What the Activity is for

This activity provides a memorable experience relating to the gravitational pull of the Earth and establishes that objects fall because of the external action of gravity.

What to Prepare

• horizontal beams (this activity involves pupils hanging by their hands and arms from a horizontal beam and the best place for the lesson is the school gymnasium or sports hall – remember to check availability).
• stop watch
• clip-board

Safety note: Check with the PE department as to the safe working load of the beam and how to secure it safely in position. Use the minimum height possible. Beware of excessive competition leading to muscle damage. Do not attempt to improvise a beam in any other location (e.g. using the suspension beam in a physics laboratory).

What Happens During this Activity

The build up to the beam-hanging activity involves introducing and talking through the strange property of gravity, that it acts at a distance. The idea then is to get the pupils into a situation where they are away from the Earth's surface and being attracted towards it.

This is easily achieved by getting the pupils to hang on to a horizontal beam with their hands, such that their feet are just clear of the ground. You might organise the activity such that five or so pupils of similar height hang from the beam at once and the big challenge is to:

Battle the pull of gravity for as long as you can!

The spectacle is quite something! As the pupils hang from the beam, sweat pouring from their brows, pass up and down the line asking:

Teacher: Can you feel the pull of gravity? Can you feel it pulling you down?

(Some might refer to this as kinaesthetic learning.)

Many classes like to engage in this activity as a competition, with no shortage of volunteers to participate. Experience has shown that the competition is often won by relatively small and wiry pupils who appear to have the capacity to withstand the pull of gravity for as long as they like! It is the stronger boys who struggle to withstand the greater pull of the Earth on their more massive bodies much to their obvious embarrassment, and to the delight of others.

What the Activity is for

This activity is to remind pupils that to escape the Earth's gravitational field a force is needed. Establish that the bigger the mass of the rocket the bigger the force needed.

The activity also provides the opportunity to introduce the principles of rocket propulsion, showing that the force of the rocket on the water produces a force of the water on the rocket (an example of Newton's third law), which lifts it off the ground in a spectacular fashion.

What to Prepare

• film canisters
• postcards
• coloured pens
• indigestion relief tablets
• water
• support sheet
• A clip of a rocket lifting off

  download clip to view

• eye protection

Support sheet

Safety note: Pupils should wear eye protection and stand well back (at least two metres) from the rocket once it has been primed. If the rocket does not lift off the teacher should knock it over with a metre rule before dealing with it.

Building the rockets

What Happens During this Activity

Introduce the activity by showing some clips of rockets taking off. Ask why so much fuel is needed and point to the fact that the gravitational force at the surface of the Earth is large. Generating a rocket thrust to overcome the pull of gravity requires a lot of fuel. Tell the pupils that this activity is going to involve making a rocket on a much smaller scale.

Explain that the fuel you are going to use is compressed carbon dioxide. Demonstrate the effect of adding the indigestion relief tablet to the water and ask the pupils what would happen if this was carried out in a sealed space where the carbon dioxide could not escape. Then show what happens if bits of indigestion relief tablet are dropped into a canister filled with water and the lid put on. Gas is released inside the canister and pressure builds until the lid is pushed off and the water shoots out backwards, at high speed. This provides the thrust for the rocket.

You might use an inflated balloon to model the effect and ask why the rocket will be better than the balloon. This is because the water is much more massive than the air inside the balloon and will produce a greater force on the rocket.

Instructions:

1. Roll the postcard into a tube. Slide the empty film canister in at one end with its top on. Now tape the seams of the postcard together and tape the postcard to the film canister so that the two do not separate.
2. Cut two triangular, paper fins and tape them to the rocket.
3. Make a small paper cone and tape it to the top of the rocket for the nose cone.
4. Hold the rocket upside down and fill one quarter of the canister with water.
5. Add half a tablet to the film canister (you will need to experiment with the amounts of tablet used with different size canisters) and quickly snap on the lid.
6. Place the rocket on the ground, lid down, stand back and count down while you wait for the launch.

What the Activity is for

The aims are to establish the strength of the gravitational field at the Earth's surface and to appreciate that the strength of the gravitational field will be different at the surface of other planets.

According to Newton's model, all objects with mass exert a gravitational force on all other objects with mass.

At any point the strength of the gravitational field due to any object with mass (such as the Earth) is given by the gravitational force per kilogram.

At the surface of the Earth the strength of the gravitational field is about 10 N on each kilogram (10 newton / kilogram). On the surface of the Moon the strength of the gravitational field is about one-sixth of what it is on the Earth (2 newton / kilogram). The strength of the gravitational field is different at the surface of all of the other planets because they have different masses.

What to Prepare

• spring balances 0–10 newton
• 1 kilogram of 100 g slotted masses
• selection of 200 g baked bean tins (with labels retained) of different masses

To prepare the tins use 200 gram tins, with the labels kept on. Drill a large hole (30 mm diameter) in the base, drain the contents, wash thoroughly and add sand and masses as required. Reseal the hole with a metal or plastic patch attached with epoxy resin adhesive (a false patch can be added to the Earth can of beans as well!).

Combinations of 100 gram masses and sand can be used to obtain the appropriate masses, as follows:

Venus 180 gram

Earth 200 gram

Mars 80 gram

Jupiter 520 gram

Label the top of each tin with a letter so that the order is not obvious.

Safety note: If can openers are used, all sharp edges must be sealed effectively. If tins are not washed out thoroughly, decomposition of the food residue will introduce a health hazard. Alternatively, discard the tins after use.

Instructions

What Happens During this Activity

In this activity, pupils measure the strength of the Earth's gravitational field, and begin to explore the difference between the mass of an object and the gravity force acting on the object. After this activity, they should also begin to appreciate that placing the same mass on other planets will result in different gravity forces acting on those masses.

Instructions:

1. With a spring balance and a 1 kilogram set of 100 gram slotted masses in front of them, ask the class to measure the strength of the force with which the Earth pulls on 100 gram (the wording is important here). Confirm that it is about 1 newton, the gravity force (as it happens) of a typical apple. To get a feel for the gravity force of a 100 gram mass encourage the pupils to hold it in their hand. Explain that since gravity force is a force, we say that its gravity force is about 1 N.
2. Ask pupils to then predict what the force of the Earth will be on 200 gram, and then perhaps 600 gram.
3. Next ask pupils to measure the strength of the force with which the Earth pulls on 1 kilogram. Confirm that this is about 10 newton. This is a special value, which we call the gravitational field strength of the Earth.
4. The gravitational field strength at the surface of the Moon is different, because the Moon has a smaller mass. It is about one sixth of the value, around 1.7 newton / kilogram. Ask the pupils to put just 200 gram on the spring balance and feel the gravity force acting on that mass with their eyes closed. If they were on the Moon, this is roughly how heavy a kilogram would feel.
5. Attention should now be drawn to the selection of baked bean tins, all nominally 200 gram. The pupils should be told that one of them is how a can of baked beans would feel on Earth, one on Venus, one on Mars and one on Jupiter. They should identify which is which (it is likely you will need to share sets of tins between several pairs).

The whole activity should take no more than 40 minutes.

The important thing is for the pupils to get a feel for the concept of gravitational force and an appreciation of what gravitational field strength means and how it varies from place to place.

For reinforcement, a set of simple calculations could be performed:

Teacher: Calculate the gravity force acting on an object of mass Z on planet Y, where the gravitational field strength is X newton / kilogram.

Here's an example:

Calculate the gravity force acting on an object of mass 12 kilogram on Neptune, where the gravitational field strength is 14 newton / kilogram.

Answer: the gravity force is 12 kilogram  ×  14 newton / kilogram, which is 168 newton.

As an extension, you might mention the effect of distance on the value of the gravitational field strength – nothing quantitative, just an appreciation that it will decrease with distance. So, in the case of the Earth, as you go into space gravity gets weaker.

However, you have to go a very long way from the Earth for gravity to become insignificant. At the height of most orbits for spacecraft, there is still an appreciable gravitational field. That is what holds the spacecraft in orbit.

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 and unzip files to use

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.

What the Activity is for

The aim is to provide a framework to discuss the two effects that mass has on acceleration when something falls.

What to Prepare

• the modelling program VnR

This file shows how the model is built, and will help you to prepare your support for children thinking through their own constructions.

download clip to view

What Happens During this Activity

Using VnR, build up a model like this one. Probably you'll want to have a couple of masses to hand – 1 kilogram and 5 kilogram – to use to prompt questions, so making the whole experience more interactive for the class than is possible with this clip, which just lays out the argument.

What the Activity is for

Here pupils check their understanding of the connections between the central ideas in this episode.

What to Prepare

• this interactive object

  download and unzip files to use

or

• printed copies of this prepared sheet
• scissors, glue, paper

Support sheet

What Happens During this Activity

This is rather a convergent activity. Children working in small groups around a computer screen, or collectively on a large shared display, can make up the map by choosing the statements in the drop-down menus. The display can be checked, then printed to provide a summary of their understanding.

A more divergent activity can be enjoyed by using paper – cutting the pieces out and arranging them on paper, before writing in the links. Alternatively, a number of concept mapping pieces of software are available, and you can prepare your own files as starters, adjusting these to suit your classes. The key is to think of what pieces of thinking you want them to do. These correspond precisely to the links you leave out on the concept map.