Explain how gravity forms a star from dust and gas, initiating fusion and creating equilibrium between gravitational collapse and expansion from fusion energy.
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GCSE Physics Revision
Learn it. Recall it. Revise it.
GCSE Physics revision
Our solar system
Solar system; stability of orbital motions; satellites (physics only)
Your specification
AQA student objectives
Learning pathway
All · Most · Some
Apply the scientific explanation of our solar system to a relevant example.
Analyse a new situation involving our solar system and explain the scientific reasoning.
Revision summary
Key knowledge
Read on screen, then print for Cornell-style active revision.
What Is an Orbit?
- An orbit is the curved path of one celestial object or spacecraft around another, such as the Earth orbiting the Sun or the Moon orbiting the Earth.
- Two key factors determine a stable orbit: the instantaneous velocity of the orbiting object and the gravitational pull of the larger central object.
Newton's First Law and Orbital Motion
- Newton's first law states that an object will continue travelling at the same velocity unless acted upon by a resultant force.
- Without gravity, the Moon would travel in a straight line at its instantaneous velocity of approximately 1,023 m/s forever.
- Earth's gravitational force acts as an attractive force on the Moon, continuously pulling it towards the Earth and preventing it from flying off in a straight line.
Why the Moon Doesn't Crash Into the Earth
- The Moon has a large amount of momentum in its forward direction due to its mass and speed, so gravity cannot pull it straight in.
- Gravity is only strong enough to slightly change the Moon's direction rather than pull it all the way in towards the Earth.
- Because Earth continuously applies this gravitational force, the Moon's direction is constantly adjusted, resulting in a continuous curved orbit.
Speed, Direction, and Velocity in Orbits
- Although the Moon's speed remains constant throughout its orbit, its direction is always changing.
- Since velocity measures both speed and direction, a change in direction means the Moon's velocity is constantly changing.
- Using the equation for acceleration a = Δv t
- , any change in velocity means the Moon is always accelerating even though it is not speeding up or slowing down.
Centripetal Acceleration in Circular Orbits
- The acceleration experienced by an orbiting object due to its continuously changing direction is directed towards the central object and is called centripetal acceleration.
- This centripetal acceleration is caused by the gravitational force acting between the two objects, keeping the orbiting body on its curved path.
Orbit Size and Orbital Speed
- The smaller an object's orbit, the closer it is to the central body, meaning the gravitational force acting on it is much greater.
- To maintain a stable orbit at a smaller radius, the orbiting object must travel at a greater speed so its instantaneous velocity is large enough to avoid being pulled inward.
- The larger an object's orbit, the weaker the gravitational pull, so the object can maintain a stable orbit at a lower speed.
Key Takeaways on Orbits
- A stable orbit is the result of a balance between the instantaneous velocity of the orbiting object and the gravitational attraction of the central body.
- Objects in smaller orbits must travel faster to maintain stability, whilst objects in larger orbits travel more slowly.
- An orbiting object is always accelerating due to its constantly changing direction, even when its speed is constant.