Asking Questions and Making Predictions Gravitational Force and Celestial Bodies
A group of students has been asked to develop a model to represent the gravitational force and to compare the size of gravity among different celestial bodies.
Before developing an idea or a model, it is useful to make a note of all the information you already know. This will help you to make predictions about how the model will work to simply demonstrate an idea or process.
1 Make notes about everything you know about gravity. What is it? What is its range? Does it vary for different celestial bodies? If so, in what ways?
The students develop their model using a tightly stretched rubber sheet. The rubber sheet represents the gravitational field in that region of space.
z Fig. 21.5.1
Tightly stretched rubber sheet
2 Predict what happens when a weight is added to the rubber sheet. What does this tell you about the effect a large mass has on the gravitational field?
The students want to use the model to compare the gravitational force around four celestial bodies: Earth, Earth’s moon, Venus and Pluto. They took the relative mass of Earth to be 1 and noted the mass of the other bodies compared to this in the following table.
Celestial body Earth
Venus
Earth’s moon Pluto
Relative mass 1
0.815
0.0123 0.0022
The students use a mass of 500 g to represent the mass of the Earth. 378
