Variables - There are a number of different variables which can be investigated in the experiment. They include:
- no. of rubber bands used
- different masses used (100g masses stuck together)
- the lengths of how far back you pull back the rubber band(s) (different forces applied)
- carrying out the experiment on different surfaces
- different temperatures of rubber bands (this wouldn’t be a very good variable to investigate because it has very little effect and it would be very hard to investigate this variable)
I am going to investigate variable no. 1 (no. of rubber bands) and no. 3 (different forces applied when catapulting the mass).
Equipment list -
- 100g mass. A 10g mass would be too light and wouldn’t stay on the surface and something like 1kg would be far too heavy to even more far
- Long sheet of paper (around 1.5m long). I will only use this as the surface on which the mass travels so the surface is always going to be constant
- 1-3 elastic bands (I will make sure they are all the same size/length/thickness)
- 1m rulers to measure how far the mass travels. If I use a 30cm ruler the for some of the further measurements I would not be able to measure in a straight line
- Tripod to hold the elastic band
- Blue-tack on each of the tripod’s legs to stop it from moving
- Scale to measure the length from which I am catapulting the mass from
- A long board. I will place my long sheet of paper on it and when catapulting the mass using a great force, the mass will keep travelling in a straight line and will not fall off.
Safety – Safety is always important. To carry out this experiment out safely, I will wear goggles and when catapulting the mass, I will make sure nobody is standing nearby. Careful handling must also be taken into account.
Method -
- Set up equipment as shown below.
- Attach the elastic rubber band to tripod (make sure blue-tack is fixed well).
- Place mass against centre of elastic band and make sure it isn’t twisted.
- Pull back mass so that edge is at 1cm and release it so that it travels down the centre of the track.
- Measure the length that the mass has travelled using the 1m ruler.
- Repeat this for the rest of the lengths (1-14cm). (if mass leaves the surface, discard the result and repeat).
- Now attach another rubber band so that there are 2 and do the experiment again using all the lengths of the scale to catapult the mass.
- Do the same thing using 3 rubber bands,
- Repeat numbers 3 – 8 (all the 3 sets of experiments) twice so that in the end I will have 3 sets of each experiment using 1 – 3 rubber bands (if time allows).
- Record all my results in a table that looks something like this:
Diagram - bird’s eye view of the experiment
Fair test - To make my experiment a fair test I must keep all of the factors constant except for extension and no. of rubber bands which are my two variables that I have chosen to investigate. I will try to keep the temperature of the elastic bands the same by stretching them a couple of times before using them to catapult the mass. As they get hotter, they stretch more easily. This affects the force used. If I was to start straight away then my first time results are likely to be different from the others because they haven’t warmed up yet. I will also keep my mass of 100g constant, keep the surface the same and make sure that the elastic bands are the same size/length/thickness. Therefore, if I do all this then my experiment will be accurate and a fair test.
Predictions - After carrying out preliminary experiments, I am able to make my predictions for what will happen in my results for the real experiment. I predict that as extension increases, so will the distance that the mass travels. This is because as extension increases, the force increases as they are proportional to each other according to Hooke’s Law. Hooke’s Law states that if you stretch something with a steadily increasing force, then the length/distance will become steadily greater too. In this case the more force you apply to the spring, the further the mass will travel. However there is also the elastic limit that stops the elastic band behaving normally and so once it gets stretched too much, the line of the graph will go straight horizontally.
The way I will expect my graph to be will be a slightly ‘S’ shape. I predict that at the start of the graph, the line will be slightly increasing and when the extension of the rubber band gets to about 6 or 7, the distance that the mass travels will be a lot greater but when it gets to around 14cm extension, the distance will stop going up in a straight line and will go down a bit. My predicted graph is shown below.
Obtaining Evidence:
Safety - See bottom of page 3.
Results -
In the last two tables, the extension of the rubber band was less than in the first one which went up to 14cm because when I did catapult the mass, it went off the board which meant that it land on the floor and stopped moving. Therefore I had to discard those results and leave them out. Otherwise it would have been an unfair test and the results that I would have got would be totally incorrect. So I always kept the surface on which the mass travelled constant. I only changed the variables but kept all the rest of the factors constant all the time. (Look back to the bottom of page 4 – Fair test section to see all the things that could affect the experiment constant and what I changed as my variables I chose to investigate).
I have ensured to collect reliable evidence by repeating the experiment times which meant that I would still get an enough sufficient result for each individual experiment that I carried out even if one of the results was quite inaccurate. The averages that I calculated using the results tables are transferred into graphs as shown below.
Analysing and Considering Evidence:
Patterns + conclusions - As you can see from the graphs, they are not all the same and haven’t exactly come out as I have predicted but I wasn’t able to completely carry out the whole experiment as the mass kept flying of the board because the distance travelled was too far due too a high extension of 10 – 14 cm, especially when three rubber bands were used. However I got a very reliable and correct results for the set which I used only one rubber band. I more or less got an ‘S’ shaped line which means that my predictions were correct. There is an elastic limit from what I see and therefore the distance that the mass travelled didn’t keep going up. But when I used two rubber bands, my results showed me that the line didn’t curve at the end. That was probably because I couldn’t finish doing the experiment up to 14cm extension so I didn’t get any sign of a curve which would mean that the rubber band is losing its normal elastic behaviour. The same happened for the experiment involving three rubber bands. Although I hoped for more, I still made a prediction on one of the graphs which compares the three experiments by showing what I would expect to happen if I were to keep extending the elastic rubber band. This would be something I would like to do if I could expand the investigation even more and spend more time on it. My prediction was right according to the first experiment that the line would curve down like Hooke’s law states even though I am using an elastic band rather than a metal spring which explains why there is a curve at the start of every graph and not on a graph involving a metal spring. There is also a slight clue in the graph that the force is sometimes proportional to the distance travelled but this isn’t fully true. Otherwise I would have ended up with straight line.
Evaluating:
Accuracy + suitability + improvements - The results that I got were accurate but there wasn’t enough and so there are a lot of things I could improve. Firstly I wasn’t able to say much about the last two sets of results, as they weren’t complete. The experiment I did using one elastic rubber band went very well and I got reliable evidence (I repeated the experiment 3 times using the whole scale of 14cm extension) as well as my prediction was right. But later as I already said the last two sets of experiments were incomplete because the mass, when catapulted using a high force and more than one rubber band made the mass fall of the board and stop moving. Therefore I only got a line in the graph curving upwards and that was it. So it did not fit my prediction to the extent, as the line didn’t curve down. If I did do the extension of up to 14cm in each of the three sets then I think that the line would be similar to the first set of results + first graph I got. But there is no proof that my prediction was wrong.
As for the suitability of my method the only thing I could improve is to extend the scale further to a maximum of 18cm extension. This is because I would see what happens when I do use an even greater force than I already have and therefore I could extend my investigation of my chosen variables by getting even more results. I would also repeat each set of experiments five times in total which would mean that I would get even more reliable evidence than I already do and I would conduct the whole experiment on a very long board or make sure I have much more space to conduct my experiment so that I could finish getting the results for the last two sets of experiments and even go back further to see what happens when catapulting the mass back from 15, 16, 17, or 18cm.
Another factor worth thinking about in order to extend the whole investigation is to investigate other variables and if they have anything in common with those which I have already investigated (whether their effects are similar to my chosen variables).
I don’t think that I got a firm conclusion because I did not fully finish the experiment except for the first set of results which matched my prediction. The factors which I could improve in order to get a firm conclusion are stated on the previous page.