Unit 4.3 - Orbits and the wider universe Flashcards
What do we apply in this unit?
What we learned about gravitational fields to the behaviour of objects in orbit
What happens if the initial speed of a cannonball is increased? Why?
The path becomes longer since it travels further
If a cannonball was fired from the top of a mountain when would it be in orbit? Explain
If it was fired at a certain speed at a certain speed, which is when it’s travelling long the equipotential since no work is done nd so no energy is lost
Is there friction in the atmosphere?
No
When is something in orbit? Explain
When it’s travelling along the equipotential since no work is done and so no energy is lost
What would you feel if you were inside a cannonball fired in space? Explain
You would constantly be falling but in addition the air around you would be falling at what same speed - you would feel an absence from a gravitational field and so feel weightless
What did Kepler based his work on?
The observations of Tycho Brahe
What are all of Kepler’s laws - why and what does this mean?
Empirical
He described what he saw based on observation
They weren’t agreed physically, only agreed with observation
How did Newton explain Keplers laws?
Using his laws of motion and the law of general gravitation
When was Kepler’s 3rd law no longer empirical?
When newton explained his laws using his laws of motion and the law of general gravitation
Kepler’s first law
The planets move in elliptical orbits with the sun at one focus
Kepler’s second law
The vector radius (an imaginary line connecting the planet and the sun) sweeps out an equal area in an equal time
Kepler’s third law
The square of the period of orbit of a planet is directly proportional to the cube of its average distance from the sun
T^2 ∝ r^3
How do we know that Kepler’s 1st law is true?
The sum of the distance between 2 foci is always constant
How would we describe an eclipse if the 2 foci are far away?
Eccentric (stretched out)
When do planets travel fastest in their orbit and why?
When closer to the sun
Angular momentum is conserved and is constant
Explain why planets travel faster when closer to the sun in terms of Kepler’s 2nd law
Close to sun = small distance to sun but moves faster so larger radius to make up for it
What shaped orbits are studied in this course?
Circular
Why do we use circular orbits in this course?
We will have a constant velocity since planets won’t get further/closer to the sun
What kind of motion do we have if we only study circular orbits?
Circular motion
Explain why Kepler’s first 2 laws are trivial for circular orbits?
An ellipse has two foci - a circle has both at the centre (where the mass causing the orbit lies)
The orbiting body moves at a constant speed and therefore the area swept out in a given time
Why were Kepler’s laws empirical?
Since they described the motions but didn’t give a theoretical underpinning to them
Why were Newton’s theories required for Kepler?
To explain why the planets moved in the way described by Kepler
How did Newton derive an equation for Kepler’s third law?
Using his universal law of gravitation