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:

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 elastic band the further the weight will travel the further the elastic band is pulled back the more potential energy it stores when released more potential energy is changed into kinetic energy and therefore more kinetic energy is used to move the weight. I predict that as the elastic band gains more potential energy (pulled back further) it will fire the weight further (more kinetic energy). 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.”

Work         =         Force X         Distance

X4                          X2                X2

From looking at my preliminary graph I am going to investigate the straight area (0-10N) as this seems to be a good reflection of the above rule. Also when the band is stretched too much its potential and kinetic energies vary. (to extend the experiment I could investigate this section of the graph (15+N)

  • Final experiment work

For our final experiment we would fire just one 100g weight but this time the variable would be the force pulled back on the elastic band not the force put on it like in the trial experiment.

Here is what I think the graph will look like.

Apparatus list:

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Here is a diagram of our final experiment:

The elastic band was pulled back at a force of 1 Newton in the first experiment. This was then repeated 5 times. We did the experiment 5 times so that we would have an accurate set of results and so that we had lots of results to compare. The more results taken give more reliable results. The elastic band was then pulled back at a force of 2 Newton’s. The elastic band was ...

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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.

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.

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.