Harry Sedgwick 25/2/08 Physics practical – catapults

Introduction/aim – In this practical the aim is to investigate how the mass of a projectile will affect it range when fired from a catapult device. For this I had to construct my own catapult device, this will be made by using wood and screws to make a structure and then using an elastic band to fire the projectile.

Method – The first step is to set up the experiment. This will consist of making sure that the catapult is firmly attached to the floor using blu-tac or plasticine, so that it will not move back or forward and ruin the experiment, because it may change the distance the projectile has travelled. Measuring the masses of plasticine is vital so I can see the changes of horizontal distance compared to the projectiles mass. Using an electronic balancer scales is the best way to get the most precised measurement for the mass. The range of masses will be the following – 5g, 10g, 15g, 20g, 25g, 30g. When fired, to measure the distance travelled from the catapult, It is useful to use a tape measure so to record down on a table of results including mass and range with 2 repeats. To make this a fair test I have to keep all variables the same except the mass of the projectile, because it is testing the mass and the mass of the projectile only. This means I used the same catapult for each one and used the same vertical and horizontal distance when the catapult is pulled back. Results are recorded down to the nearest centimetre so it makes the results much more accurate and repeating this method twice is much better as an average can be taken from the three recorded results. The shape of the projectile is also very important because of air resistance so I need to use a template and make them all the same shape using lead shot so that can use the same shape and vary the mass. For this experiment, using a sphere made from plastacine seems to be the easiest route to choose and adding lead balls to the plastacine will vary the mass but not shape.

Diagram –

I pull the catapult to the same place every time because the wood stops show me when to stop.

Prediction and hypothesis - My hypothesis is the lighter the mass of the projectile, the further that the projectile will fly. This is because that the mass is inversely proportional to the distance travelled. For instance, the 5g projectile will fly much further than the 50g projectile. The theory for this idea is - Ek = ½ x m x v2 and V2 is inversely proportional to 1/mSo if that is true it must mean that V = 1/√mAnd if we are attempting to find the displacement then we need the equation S = V x TIf this equation is true then S is proportional to V which is inversely proportional to 1/√mIf s is then inversely proportional 1/√m this means I will have to plot a graph with range and 1/√m and it should end up with a negative correlation if my hypothesis is correct.

Safety – The main accident that could have happened in this experiment is that someone could have been hit by a projectile, so the only safety hazard was to make sure that everyone is behind a certain line when catapult has fired the projectile.

Apparatus – - A catapult (made from wood, screws and elastic bands) - Plasticine and lead balls (for the projectile and varying the weight) - A tape measure (for measuring the distance that the projectile has travelled) - Electronic balancer (for measuring mass of projectiles) - A clipboard and pen (so I can easily take down results outside)

Observations/results –

Table of results - Mass (grams)+/- 0.1g

5 10 15 20 25 30

Range (cm) +/- 10cm

337 295 265 243 210 195

Repeat range 1 441 330 273 245 225 200Repeat range 2 410 385 319 223 217 198Average range (m) 3.96 3.36 2.85 2.37 2.17 1.971/√mass 0.4472 0.3162 0.2582 0.2236 0.2000 0.1826

Key for graph next page^

Graph – title – graph to show the distance travelled according to 1/square root of mass

Conclusion/evaluation – To show that my prediction was correct I produced a graph with range on the vertical line and the reciprocal of each mass along the horizontal line. If my prediction was correct there should be a negative correlation to the graph, which there is, proving my prediction. The procedure went to plan except for a few minor mistakes. One of these being the shape of the projectiles, this is because the lead balls were unavailable at the time so I just changed the amount of plastacine to vary the weight which was against my method but it didn’t affect my results as much as I thought. Another mistake was how I interpreted the projectiles movement when it had come into contact with the floor. The projectiles bounced along the tarmac so it was harder to measure where it landed exactly, but again, this was not a major problem. If I had to do this again I would take more time building the catapult as it was slightly uneven at times. I would also take the experiment inside a large room so that it was not affected by the wind instead of doing it outside. Finding a better way of measuring the distance travelled by the projectile would also aid my experiment because I could use more accurate units on my graph and show it in much more detail. This procedure could have included much more detail around the mass to range connection. It could have measured the speed connected with the projectile as well; this could have given us more of an insight about this subject.