Intrigue your students with some surprising experiments – it’s a great way to challenge their intuitions and explore the laws of mechanics.
Why do we teach science? Is it to give students new ideas – or to remove ideas they already hold? In some areas, such as the physics of free fall, it seems the latter is the real task: while the ideas described by Newtonian mechanics are simple enough, our intuitions can make them difficult to accept.
When students arrive at physics classes, they already possess firm ideas about many physical situations. Research in science education has shown that students often hold a set of intuitive rules about the mechanisms underlying physical changes (Viennot, 2014). Some of the most widely held of these rules include:
We use the term ‘type’ here to mean any of the following:
In this article, we analyse three simple experiments that involve the rule of type. They can be used to reveal how students reason and to enable teachers to present physics concepts in a more accessible way.
The first experiment is an extension of the popular textbook problem that asks students to draw the forces acting on a coin tossed straight up into the air. Students have difficulty realising that (if we ignore air resistance) once the coin has left the hand, the only force acting on it is its weight. In fact, the coin is in free fall while going up! Replacing the coin with water demonstrates this fact beyond doubt.
The other two experiments explore ideas of balance: the second using a pulley, and the third using buoyancy. All the experiments are suitable for science students aged 14–16.
This is a brilliant experiment to demonstrate weightlessness. We suggest doing this as a teacher demonstration (rather than student experiment). Allow 30 minutes for the experiment and discussion, plus 10 minutes for preparation.
When the bottle is at rest, water flows out through the hole in the base. This is consistent with the rule of type: the cause (the water’s weight) is directed downwards, and the effect (the downward flow of water out of the bottle) is similar. Most students correctly predict that when the bottle is falling, the outflow of water will be ‘cancelled’ during the fall because both the bottle and the water are moving downwards in the same way.
But what happens when the bottle is thrown upwards: will water still flow out? Most students (more than 80% of ours) think it will. Their reasoning is often based on what we experience when standing in a lift (Corona et al., 2006). As it moves upwards, we feel heavier, so the water should ‘feel heavier’ and thus flow out faster.
This prediction is wrong. As the demonstration shows, no water flows out during the upwards or downwards motion of the bottle. After the bottle leaves the hand, the only force acting on it is its own weight, so it is in free fall – even when the bottle is moving upwards (because it is decelerating). Like orbiting astronauts, the water in the bottle ‘feels’ weightless – and no weight means no water flowing out.
The students’ erroneous reasoning is in line with the rule of type: here, the imagined cause is the supposed increase in the water’s weight due to being launched upwards, so the effect should be an increase in water flowing out. The experiment shows that what happens is the opposite of what’s expected, challenging the students’ intuitive reasoning.
In this experiment, two weights are balanced – but, unlike with a set of balance scales, they are not on the same horizontal level. Allow 20 minutes for the experiment and discussion, plus 10 minutes for preparation.
Steps 1–4 are preparation; the demonstration is steps 5–6.
When we ran this experiment, almost 50% of our secondary school students predicted that the raised block would return to its initial position – despite the fact that if the two objects are in static equilibrium, the net force acting on each object is zero and the block will not move.
When students see the block and bucket at different horizontal levels, they seem not to notice that the two objects are balanced. Instead, they focus on the most striking observable feature of the situation: the different heights of the two objects. This appears to represent an effect – so, in line with the rule of type, there must be a cause of the same type: unequal forces. As a result, many of the students conclude that one object (the one nearer the ground) must be heavier than the other one.
Steps 1 and 2 are preparation; the demonstration is steps 3–6.
When we ran this experiment, more than 40% of our students predicted that the higher the bottle is in the tank, the greater the buoyancy acting on it. In fact, the bottle is in static equilibrium in both positions, so the net force acting on it is zero.
When students see the bottle floating at different levels, some of them seem to ignore the fact that the forces on it are balanced. As in the previous experiment, they focus instead on what they see – unequal levels – and infer that there must be a cause of the same type: unequal forces. The weight of the bottle is the same throughout, so they conclude that when it is higher up, it must have greater buoyancy.
We believe that these counter-intuitive demonstrations are more than just curiosities: we can use these mismatches between intuition and experiment in the classroom to help understand students’ misconceptions. And by integrating these experiments into teaching in a structured way, we can provide some real benefits for learning.