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Catapult Investigation

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Physics coursework

Mark Cranshaw


Physics coursework

Catapult Investigation


  • Preliminary work

The preliminary part of my catapult investigation was to see how far I could stretch an elastic band without breaking and also to test to see what readings I could use in the final experiment.

I am going to plan an experiment where I shall investigate the firing distances of 100g weights fired by two elastic bands wrapped around a stool. First of all we did our preliminary experiment. In this we investigated elastic bands to see which would be most suitable to use in our final experiment. We tested the elastic bands with different forces (1-10 Newton’s) and recorded the distances of which they were stretched. I realised that if I stretched the elastic bands with more than a force of 10 Newton’s then they would probably break or loose their elastic energy. Here is a diagram showing our trial experiment:

The results of this experiment are shown on the graph on the next page and also below:

Force (Newton’s)

Distance stretched (cm)































From the results it is quite easy to see that the bigger the force on the elastic band the further it will stretch. From this I will make a prediction:

“The more force put on the

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The data collected in the preliminary experiment helped us a lot in the final experiment. Without the trial experiment we would not have known how far to pull the elastic band back.

A rubber band stores more energy when you stretch it because the particles within the rubber band are stretched, it has potential energy. Kinetic energy is formed when these particles return back to their original state, this cause a pushing motion and kinetic energy is released. What potential energy is, is things that are out of position and when released the will do work. In this case the particles in the rubber band are out of position and when released will do work / change to kinetic energy / fire the weight. Elastic molecules wrap themselves around each other and when stretched are moved away from each other. There is a strong hold so when the force has stopped stretching the molecules return to there original state i.e. they pull back to where they were. Here is a picture of elastic molecules:


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  1. The same person firing.
  2. Something to pull the elastic band back, a piece of wire for example. This would help the person firing.
  3. Accurate markings on the floor.
  4. Measuring the distance fired with a tape measure – this would stop any measuring difficulties
  5. We could use one long strand of elastic and mark middle with pen – this would ensure that the weight was being fired in a straight line and from the centre of the band.
  1. Measuring the distances to 2 decimal places – this would help get a more accurate result.

Obviously I could extend my investigation buy trying out different weights, 200g or 50g for example or by using different elastic bands or by just simply firing weights at different forces 1 – 15 Newton’s for example. A good investigation that I could do is to see if when the elastic band is stretched its furthest does the weight travel the furthest or does it slow down. If extending this experiment further the first thing that I would do would be to investigate the curved region on my preliminary graph. This would then enable me to investigate the difference in the potential energy of bands with 1-10 N and then 15+N.

Mark Cranshaw 10P

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Response to the question

The author has answered the question well – they have carried out an experiment and used their results to explore the relationship between the force applied to a weight and the distance it travels along the floor. They have come ...

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Response to the question

The author has answered the question well – they have carried out an experiment and used their results to explore the relationship between the force applied to a weight and the distance it travels along the floor. They have come to a conclusion and evaluated their experiment well.

Level of analysis

The author has discussed their results and used them to test their hypothesis. They have suggested caused of uncertainties and anomalous results, although this could have been done in more depth. However, they have not shown their workings when analysing their results – for example showing the formula for calculating anomalies (results > mean + 2 x interquartile range, results< mean – 2 x interquartile range), or included uncertainties in their measurements – which we were advised was essential to gain good marks. These are usually calculated by halving the smallest measureable value of the equipment, e.g. for a ruler measuring to the nearest millimetre, the uncertainty would be 0.5 mm, or 0.0005m.When they made their hypothesis, there is very little explanation as to how they reached this. “If you double the force put on the elastic band and if you double the distance that it is pulled back then the distance that the weight is fired will quadruples” I would then have used various known formulae to explain the hypothesis: F=ma so when the force is doubled, the acceleration doubles, so the speed when the weight leaves catapult quadruples because V2=u2+2as (v= speed when weight leaves catapult, u= initial speed=0, a= acceleration, s= distance travelled within catapult (a constant as the weight moves from a fixed position). However, in this example, I would have been much clearer about what I am measuring and what I am changing in the experiment – the distance the elastic band is stretched or the force applied to the elastic band? This would completely change the experiment, as the elastic band is trying to pull back to its original position, so some of the potential energy is being used for this, rather than it all being directed at the weight. The distance the elastic band is pulled back, however, is a measure of the force in the desired direction. They have also occasionally shown confusion over their experiment: “Without the trial experiment we would not have known how far to pull the elastic band back” – this is incorrect, as they measured the force, so should have been ‘would not have known how much force to apply to elastic band’. Despite these mistakes, however, the author has analysed their results well, using graphs and calculations to test the hypothesis and discussing unusual results well, considering the cause of anomalies.

Quality of writing

The author’s quality of written communication is acceptable, and unlikely to lose too many marks. However, they have occasionally used unusual phrases such as “an extraordinary result” – it would be better to say ‘an anomalous result’. It would also have been preferable to use standard units throughout. Occasionally their descriptions are rather long-winded, for example: “The elastic band was then pulled back at a force of 2 Newton’s. The elastic band was then pulled back at a force of 3... The elastic band was then pulled back at a force of 9 Newton’s. The elastic band was then pulled back at a force of 10 Newton’s” This sentence does not need to be repeated like this – it is sufficient and easier to read to say ‘the elastic band was pulled back with a force of 1 N, increasing by one up to 10N as shown in the results table’. Another example is when the author attempts to explain the science behind the experiment; however it has been rather clumsily phrased. I would have given the following description: ‘As a rubber band is stretched, the molecules within it are pulled further apart. Sulfur cross-links between these molecules attempt to pull them back to their original position. Thus, the energy required to stretch the elastic band has been turned into potential energy. This is turned into kinetic energy when the force is removed, causing the elastic band to be pulled back into its original shape by the sulfur cross-links and pushing the weight forwards, giving it kinetic energy and causing it to be fired across the floor.’ This is clearer and therefore easier to understand, hopefully gaining more marks as the examiner can follow the explanation and see an understanding of the physics of the experiment. I find it very important to be clear in a report in order to show to an examiner that I understand the experiment rather than just repeating what the teacher has told me to and not being able to explain it in context. They could also have improved the presentation by using clearer headings. However, their spelling and grammar seems good throughout, and they have included graphs and diagrams which makes it easier for the reader to follow the experiment.

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