Before starting this unit, check the children can recall the following ways of using forces to change the shape of materials from the unit Science, Year 3, Forces and magnets:

• Forces (push, pull and twist).
• Contact forces (friction).
• Non-contact forces (magnetism and gravity).

Use the Activity: Unbalanced forces mind map (one each) to elicit the children’s prior knowledge if required.

Show the Pupil video: Gravity, which covers the work of Galileo Galilei and Isaac Newton, who helped develop the theory of gravitation.

Ask the class to discuss the following questions with a partner:

Take feedback and clarify that mass is the amount of matter in an object: it tells us how much stuff is in that object.

Display slide 1 of the Presentation: Investigating gravity and inform the class that they will conduct a pattern seeking enquiry to investigate the question:

• Does the size of an object affect its gravity?

Continue to slide 2 and read through the three options for predicting the results of the investigation.

Ask the children to discuss the predictions with a partner and then vote on which they think is true. Record the class prediction (the most popular choice) using the pen tool, which should be choice three: the larger the object, the larger the gravity because it has more mass. 

Arrange the class into ten groups and assign each group a celestial body and its relative diameter from the Resource: Diameters of celestial bodies (pre-cut, one planet per group). Explain that the diameters are calculated relative to the Earth if the Earth is one metre in diameter. Provide each group with a 15-metre tape measure, a piece of chalk, a pencil and the Activity: Celestial bodies data table.

Take the class to a large outdoor space where they can draw on the floor with the chalk. Instruct the groups to measure the correct diameter of their celestial body using the tape measure. Explain that they should then use the chalk to draw a circle of that diameter to represent their celestial body. Ask them to label their celestial body (e.g. Moon, Earth, Mars, etc.).

Allow the groups to draw and label their celestial body and instruct them to rotate around to each of the ten and measure and record the diameter of each on their data table.

Return to the classroom and show slide 3, which shows the correct data. Allow the groups to check and correct their data. Ask the groups to discuss the following questions and take feedback:

Explain to the class they will use data from NASA to find out if, as predicted, there is a relationship between gravity and diameter; specifically that gravity increases as diameter increases. Ask them to find the data from the Resource: NASA relative gravity data stuck around the room (see Teacher knowledge) and record the gravity for each celestial body in their data table.

Click the ‘Gravity’ data column to reveal the correct data and encourage the groups to check their data.

Analysing and conclusion writing

Hand out a green and a red (or orange or pink) highlighter to each group and ask them to look through the data in the third column (gravity) carefully and highlight the data as follows:

• If the number is larger than the preceding number, highlight it in green because it fits the predicted pattern of showing an increase in gravity with size.
• If the number is smaller than the preceding number, highlight it in red (or orange or pink) because it does not fit the predicted pattern and shows that gravity does not increase with size.

Click the data in the gravity column again to reveal the correctly highlighted data and ask the class to discuss the following questions:

Continue to slide 4, which uses questions to prompt how to structure a conclusion. Ask the groups to discuss the questions and write a conclusion in their books.

Click on each question to build the conclusion; read through the model conclusion. Allow time for the pupils to compare to their own conclusions.

Display slide 1 of the Presentation: What affects gravity?, which has a graph showing relative mass compared to the relative gravity of celestial bodies.

Ask the groups to discuss the following questions:

Explain to the class that neither the size (diameter) of the planet nor the mass of the planet showed a pattern that was always true.

Remind them that Newton theorised that gravity was affected by both mass and distance. Continue to slide 2, which shows the data tables for mass and diameter compared to gravity. Ask the groups to discuss the following question:

Take feedback from the class and continue to slide 3, which shows a diagram to explain why the gravity on Mars and Mercury is similar.

Explain that the gravity you feel on a planet depends not only on its mass but also on its size, specifically the distance from its centre to its surface. Mars has about double the mass of Mercury, which would make its gravity stronger if both planets were the same size. However, Mars is also much larger than Mercury, meaning its surface is farther away from its centre than Mercury. This greater distance reduces the gravity you would feel on the surface of Mars.