2. Gravity and motion Flashcards
In the Star Wars Episode IV movie, Han Solo states about his spaceship, the Millennium Falcon: It’s the ship that made the Kessel Run in less than twelve parsecs. Explain if this statement makes sense or not.
Parsec is a measure of distance, not speed.
Why do objects move at a constant velocity if no force acts on them?
Newtons first law
What are Kepler’s laws of motion?
- The orbit of a planet is an ellipse with the Sun at one of the two foci
- A line segment joining a planet and the sun sweeps out equal areas during equal intervals of time
- The square of a planets orbital period is proportional to the cube of the length of the semi major axis of its orbit
What is velocity?
Velocity is the rate and direction of an objects movement
Example: if you move 5kmph in a circle and finish where you started, your speed is 5kmph but your velocity is 0
Explain how we can use Newton’s version of Kepler’s third law to measure the total mass of two stars in a binary system.
We can measure how far they move apart from each other (their orbital distance) and how long it takes them to move around each other (their orbital period). Then you use the formula to calculate M1+M2.
What is the centre of mass?
The unique point
Weighted Relative Position
Distributed mass
Sums to zero
What is mass?
Mass is the amount of matter in an object
What determines the strength of gravity?
The masses of objects and the distance between them
Standard time is defined…
With respect to the time zones
What is weight?
Weight is the the force acting on an object due to gravity
What are Newtons 3 laws?
- Every object will continue to be in a state of rest or move at a constant speed in a straight line unless it is compelled to change by an outside force
- The change of motion of a body is proportional to and in the direction of the force acting on it
- For every action there is an equal and opposite reaction
What is latitude an longitude?
A system of lines used to describe the location of any place on Earth. Lines of latitude run in an east-west direction across Earth. Lines of longitude run in a north-south direction
What is force?
Force is a physical influence, which when applied to an object causes it to accelerate in the direction from which it was applied
Newtons version of Kepler’s 3rd law
Newtons formulation introduces the additional factor of the masses of the Sun (M1) and the planet (M2), both expressed in units of mass.
a^3=(M1+M2)xP^2
How does the tilt of the Earth cause the seasons?
The Earths spin axis is tilted with respect to its orbital plane. When the Earths axis points towards the Sun it is summer for the hemisphere
Mean solar time
Defined locally based on the average solar day over the course of the year. Contains exactly 24 hours and is what we use in our every daytime keeping
Describe the shape of planetary orbits in our solar system
Elliptical
What is angular momentum?
Angular momentum is the rotational analogue of linear momentum. It is a product of mass, velocity and radius.
What is acceleration?
Acceleration is the rate of change of velocity
Explain why orbital period is independent of eccentricity
Eccentricity is the ratio of the distance between the foci to the length of the semi major axis of the ellipse.
Orbital period is the time it take a planet to travel once around the Sun.
Orbital period depends on the distance to the centre of the object being orbited and the mass of that object.
What keeps a planet rotating and orbiting the Sun?
Gravity
Formula for Newtons law of gravity
If you could somehow compress the Sun so that it still had all its mass but had a radius of only three kilometres, it would become a black hole!
What effect would this have on the Earth’s orbit?
While 8.3 minutes after this occurred the Sun would disappear from the Earth’s view, its gravitational force would remain. Gravity simply depends on masses of objects and their separation. Neither the size nor density matter, and so indeed there would be no effect on the orbit.
Explain how planets are under constant acceleration from the force of gravity from the Sun
The gravitational attraction between two bodies is proportional to their masses. The more mass an object has, the stronger the pull of its gravitational force
Suppose a solar system has a star that is four times more massive than our Sun. If that solar system has a planet the same size as Earth, orbiting at a distance of 1 AU, what is the orbital period of the planet?
From Kepler’s law, we see that the period depends on the inverse square root of the object masses. Thus, if we have a star four times as massive as the Sun, the period of a planet orbiting at 1 AU will be half that of the Earth, or 6 months
What causes eclipses?
An eclipse happens when one astronomical body blocks light from or to another
What is gravitational acceleration?
Gravitational acceleration is the acceleration of an object in free fall within a vacuum. This is the steady gain in speed caused exclusively by the force of gravitational attraction
What is an ellipse? How is it different from a circle?
An ellipse is a plane curve surrounding two focal points, such that for all points of the curve the sum of the two distances to the focal point is a constant
Sidereal day is…
4 minutes shorter than a solar day
How long does the cycle of lunar phases take?
29.5 days
How do we mark the progression of the seasons?
We define 4 special points: summer and winter solstice, spring and fall equinox
Describe the components of an ellipse
Centre
Two foci
Major and semi major axis
Minor and semi minor axis
How do we locate objects on the celestial sphere?
Right ascension and declination
What is momentum?
Momentum is the product of the mass and velocity of an object
Why do we see phases of the Moon?
Its orbit around Earth and Earths orbit around the Sun
What is speed?
Speed is the time rate at which an object is moving along a path
What is a sidereal day?
The time it takes for the Earth to complete one rotation about its axis with respect to “fixed” stars
4 minutes faster
Particular star will rise 4 minutes earlier every night
Reason why different constellations are only visible at specific times of the year
P^2=a^3
P = orbital period in years a = distance in AU
Kepler’s third law, says that a planet’s orbital period squared is proportional to the semimajor axis of its orbit cubed.
