Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How to:

• emphasise that the pull of gravity depends on the environment
• keep separate the effects of atmosphere and the pull of gravity
• establish gravity as a universal force, occurring everywhere
• establish gravity as a universal force, acting on stationary and moving objects
• use the term force of gravity consistently
• connect interactions on this Earth with those occurring elsewhere in the universe

Teacher Tip: These are all related to findings about children's ideas from research. The teaching activities will provide some suggestions. So will colleagues, near and far.

Focusing on the physics:

Representing–noticing–recording. How to:

• keep force and energy separate
• explicitly model the action of gravity as a force exerted by the environment
• introduce a variety of contexts to explore how the pull of gravity varies
• draw the force of gravity acting on objects
• describe the variation in the force of gravity with separation
• account for the variation in the magnitude of the force of gravity
• use the idea of a field
• separate the force of gravity acting on an object from the mass of the object
• account for objects with different masses falling at the same rate
• communicate an accurate understanding of mass
• provide a coherent account of the experience of weightlessness

Teacher Tip: Connecting what is experienced with what is written and drawn is essential to making sense of the connections between the theoretical world of physics and the lived-in world of the children. Don't forget to exemplify this action.

Starting points

What would you expect children, on entering secondary school, to know about the force of gravity?

We asked a group of 11-year-old pupils the following question:

Teacher: I throw an object up into the air. Are there any forces acting on it after it has left my hand?

Here are a couple of sample answers.

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Anya (clip 1) states that gravity is pulling the object down. She then goes on to suggest that there is also an upthrust acting on the object and that if the gravity overpowers the upthrust the object will fall. In reality the object is accelerated by the upwards force of the hand (during the act of throwing), but after that the only force acting on the object is the downward pull of gravity (ignoring air resistance).

Sarah (clip2) knows that gravity is acting on the object at the top and as it comes down, but is not sure about what happens on the way up. Pupils often struggle with the idea that a force can act in the opposite direction from the direction of travel of an object (gravity acts down as the object moves up).

Falling because of gravity

Wrong Track: When you drop a ball it just falls. It falls to the ground 'cause it's heavy.

Right Lines: When you release a ball from your hand it accelerates to the ground because of the gravitational force of the Earth pulling on it.

Objects fall because of the gravitational pull of the Earth

Thinking about the learning

A key step on the journey is to move away from thinking that objects fall because they are heavy. In other words, believing that it's something within the object (its heaviness) that makes it fall. The Newtonian view is that objects fall because of the action of the external gravitational pull of the Earth.

Thinking about the teaching

A possible activity is to set the pupils the challenge of battling the pull of gravity. This involves taking the class to the school gymnasium and getting volunteers (there's never a shortage) to hang from a horizontal bar so that their feet are just clear of the ground. You can then time how long each can withstand the pull of gravity.

The idea here is to set up a situation where the pupils can get the idea (and feel) that gravity is pulling them down towards the centre of the Earth. It is always a memorable lesson as you encourage the pupils to battle against the pull of gravity and ask can you feel the pull of the Earth?.

Interestingly, its often the little, thin pupils who can withstand the pull of gravity the longest, with the more likely stronger boys struggling against the larger gravitational pull on their greater masses.

Gravity without an atmosphere

Wrong Track: On the Moon things just float around because there is no gravity. This is because there is no air on the Moon.

Right Lines: Gravity exists on the Moon but it is less than on the Earth because the Moon is smaller. There is no atmosphere on the Moon, but this does not affect the gravitational force in any way.

Gravity on the Moon

Thinking about the learning

A common, but incorrect, line of reasoning among pupils involves making the link between gravity and the atmosphere or air.

Thinking about the teaching

The obvious first step here is to tackle the idea that there is no gravity on the Moon. Sometimes pupils will tell you that astronauts just float around on the Moon. Is this really the case? Show some pictures or videos of the Moon landings. Agree that there is no atmosphere on the Moon and make the point that this has no effect on the gravitational force.

A universal force

Wrong Track: There is no gravity out in space, so things just float around.

Wrong Track: Astronauts in spacecraft are weightless, you see them floating around. This is because there's no gravity.

Right Lines: Gravity is a universal force acting throughout space. For example it's the gravitational pull of the Sun that keeps Pluto in its orbit. It is impossible to find even the remotest part of the universe where gravitational forces are not acting. Astronauts orbiting the Earth in craft such as the International Space Station do experience weightlessness. They do float around. But bear in mind that the International Space Station orbit is only about 400 km above the surface of the Earth. This is relatively close to Earth and the Earth's gravitational pull certainly works there. In fact it is the Earth's pull that keeps both the astronauts and the Space Station in orbit and gives rise to the condition of weightlessness. As both the astronauts and the Station orbit the Earth, they are falling towards the Earth at the same rate and the astronauts appear to be weightless (the floor of the space station does not support the astronauts).

Gravity is everywhere

Thinking about the learning

A major challenge is getting across the idea that gravity is a force that acts everywhere in the universe and is not simply restricted to the surface of the Earth. A more limited view of gravity takes pupils down various wrong tracks.

Gravity acts between all objects

Wrong Track: If the book falls off the table, it's because of gravity pulling it. If the book is on the table, it's just sitting there, gravity doesn't come into it.

Right Lines: Gravity is a universal force that acts between all objects with mass in the universe. An object does not need to be falling to be under the influence of gravity. If you are asleep in bed you are being pulled down onto the bed by the Earth's gravitational force. The bed springs push back up on you and you stay in your sleeping position.

Gravity is always acting

Thinking about the learning

Some pupils only associate gravity with situations where something is falling.

Thinking about the teaching

It is worth considering a range of objects in a whole variety of situations and asking whether or not gravity is acting on them. The answer is always yes.

For example, try water at the top of a waterfall compared with water in the pool at the bottom of the waterfall.

Higher means stronger

Wrong Track: Gravity gets stronger the higher up you get.

Right Lines: If this statement was true, it would get harder and harder to escape from the Earth. However, pictures of a Space Shuttle taking off show that the largest force is required at the beginning and as you get farther from Earth, smaller rockets are required. The force gets weaker the larger the distance between the objects.

Height and falling speed

Thinking about the learning

Some pupils believe that gravity increases with height.

This wrong track idea is likely to be based on observations that if an object (such as a brick) is dropped from a greater height, then it has a bigger impact on hitting the ground. This is true but it's because the additional height increases the energy in the gravity store (which is augmented as the brick is dragged away from Earth) and not the gravitational force acting on the brick.

There's a good chance you could improve your teaching if you were to:

Try these

• separating the pull of gravity acting on an object from any compression or tension forces supporting the object
• exploring situations where gravity still pulls, but there is no atmosphere
• modelling the action of the pull of gravity as a force
• drawing forces precisely

Teacher Tip: Work through the Physics Narrative to find these lines of thinking worked out and then look in the Teaching Approaches for some examples of activities.

Avoid these

• conflating mass and weight
• restricting drawing the pull of gravity on objects to situations where these objects are falling
• being casual with the term weightless
• stating that a force is zero, when it's only very small

Teacher Tip: These difficulties are distilled from: the research findings; the practice of well-connected teachers with expertise; issues intrinsic to representing the physics well.