you are in the air, plummeting towards the water headfirst – it feels like forever, but in only a few seconds you feel the bungee cord pulling at your legs slowing you down as your body dips into the water up to your waist and you get pulled back up. Bungee jump, done! It takes a lot of maths to be an expert thrill seeker, you need to know where the limits are, so you don’t die. In this activity, well will be looking at some of the science that allows us to perform bungee jumps – Hooke’s law and elastic forces.
A bungee cord is basically a big rubber band, and it might help some of this to sink in if you have a rubber band to play with while you read this. If you apply just one force to a rubber band that it won't stretch, it will just move in the direction the force is applied. You need a minimum of 2 forces that will act in different directions to stretch anything. One of these forces could be stationary, you finger, and one could be moving, like the hand pulling the band backward. This will apply a force to the band and make it stretch. If you release the forces, then the band will return to its original shape. All good so far?
Okay, so Hooke’s law is a law that defines stretching things. It says that ‘The force applied to an object is proportional to the length of the extension’. This basically means that if you were to plot force and extension on a graph, it would make a straight line – just like the one shown below.
GRAPH IMAGE HERE
Hooke realized that he could make an equation out of these graphs that could tell him in any given circumstance what the extension of an object would be.
INSERT EQUATION HERE
F = force (Newtons (N))
k = spring constant – a number that tells you how hard it is to stretch the object (Newtons per meter (N/m))
e = Extension – how much the spring has stretched (meters (m))
So, the person putting the numbers into the computer at the start of the activity was just working out how much you would stretch the bungee using this equation.
Let’s have a go at this ourselves.
Q – A person stretches the bungee 15 m when the bungee cord has a spring constant of 40 N/m. Calculate the force applied by that person.
Step 1 – Highlight the number son the question:
A person stretches the bungee 15 m when the bungee cord has a spring constant of 40 N/m. Calculate the force applied by that person.
Step 2 – Write out the numbers:
F = ?
k = 40 N/m
e = 15 m
Step 3 – Put the numbers into the equation:
F = 40 x 15
Step 4 – Do the calculation and write out your answer.
F = 600 N
Don’t forget your units!