This Subject Update is designed to guide the non-specialist science teacher in developing investigation skills for younger students. It uses as an example the Submarine STEM Submarine shape investigation.
This investigation involves students investigating the best shape for a “submarine”. They do this by dropping different modelling clay shapes into a cylinder of water and timing how long they take to reach the bottom.
It is important to frame the question in the right way, so that it is a testable question. A testable question is one that can be investigated using a scientific experiment.
Consider the question ‘What is the best shape for a submarine?’. How would students test this?
For this question to be testable, we need to define what is meant by “best”. In this case, it is that the “submarine” falls at a slow rate so that it reaches the bottom of a cylinder in a certain number of seconds.
So, the question could be reframed ‘What submarine shape falls at the right rate?’.
If the opportunity arises, before the investigation begins in earnest, give the students an opportunity to play and have a dry run with the equipment. This can help the students become familiar with the method, as well as the chance to identify any problems with the method that can be corrected before the experiment starts.
This experience will also allow the students to more easily identify the variables involved. What can they change? What must they keep the same?
There are a number of input variables (independent variables) that the students could change in this experiment, but the shape is the variable assumed to be investigated for the purpose of this lesson. It is what our question is testing.
Some possible independent variables are:
There may be more, but only one independent variable should be selected and tested.
Output variables (dependent variables) are variables that can be measured to get a result. There are usually fewer output variables than input variables. The time taken to fall through the cylinder is the suggested output variable to measure.
Control variables are variables that are kept the same throughout the whole experiment. Usually, they are made of all the other possible independent variables that were not chosen. So, in this example, they would be:
It is strongly advised that the teacher trials the experiment before the lesson, as the parameters of the investigation will depend on the practical equipment and apparatus available in the school.
Suggested equipment and sizes are given in the investigation Activity Overview, but this experiment can be conducted with similar equipment that may be more readily available.
The idea is for the student to find a shape of modelling clay that takes seven seconds to fall through a cylinder of water – not too fast, not too slow. Submarines need a controlled descent for safety reasons.
A piece of modelling clay around the size of a marble is suitable for a 250ml cylinder. You can adjust accordingly for the equipment you have. Moulding the clay until it is broadly the shape of a rugby ball gave reasonable results in tests, but you may find something more appropriate in your tests.
For each test, the shape should be released from the same position – preferably submerged in the water upon release, and not released from above the surface of the water. This will prevent it speeding up in the air above the water before the test has begun.
On the cylinder, a start and end point should be identified and marked. The start point should be a few centimetres below the surface of the water – low enough to allow the shape to be submerged without crossing the start line. The end point should be above the bottom of the cylinder, to allow some space for previous attempts / pieces of modelling clay to collect without getting in the way of the falling pieces of clay being tested. You should start the stopwatch as the bottom of the shape touches the start point, and stop the stopwatch as the bottom of the shape touches the end point.
Once you have identified a suitable size of modelling clay, students are encouraged, through trial-and-error and adjustments to their shapes based on observation and reasoning, to find a shape that takes exactly seven seconds (or whatever time the teacher selects) to fall through their given cylinder of water.
The following variables should be kept constant for each test:
The only thing that should be changed for each experiment is the shape of the modelling clay, the input or independent variable. This ensures that the selected variable is the only thing being tested.
Encourage your students to make predictions. It’s what real scientists do! Ask your students to think about why scientists make predictions.
A prediction is something a scientist thinks will happen. A prediction is not just a guess but based on previous experiences and observations. This past experience gives them an interesting idea, or a pattern they want to investigate.
Scientists will then design a test to find out if their prediction was right or wrong. If their prediction was wrong, and they made no mistakes in their test, the scientist will change their prediction or idea, and design another test.
For students predicting what shape of “submarine” might fall at a slower rate, they may consider some of the observations they might have made about objects moving through the air. Aircraft are designed to be more streamlined or pointy and move through the air more easily.
Flatter shapes, such as leaves or parachutes, fall through the air more slowly. Using observations such as these will turn students’ guesses into predictions. Scientists actually like their predictions being wrong, because it gives them something new to investigate or find out!
Students should also be encouraged to develop a hypothesis. You can think of a hypothesis as a statement that can be tested. A good hypothesis has a good, scientific reason behind it. For example:
“Flatter shapes will fall slower than streamlined or pointy shapes. This is because flatter shapes experience more water resistance due to their larger surface area.”
Scientists record the results of their experiments to see if their prediction or hypothesis is correct. These results are often in the form of data, in this case the time it takes for the “submarine” to fall through the water.
Students should record each shape, either by drawing it or naming it, and the time it has taken to fall through the cylinder. They should also leave space to record three results for each shape tested.
It is important to re-test each shape, to make sure no mistakes have been made. Imagine if you tested a shape three times, and it recorded times of 15 seconds, 8 seconds and 8 seconds. It is clear from this that the time of 15 seconds was probably caused by accident, perhaps an error using the stopwatch. If, however, this test was only conducted once, the student would have only had the time of 15 seconds recorded, and no way of knowing this was an error.
These results are known as anomalies. If an anomaly occurs, students should consider conducting the test again to see whether the anomaly was a one-off or if it occurs more often. If after retesting it appears that the anomaly was a one-off, then this result can be discarded from the overall results.
An example of a results table is included below, which teachers may wish to re-draw or use for all students. Teachers are encouraged to model how to record results to the students before their tests.
Students should be encouraged to further their experiment in a manner of their own choosing.
Perhaps the students didn’t find a shape that hit the target time, but found two that were close? They could adjust these shapes based on the other results they have obtained. If they need to slow a shape down, make it slightly flatter. If they need to speed it up, make it slightly more streamlined!
If possible, the students could repeat the test using a different type of modelling clay or other shaping material to see if they get similar results. If they can show that streamlined shapes made out of different materials travel faster, they have more evidence to prove that shape makes a big difference to the speed of fall. If they try a different colour modelling clay, but keep everything else the same, they should hopefully prove that colour makes no difference, and that shape is the factor that affects the speed.
Scientists like having all the evidence they can get – it helps them to draw better conclusions.
Students should use the results they have obtained in their experiment to test their hypothesis.
After deciding if their results show if their hypothesis is true or not, they should consider whether or not their results were obtained fairly. After all, results that prove or disprove a hypothesis mean nothing if they were not obtained with a fair test!
We call this being valid. Results need to be fairly obtained to be valid. Invalid results mean nothing, and cannot prove or disprove a hypothesis.
Fair tests are all about making sure that only the variable that needs changing (in this case the shape) is changed, and nothing else. All the other variables named above should stay the same.
Ask the students to consider how they kept each variable the same each time. For example:
If the students could not answer these questions, then they have suggestions on how to improve their test next time to make it fairer.
Students should also consider if they have repeated tests for each shape, and if the repeat results agree with each other (see the second paragraph in “Recording Results” above). If they have not repeated tests, this is another suggestion for improvement next time.
Conclusions should be drawn from the observations and results the students have obtained, and the scientific principles taught in the lesson.
Do a students’ results agree with the scientific theory? Are ‘flatter’ shapes slower overall? Is this because of the surface area, and forces involved?
Students are expected to draw a conclusion backed up by theory. For example, “In my experiment, flatter shapes were slower than streamlined shapes. This is because they have a larger surface area, meaning that the force of water resistance is higher.”
Where students’ results do not match the scientific principles, students should try to determine why this is not the case, and perhaps analyse where they did not conduct a fair test. For example, “In my test, flatter shapes seemed faster or the same speed as streamlined shapes. This may have been because I did not start and stop the stopwatch at the right times, because the start and stop points were not clearly marked. Next time, I will make sure I have clearly marked points, and re-test each shape three times to make sure my results agree.”