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Gravity provides the inward resultant force for an orbit. An object in a circular orbit can have constant speed while continually changing velocity direction and therefore accelerating. A stable circular orbit does not mean gravity is balanced by an outward force acting on the same object in the ordinary inertial description. Satellites at different orbital radii have different relationships between speed and period. Use the model and quantities supplied.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Explain orbital motion
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What supplies the inward force in an orbit?
Gravity. The force changes velocity direction.
After watching 2: What is the main energy-releasing process in a main-sequence star?
Nuclear fusion. Light nuclei combine in the core.
What you'll be able to do
Gravity provides the inward resultant force for an orbit. An object in a circular orbit can have constant speed while continually changing velocity direction and therefore accelerating. A stable circular orbit does not mean gravity is balanced by an outward force acting on the same object in the ordinary inertial description. Satellites at different orbital radii have different relationships between speed and period. Use the model and quantities supplied.
A star forms when gas and dust collapse under gravity, heating the core until fusion becomes important. A main-sequence star has a long-lived balance between inward gravitational tendency and outward pressure associated with energy release. Later evolution depends strongly on mass. Sun-like stars can become red giants and then white dwarfs; sufficiently massive stars can undergo supernovae and leave neutron stars or black holes. Fusion in stars and explosive events explain the origin and dispersal of elements.
Light from distant galaxies is often redshifted: spectral features are observed at longer wavelengths. The relationship between distance and recession supports an expanding-universe model. Cosmic microwave background radiation is another important line of evidence for a hot early universe. A redshift observation is data; the model connects that data to a broader history. Do not claim it identifies a unique spatial centre from which all galaxies were launched into pre-existing empty space.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: What does a spectral line shifted toward longer wavelength indicate in the simple recession model?
The source is receding in that model.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.