Gravity

This focus idea is explored through:

Contrasting student and scientific views

Student everyday experiences

Things falling towards the Earth are such familiar occurrences that students may consider these events as ‘natural’ with no need for further explanation.

Even students who use the word ‘gravity’ in an appropriate context may not be able to be able to explain what it is, or will be inconsistent in their explanations.

Research: Skamp (2004)

Students’ views about gravity, the shape of the Earth, and the direction of ‘down’ are often intertwined.

This idea is also developed in the focus idea Forces without contact.

Students may have a range of views about gravity:

• gravity is a push from above (in some instances due to air pressure)
• gravity is related to the presence of air, or is something in the air, so if there is no air, there is no gravity. (Hence there is no gravity on the moon, in Earth satellites or in space; as one rises above the surface of the Earth gravity lessens because the atmosphere thins)
• gravity increases with height
• gravity is significantly less on high mountains or tall buildings and increases as we lose height (which is why falling objects speed up)
• gravity is caused by the Earth spinning
• gravity affects things while they are falling but stops when they reach the ground. It does not operate on things that are moving upwards
• gravity acts upwards on things that are moving upwards
• gravity is a large force
• there is no gravity in a spacecraft orbiting the Earth.

Research: Skamp (2004), Palmer (2001), Gunstone & Mitchell (1998), Gunstone & Watts (1985), Watts (1982)

Scientific view

Gravitational forces are considered to be inherently linked to what we call ‘mass’. There is a gravitational force of attraction between every object in the universe. The size of the gravitational force is proportional to the masses of the objects and weakens as the distance between them increases. Both objects exert an equal attractive force on each other: a falling object is attracting the Earth with the same size force as the Earth is attracting it. The vast difference in mass between the Earth and the falling object means the Earth’s movement is imperceptibly small.

We only notice gravitational forces if one of the objects involved has a huge mass (such as the Earth). In all attempts to compare gravitational forces with other forces, they are relatively much weaker than magnetic and electric forces.

Critical teaching ideas

• Gravitational force is an attraction between masses.
• The greater the size of the masses, the greater the size of the gravitational force (also called the gravity force).
• The gravitational force weakens rapidly with increasing distance between masses.
• The gravitational force is extremely hard to detect unless at least one of the objects has a lot of mass.
• Because the Earth is so big, you have to travel to a very great height above the Earth’s surface before there are any detectable changes in the gravitational pull of the Earth (there is only about 0.25% reduction at the top of Mt Everest).
• The weight force on an object tells us the size of the gravity force from Earth acting on the object.

Explore the relationships between ideas about gravity in the Concept Development Map - (Gravity, Stars)

It is useful to focus separately on two areas where gravity is important:

1. Near the Earth’s surface

Students need opportunities for discussion that draws out the idea that gravity forces on objects are exerted by the whole Earth towards its centre. A gravity force acts on an object regardless of whether it is moving or not moving. In everyday situations, the size of the gravity force on something does not change significantly as it rises above the Earth. (An object needs to go much higher than a jumbo jet for major differences to occur. The size of the gravity force at an altitude of 200km is still about 94% of what it was at sea level.)

The gravity force on an object from the Earth is the same regardless of whether the object is surrounded by air (or water or anything else).

Research: Mitchell (2007)

2. The universe

It is helpful if students understand that the Earth and other planets orbit the sun, and that when things change direction (as the planets are constantly doing) they have a force on them.

This idea is also developed in the focus ideas Pushes and pulls; What is a force? and Day and night.

The idea that there must be a force on the planets that changes their direction can be linked to the gravity force of the Earth on objects near its surface, thus helping students to understand the generalisation that gravity forces exist everywhere in the universe.

Film or videotapes demonstrating the gravitational forces between objects, and that astronauts were able to walk on the moon and drop objects on it because of the moon’s gravity, can help make these ideas plausible for students.

Teaching activities

Challenge some existing ideas

POE (Predict-Observe-Explain): a spring balance with a weight attached hangs inside a sealed bell jar connected to a vacuum pump. Ask students to predict whether, and how, the spring balance reading will change when the air is pumped out. Then ask them to explain their observations. Note: It is helpful to show beforehand that the reading registers a reduction in a net downward force if the weight is placed in water due to the upward push of water. See the diagrams.

Bring out students’ existing ideas

Get students to think about whether the gravity force on them now is a lot more/a bit more/the same/a lot less/a tiny bit less/zero when they are:

• in the same classroom with all the air pumped out
• on top of Mt Everest
• on top of the tallest building in Melbourne
• on the moon (1/6 the size of Earth)
• in the classroom but the Earth has stopped spinning
• next to another astronaut in deep space
• the only astronaut in deep space
• in free fall after jumping from a plane.

Collate responses and conduct a whole class interpretive discussion to clarify students’ thinking and inform further teaching.

Research: Gunstone & Mitche​ll (1998)

Promote reflection on and clarification of existing ideas

Encourage classroom discussions (interpretive discussions) which explore the interaction of gravity forces with air resistance and frictional forces. An approach is to compare the motion of two sheets of paper, one flat and the other crumpled into a small ball, when they are released at the same time from the same height. The gravitational forces acting on each are the same but their relative motion is very different due to the action of air resistance. Extend the discussion to explore the forces acting on skydivers during free fall and when using parachutes.

POE (Predict-Observe-Explain): to help students think about their relative sizes, ask students to predict which of these forces is smallest: magnetic, electric or gravity forces. Then ask them to observe a hair sticking to a charged comb and a hair clip sticking to a magnet. In the ensuing discussion, it may be helpful to point out that the whole of the Earth is pulling on the hair/hair clip. While this activity is imprecise it opens up discussions about the relative strengths of these forces.

Help students to work out some of the ‘scientific’ explanation for themselves

Using the internet, students can look for examples of ‘weightlessness’ and develop explanations using ideas about universal gravitation.