Falling: an investigation

What the Activity is for

This activity provides direct evidence that objects of very different masses fall at the same rate, regardless of how heavy (massive) they are, and a way of explaining this phenomenon.

As long as air resistance is negligible, all objects fall at the same rate. Galileo famously, though almost certainly apocryphally, dropped two different things from the top of the Leaning Tower of Pisa and, to the amazement of the watching crowd, the things reached the floor at the same instant. Amazement because the common wrong track thinking is that heavier things fall faster.

Whilst this is true if you compare the fall of a piece of paper with that of a stone, it is not true if you scrumple the paper into a small ball. Much to the amazement of most, if not all, pupils they both arrive at the ground at the same time, although many will swear blind that the stone got there first.

However, by thinking through a simple thought experiment it is possible to understand that this result is not amazing at all, but is to be expected. The conclusion is reinforced by seeing it happen in the lab, or on video, or by a dramatic reconstruction of the Pisa experiment (or maybe all three).

What to Prepare

• spring balances 0–10 newton
• a melon and a peach to be dropped safely from a great height
• guinea and eather demonstration
• two files, one a clip of falling on the Moon, and one to support a discussion (download below)

Safety note: The glass tube for the guinea and feather demonstration must be intended for reduced pressure. All tubes should be checked for cracks and scratches, because these seriously weaken them.

To help you, here is a teacher showing the guinea and feather demonstration.

download clip to view

Teacher presentation of the argument

What Happens During this Activity

Use the interactive object you have downloaded to support your discussion.

Show steps 1 and 2: Knowing the gravitational field strength of the Earth, pupils should be able to say what the value of F is (10 newton). When released from rest, the subsequent motion (uniform acceleration) is determined by the size of the mass and the force on that mass. This should be discussed with the class.

Show step 3: No surprises, these two masses will fall at the same rate.

Show steps 4 and 5: Ask what difference it would make if we placed the two 1 kilogram masses so that they just touched and then released them (the logical answer is that it would make no difference, the two masses will fall side by side). Now ask whether it would make any difference if the two masses were joined at the point of contact (there is no reason why it should make a difference).

When the pupils have agreed that it will make no difference put up step 6. This is the conceptual leap. Point out that the mass is actually two kilograms and that they have just argued that it will fall at the same rate as a 1 kilogram mass. The argument can then be extended to as many masses as you like.

We'd suggest you discuss the idea of a thought experiment here – this is a classic example. By imagining a model or picture of the world, we can begin to realise that some of our ideas about that physical world (such as heavier objects falling faster) are, in fact, incorrect.

The guinea and feather demonstration

This is a classic physics demonstration that shows how, in the absence of air, both a guinea (a small coin) and a feather will fall at the same rate. The most famous demonstration of this was carried out on the Moon (with a hammer and a feather) during the Apollo 15 mission. You might show the video clip of this event.

Alternatively it is possible to carry out the demonstration yourself with a long tube which has a rubber bung at each end (see video clip). One of the bungs has a tube in it that is connected to a vacuum pump.

Safety note: You must use an appropriate piece of low pressure specified glassware for this work. All tubes should be checked for cracks and scratches; these seriously weaken glassware and so such tubes should not be used. All present will require eye protection. The teacher should wear gloves to hold the glass tube in case it implodes.

Begin by showing the pupils the tube with air in it. Turn the tube upside down and let the pupils watch how the guinea (coin) and the feather fall. The feather will fall much more slowly.

Now connect the tube to a vacuum pump for 15 second or until the note of the pump changes. Invert the tube again, telling the class to watch the feather and the guinea closely. The feather will now fall like a stone, reaching the bottom at the same time as the guinea. Repeat as often as is necessary to convince your pupils.

The melon and peach demonstration

Dropping a melon and a peach simultaneously from a great height (second-floor window) will provide memorable proof that the mass of an object makes no difference to the rate of fall.

You should be aware of the mess that will be created by this demonstration. This can be minimised by placing a large plastic sheet on the ground. You also need to be aware of the fact that, from too small a height the fruits will remain intact, while from too great a height air resistance might become more significant and the two will not reach the ground simultaneously. The moral here is that practice makes perfect.

This is certainly one of those demonstrations that makes physics good fun and simply unforgettable.

Safety note: Do not allow pupils to lean out of the window. An external fire escape staircase with a hand-rail will reduce the risk and allow good viewing. No one should be present in the drop zone – a melon has a similar mass to a brick.