Carrying out an Earth and Space Science investigation

What the investigation must include

• AS 91187 is an internal: you plan and carry out your own practical investigation, then write a report on it.
• The report needs a purpose that comes from an Earth and Space Science context, and a method that gives the values you will use for the independent variable, how you will measure the dependent variable, how you will keep other variables steady and how you will record the raw data.
• Then come your raw results, the processed data (means, a graph), a conclusion that answers the purpose, and the Earth and Space Science that explains what happened.
• Merit asks for more: a method you have trialled and then confirmed or improved so the data are valid and reliable, a conclusion the data really support, and an explanation of the science, not just a description.
• Excellence adds two things: explaining how your method made the data reliable, and explaining the science in detail by tying each result, its interpretation and the conclusion back to it.
• Your teacher can give background and a broad topic, but the purpose, the method and the investigation itself are yours; there is no template to fill in.

An investigable purpose

• An investigable purpose names one thing you change (the independent variable) and one thing you measure (the dependent variable), in a context from Earth or space.
• Our class's context: the Moon is covered in craters of very different widths, made by rocks hitting it at different speeds. Their purpose: to find out how the drop height of a marble affects the diameter of the crater it makes in flour.
• A hypothesis gives a prediction and a reason: a higher drop gives the marble more energy when it lands, so it should push more flour aside and make a wider crater.
• Other purposes that work well: how particle size affects how fast sediment settles in water, how slope angle affects the mass of sand washed down a model hillside, or how salt content affects whether coloured water floats or sinks in a tank.
• Avoid purposes you cannot test with your own measurements, such as how old a crater is, or ones with no clear measured result, such as whether craters look interesting.

The method: ranges, measuring and controlling

• Independent variable: drop height, at 20, 40, 60, 80 and 100 cm. Five values spread evenly over a wide range are enough to show a trend on a graph.
• Dependent variable: crater diameter in centimetres, measured rim to rim with a ruler in two directions at right angles and averaged, because craters are rarely perfect circles.
• Height is measured from the flour surface to the bottom of the marble, and the marble is released from rest, never thrown.
• The same 20 g marble, the same tray and 4 cm of flour are used every time, and the flour is sifted and scraped level between drops so it is equally loose.
• Three drops at each height are the repeats: they show whether the results agree, and they let you take a mean.

Trial, then confirm or improve the method

• The class did a quick trial run before collecting real data, which is what Merit and Excellence reports describe.
• Problem 1: the crater rim was hard to see in white flour. Fix: a thin dusting of cocoa on top, so the flour thrown out of the crater shows up pale against brown.
• Problem 2: drops from 100 cm sometimes hit the edge of an earlier crater. Fix: smooth the whole tray after every drop, not just the crater.
• Problem 3: two students measured the same crater 0.4 cm apart. Fix: one person measures every crater, always rim to rim at the highest point of the rim.
• Each fix makes the data more valid (it measures what the purpose asks, fairly) or more reliable (repeats give closely matching values).

Graphing processed data

• Put the independent variable (drop height) on the x-axis and the dependent variable (mean diameter) on the y-axis, each with a label and unit.
• Choose even scales that use most of the grid, and plot the means, not every raw value.
• When the points curve, draw a smooth curve of best fit rather than joining the dots or forcing a straight line.
• Do not drag the line to (0, 0): a marble set down gently still leaves a dent about as wide as itself, so a 0 cm drop does not mean a 0 cm crater.
• Describe the shape in words with numbers: from 20 to 40 cm the diameter rose 1.0 cm, but from 80 to 100 cm only 0.4 cm, so the curve levels off.

The science that explains the craters

• A raised marble has gravitational potential energy, Ep = mgh. As it falls this becomes kinetic energy, so a higher drop means a faster marble with more energy at impact.
• On impact that energy pushes flour down and outwards, throws some up and out as a rim and a spray of ejecta, and the rest becomes heat and sound.
• The width grows more slowly than the energy because the crater is a bowl: its volume grows roughly with the cube of its width, so a much bigger energy is needed for a slightly wider bowl.
• Craters on the Moon form the same way, only with rocks arriving at kilometres per second, so even a small rock can blast out a crater many times its own size.
• The Moon keeps its craters for billions of years because it has no air, running water or moving plates. On Earth, weathering, erosion, sediment and plate tectonics wipe most craters away, which is why Earth looks far less cratered.

1. Ep = mghm in kg, g = 9.8 N/kg, h in m
2. 20 cm: Ep = 0.020 × 9.8 × 0.20 = 0.039 J
3. 100 cm: Ep = 0.020 × 9.8 × 1.00 = 0.196 J
4. 0.196 ÷ 0.039 ≈ 5five times the energy
5. 7.5 ÷ 5.0 = 1.5only one and a half times the width
6. five times the energy made a crater only 1.5 times as wide